U.S. patent application number 15/196323 was filed with the patent office on 2018-01-04 for methods and apparatus for continuous current limiting for fets in high inductive load systems.
The applicant listed for this patent is Texas Instruments Incorporated. Invention is credited to Sualp Aras, Abidur Rahman, Kyle Schulmeyer.
Application Number | 20180006643 15/196323 |
Document ID | / |
Family ID | 60786172 |
Filed Date | 2018-01-04 |
United States Patent
Application |
20180006643 |
Kind Code |
A1 |
Schulmeyer; Kyle ; et
al. |
January 4, 2018 |
Methods and Apparatus for Continuous Current Limiting for FETS in
High Inductive Load Systems
Abstract
An apparatus includes a FET device having a drain terminal,
source terminal and a gate terminal; a first supply voltage coupled
to the drain terminal of the FET; an output terminal coupled to the
source terminal of the FET; a bias current supply coupled to the
gate terminal of the FET; a second supply voltage coupled to the
gate terminal of the FET; a current sensing circuit coupled to
output a sense current proportional to the current flowing through
the FET; a current limit comparator coupled to the sense current
and comparing the sense current to a predetermined limit current; a
pull down current circuit coupled to remove current from the gate
terminal of the FET; a current time derivative circuit coupled to
the sense current and outputting a sense rate current; and a
circuit coupled to receive the sense rate current and coupled to
the bias current supply.
Inventors: |
Schulmeyer; Kyle; (Plano,
TX) ; Aras; Sualp; (Dallas, TX) ; Rahman;
Abidur; (Richardson, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Texas Instruments Incorporated |
Dallas |
TX |
US |
|
|
Family ID: |
60786172 |
Appl. No.: |
15/196323 |
Filed: |
June 29, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H03K 17/0822 20130101;
H02M 2001/0009 20130101; H02M 1/32 20130101; H03K 5/08 20130101;
H02M 3/07 20130101; H03K 17/687 20130101; H02M 3/156 20130101 |
International
Class: |
H03K 17/687 20060101
H03K017/687; H03K 5/08 20060101 H03K005/08; H02M 3/07 20060101
H02M003/07 |
Claims
1. An apparatus, comprising: an FET having a drain terminal, a
source terminal and a gate terminal; a first supply voltage coupled
to supply current to a current conduction path between the source
terminal and the drain terminal of the FET; an output terminal
coupled to receive current flowing through the current conduction
path of the FET; a bias current supply coupled to the gate terminal
of the FET; a second supply voltage coupled to the gate terminal of
the FET; a current sensing circuit coupled to output a sense
current proportional to the current flowing through the FET; a
current limit comparator coupled to the sense current and comparing
the sense current to a predetermined limit current; a pull down
current circuit coupled to the current limit comparator and coupled
to remove current from the gate terminal of the FET; a current time
derivative circuit coupled to the sense current and outputting a
sense rate current; and a circuit coupled to receive the sense rate
current and coupled to the bias current supply.
2. The apparatus of claim 1, and further including an enable output
coupled from the current limit comparator to the pull down current
circuit.
3. The apparatus of claim 1, and further including circuitry to
reduce the bias current to the gate terminal of the FET responsive
to the sense rate current.
4. The apparatus of claim 1, in which the predetermined limit
current is programmable.
5. The apparatus of claim 4, in which the predetermined limit
current is stored to a writeable storage circuit.
6. The apparatus of claim 1, in which the current time derivative
circuit coupled to the sense current includes a current rate
detection circuit for outputting the sense rate current when the
rate of increase of the sense current exceeds a predetermined
threshold.
7. The apparatus of claim 1, and further including: an enable
circuit coupled between the sense rate current and the bias current
supply; and a current rate enable comparator circuit coupled to the
sense current and to a reference current, and outputting an enable
signal to the enable circuit.
8. The apparatus of claim 7, in which the current rate enable
comparator circuit includes a scaling factor and the current rate
enable comparator circuit compares a difference between the sense
current and a multiplied product of the scaling factor and the
predetermined limit current to the reference current.
9. The apparatus of claim 8, in which the scaling factor is
programmable.
10. The apparatus of claim 9, and including a programmable storage
circuit storing the scaling factor.
11. The apparatus of claim 10, and including an external bus
interface coupled to address the programmable storage circuit.
12. A method, comprising: receiving an enable signal; coupling a
current conduction path of an FET between a supply voltage terminal
and an output terminal for supplying current; coupling a voltage
and a pull up bias current to a gate terminal of the FET; sensing
current flowing through the FET; comparing the sensed current
flowing through the FET to a predetermined current limit; reducing
the current at the gate terminal of the FET, responsive to
comparing the current flowing through the FET to the predetermined
current limit; comparing a rate of increase of current flowing
through the FET to a rate increase threshold; and reducing the pull
up bias current to the gate terminal of the FET responsive to the
comparing the rate of increase of current flowing through the
FET.
13. The method of claim 12, where comparing the rate of increase of
current flowing through the FET further includes: comparing the
current flowing through the FET to a threshold that is a
predetermined percentage of the predetermined current limit; and
enabling reducing the pull up bias current to the gate terminal of
the FET, responsive to the comparing.
14. The method of claim 12, and further including, after reducing
the current at the gate terminal of the FET, operating the FET to
supply current at the output terminal at the predetermined current
limit.
15. The method of claim 12, in which coupling the current
conduction path of the FET between the supply voltage terminal and
the output terminal for supplying current further includes coupling
a drain terminal of the FET to a supply voltage and coupling a
source terminal of the FET to the output terminal.
16. The method of claim 12, in which coupling the current
conduction path of the FET between the supply voltage and the
output terminal for supplying current further includes coupling the
a drain terminal of the FET to a terminal for supplying current and
coupling a source terminal of the FET to a terminal for receiving a
ground potential.
17. The method of claim 12, in which coupling the voltage and the
pull up bias current to the gate terminal of the FET further
includes supplying an increased gate voltage that is greater than a
supply voltage.
18. The method of claim 12, in which comparing the sensed current
flowing through the FET to the predetermined current limit includes
comparing the sensed current to an adjustable current limit that is
stored in a writeable storage circuit.
19. An integrated circuit, comprising: a terminal for driving a
gate of a power FET having a current conduction path coupled
between a voltage supply and a load; a voltage input terminal for
coupling a supply voltage to the integrated circuit; a pull up bias
circuit coupled to the terminal for driving the gate of the power
FET and to the voltage input terminal for coupling an increased
power supply; a current sensing circuit coupled to the voltage
input terminal to receive a voltage proportional to the current
flowing in the power FET, and having a sensed current output; a
current limit comparator comparing the sensed current to a current
limit; a pull down current circuit coupled to the current limit
comparator and to the terminal for driving the gate of the power
FET; a current time derivative circuit coupled to the sensed
current; and a current reduction circuit coupled to the current
time derivative circuit and to the pull up bias circuit.
20. The integrated circuit of claim 19, and further including an
adjustable trim current supply to set the current limit.
Description
TECHNICAL FIELD
[0001] This application relates generally to the use of power
transistors, and more particularly to the use of a power field
effect transistor (FET) device to drive a high inductive load.
BACKGROUND
[0002] Power field effect transistors (FETs) are increasingly used
to drive high inductive loads. In an example application, a power
FET drives a headlamp or headlight in an automobile or other
vehicle. The power FET is coupled between the power supply and a
high current bulb using a cable to couple current from the power
FET circuit to the headlamp. The cable and the bulb can present a
high inductive load in excess of 5 .mu.H. In an automotive example
application, the inductive load can be in a range between 5-20
.mu.H. The bulbs of some headlights presently in use require a very
high current at turn on, in an example the "inrush" current (the
current drawn at the initial turn on) for the headlight can be in
the range of 90-100 Amperes.
[0003] The power FET has a safe operating area (SOA) of current and
voltage that can be maintained without damage to the device. The
SOA is determined by the manufacturer of the FET. The SOA indicates
the current and voltage conditions that need to be maintained to
ensure reliable operation without damage to the FET and to ensure
that the FET will meet an expected lifetime of the product. In
order to protect the power FET and keep the device operating within
the recommended SOA, current limiting is utilized. However, if a
very high current through the FET occurs that causes a current
limiting circuit to shut down or to limit the current flowing in
the FET device, the energy stored in the inductive load has to be
discharged. Although the FET may be off or may be turning off, the
FET is still in the circuit between the inductive load and the
power supply, and the stored energy is discharged through the FET.
The discharge of the inductor energy through the FET can cause
undesirable device stress or can even destroy the FET because the
SOA boundary conditions can be violated. A specific characteristic
of a conventional current limit or shutdown of a power FET circuit
is that there can be current overshoot at turn off or current limit
of the FET that exceeds the boundary of the SOA for the FET.
SUMMARY
[0004] In described examples, an apparatus includes: a FET device
having a drain terminal, a source terminal and a gate terminal; a
first supply voltage coupled to the drain terminal of the FET; an
output terminal coupled to the source terminal of the FET; a bias
current supply coupled to the gate terminal of the FET; a second
supply voltage coupled to the gate terminal of the FET; and a
current sensing circuit coupled to output a sense current
proportional to the current flowing through the FET. A current
limit comparator coupled to the sense current compares the sense
current to a predetermined limit current; a pull down current
circuit is coupled to remove current from the gate terminal of the
FET; a current time derivative circuit coupled to the sense current
and outputs a sense rate current; and a circuit is coupled to
receive the sense rate current and coupled to the bias current
supply.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 illustrates in a circuit diagram a conventional high
side power FET circuit and driver device coupled to an inductive
load.
[0006] FIG. 2 illustrates in another circuit diagram a conventional
low side power FET circuit and driver device coupled to an
inductive load.
[0007] FIGS. 3A and 3B are two graphs illustrating a voltage and a
current for a conventional power FET circuit.
[0008] FIG. 4 illustrates in a circuit diagram a power FET
embodiment in a high side driver configuration.
[0009] FIG. 5 illustrates in another circuit diagram an additional
power FET embodiment.
[0010] FIGS. 6A-6D are graphs depicting an input voltage and output
currents for example power FET circuits.
[0011] FIG. 7 is a circuit diagram illustrating a power FET
embodiment in a low side driver configuration.
[0012] FIG. 8 is a flow diagram illustrating a method
embodiment.
[0013] FIG. 9 is another flow diagram illustrating a method
embodiment.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0014] Corresponding numerals and symbols in the different figures
generally refer to corresponding parts unless otherwise indicated.
The figures are not necessarily drawn to scale.
[0015] The term "coupled" may include connections made with
intervening elements, and additional elements and various
connections may exist between any elements that are "coupled."
[0016] FIG.1 depicts in a circuit diagram a conventional circuit
using a power FET to supply current to an inductive load. In FIG.
1, the power FET MN0 is arranged as a "high side" driver, in that
the power FET is coupled to a power supply at the drain terminal,
and supplies current to the load from the source terminal. The FET
MN0 in FIG. 1 is shown as an N-type FET. Note that in an
alternative arrangement the FET MN0 could also be arranged as a
P-type FET, with the drain and source terminals reversed. Various
FET devices are available for power applications and these are
applicable to the embodiments.
[0017] The gate conductor and gate insulator of a FET device form a
capacitor with the semiconductor substrate. For a power FET, the
gate capacitance can be quite large. To switch the power FET from
an initial off state to an on state, the gate capacitor has to be
charged. To then switch the FET from the on state to an off state,
the gate capacitor has to be discharged. Because the switching
speed of the FET is critical to proper operation of the circuit,
gate driver devices are often coupled to the gate terminal of the
FET to turn on, and turn off, the power FET. Substantial current
can be supplied to the gate terminal or taken from the gate
terminal of the power FET during switching operations, to charge
and discharge the gate capacitor and turn the FET on, or off, as
needed.
[0018] In FIG. 1, a conventional power FET circuit 100 is shown for
driving a load as a high side driver. Power FET MN0 supplies
voltage and current at terminal VOUT in response to a voltage and
current at a gate terminal labeled GATE. A supply voltage VBB is
coupled to the input terminal VIN for supplying the current to the
drain to source current conduction path of the FET and then to the
load at terminal VOUT. A driver circuit 105, which can also include
additional FET devices, supplies the FET gate signal at the FET
terminal GATE in response to an input enable signal labeled EN. In
this example arrangement, the driver circuit 105 has power supplied
from a charge pump 101. The charge pump 101 provides a voltage
greater than the voltage VIN for driving the GATE signal. The
output voltage at terminal VOUT can take high voltage levels close
to the input VIN. This output voltage VOUT is at the source
terminal of the FET MN-0. To turn on the transistor MN0 the voltage
at the terminal GATE has to be raised above the source voltage by a
transistor threshold voltage Vt. By raising the supply voltage to
the FET GATE input substantially above the supply voltage VIN, the
FET MN0 can always be turned on by driver 105. In one example, a
charge pump outputs a pump voltage that was boosted by 13 Volts
above VIN to the gate driver 105.
[0019] When supplying currents to a load, the FET MN0 is typically
operated in the linear mode. In operation of the power FET in this
mode, VOUT can be made approximately equal to VIN.
[0020] In FIG. 1, current limiting is applied to protect the power
FET MN0. In an example automotive application, a cable coupled to
the output VOUT at the source of the FET MN0 can be coupled to a
load 109 such as a headlamp. If the headlight or the cable is
damaged, for example due to a vehicle accident, a short to ground
can occur. This is the case drawn in FIG. 1. In a shorting case the
current drawn at the output VOUT can quickly exceed the current
that the power FET MN0 can safely conduct. In FIG. 1 the load is
shown with the bulb not shown but with a cable 109 represented as
shorted to ground. A current sensing circuit is used to detect and
limit the current flowing through the power FET MN0 to protect the
power FET. In some applications, when the current limit is reached,
the power FET can be disabled. In other applications, when the
current limit is reached, the power FET is operated while
conducting a maximum safe current, to continue supplying power to
the load.
[0021] In FIG. 1, a sense resistor Rsense is coupled between the
input voltage VIN and a drain terminal of the FET MN0. The current
flowing through the resistor Rsense is the same as the output
current (which is also the drain to source current or Ids current
that flows through the FET MN0). A voltage developed across the
sense resistor Rsense is therefore proportional to the FET current,
due to the operation of Ohm's law. The voltage across the resistor
Rsense is converted to a current by the voltage-to-current (V/I)
converter 103. As the sensed current increases, the current labeled
Iclimit in FIG. 1 modulates the gate current output from driver 105
and therefore reduces current into the GATE terminal. By limiting
the current available, the drain to source current flowing through
the power FET MN0 can be limited to a maximum level. In a critical
application such as a headlamp driver application, the system may
require that the FET continuously supply current to the load at a
maximum safe current. This condition can also occur at startup, for
example, when high inrush current has to be maintained during the
initial turn on of the headlight.
[0022] A problem with the conventional power FET circuit of FIG. 1
is that when the FET MN0 is turned off, for example in response to
a falling signal EN, the load 109 may have energy stored in it
(particularly when driving an inductive load) that needs to be
discharged. Without a path to discharge the gate capacitance of the
FET, an undesirably long time can elapse before the FET turns off.
During this time, the FET is in a series circuit with a discharging
inductor (the load) and Ids current that exceeds the SOA conditions
for the FET may occur, due to the large current flowing into
it.
[0023] The circuit 100 is shown coupled to an inductive load 109
representing, for example, a cable normally coupled to a headlamp
but shown shorted to ground. In automotive applications the load
can range from about 5-20 .mu.H. Other loads can be driven by the
FET MN0.
[0024] FIG. 2 illustrates in a second circuit diagram a
conventional power FET circuit 200 for a low side driver
application. The circuit 200 of FIG. 2 includes several components
that are similar to and operate in the manner of the circuit 100 of
FIG. 1 described hereinabove. In FIG. 2, the driver 205, the sense
resistor Rsense, the voltage to current V/I circuit 203, and the
power FET MN0 all correspond to the similar elements of FIG. 1,
with the reference labels now changed from "1XX" in FIGS. 1 to
"2XX" in FIG. 2, for convenience. In FIG. 2, circuit 200 is
supplied by a voltage at the terminal labeled "driver supply" while
the power FET MN0 has the drain terminal "DRAIN" coupled to the
load 209. The load in this example arrangement is series coupled
between the supply voltage VBB and the drain of the power FET. In
some examples this voltage VBB can be 12-13 Volts supplied from a
lead acid battery. In other examples other voltages can be used,
such as 24 Volts, 48 Volts, 6 Volts, or higher or lower voltages.
In an example of a headlight switch, the load 209 includes the
headlight and a cable connecting the battery to the headlight. The
load 209 is shown as an inductor. In example automotive
applications the inductor 209 can be in a range between 5-20 .mu.H.
Circuit 200 can drive other loads, as well.
[0025] In one difference between the high side and low side
arrangements of FIG. 1 and FIG. 2, respectively, the FET gate
terminal GATE in FIG. 2 does not need to be coupled to a boosted
voltage as in FIG. 1. The source terminal of the FET MN0 in the low
side driver configuration of FIG. 2 is coupled to ground through
the sense resistor Rsense. The gate to source voltage needed to
turn on the FET MN0 is now just a threshold voltage Vt, plus the
voltage of the sense resistor Rsense, above ground, and so in this
example, no charge pump is needed to supply the GATE voltage.
[0026] In operation of the circuit in FIG. 2, the FET MN0 allows
current to flow through load 209 in response to the voltage at the
gate terminal GATE, which is raised to a voltage at least above a
threshold voltage Vt for the FET when the driver circuit 205
receives an enable signal EN. The current flowing through the load
209 is the same as the drain to source current conducted by the
FET, and this same current flows through the sense resistor Rsense
to ground. The voltage across resistor Rsense is proportional to
the current flowing through the FET MN0 due to the operation of
Ohm's law. The voltage is converted to current by the V/I or
voltage to current converter 203. The FET current is limited by
removing current from the driver 205 using the limit current
Iclimit. Iclimit reduces the current into the power FET MN0 at the
gate terminal GATE if the sensed current exceeds a threshold
current level, in the same manner as the current limiting operation
for the high side driver circuit in FIG. 1.
[0027] FIGS. 3A and 3B illustrate in a voltage graph and a current
graph the voltage input and the corresponding current of a FET
circuit such as circuit 100 in a turn on operation. The horizontal
axis of FIGS. 3A and 3B illustrate time in milliseconds. The
vertical axis of FIG. 3A illustrates a rising voltage on the input
terminal EN in data line 301. At time 20.0 the enable terminal
receives a rising voltage of 3.4 Volts. Data line 303 illustrates
current flowing through the FET MN0 on the vertical axis of FIG.
3B. In this example the current limit is arranged to be at 90
Amperes. As can be seen in the data line 303 in FIG. 3B, the
current overshoots the specified current limit at time M3: 20.3,
rising to a maximum current of above 190 Amperes, a level that is
100 Amperes above the desired current limit of 90 Amperes. At time
M2: 21.95, the current settles at 90.89 Amperes, approximately at
the limit of 90 Amperes. The current overshoot is caused because
even though the current limiting circuitry reduces the gate signal
when the limit (here, 90 Amperes) is reached, the time it takes to
discharge the gate capacitance and reduce the current flow in the
FET allows the current overshoot to occur until the power FET MN0
can operate at a reduced current.
[0028] FIG. 4 illustrates in a circuit diagram an example
embodiment power FET circuit 400. In FIG. 4 a high side driver
arrangement is shown. Power FET MN0 is coupled to supply current
and voltage to a load at the output terminal VOUT_PAD. Power FETs
with high voltage and high current handling capabilities and having
very low on resistance (RDSon) include NexFET.TM. MOSFET devices
currently available from Texas Instruments Incorporated. NexFET.TM.
devices are a type of vertical FET that have a drain contact at a
bottom surface of a semiconductor substrate and a source contact on
the upper surface of the substrate, and which feature a shared
drain region that can be shared by many channel regions, including
one or more parallel MOSFET devices formed in the substrate.
However, the embodiments are not limited to any one power
transistor type or any semiconductor technology type, and the
various embodiments are applicable to any power FET device.
[0029] In FIG. 4, the circuit outputs output current Iout. A supply
voltage such as a battery voltage can be coupled to an input
terminal VBB_PAD. The voltage from a charge pump circuit 401 Vpump
is coupled to various components. This supply voltage Vpump is
increased above the voltage at the input terminal VBB_PAD. In FIG.
4, all of the components can be implemented on a single integrated
circuit, as indicated by the dashed line at the border of 400.
However, in alternative arrangements, some of the components can be
implemented as discrete components while other components are
arranged on one or more integrated circuits. In one example, the
power FET MN0 and the sense resistor Rsense were fabricated on a
power FET integrated circuit; while the remaining components were
fabricated on a separate driver integrated circuit. These separate
integrated circuits can be packaged individually and mounted on a
circuit board, or module. In alternative arrangements, advanced
packaging techniques such as multi-chip modules (MCMs) or stacked
die packages can be utilized to form a single package for the
system 400 even when the components are implemented on separate
integrated circuits or on separate integrated circuit dies. Power
FETs for use with the embodiments include but are not limited to:
NexFET.TM. devices, vertical FET devices appropriate for power
applications, DMOS power FETs and LDMOS power FETs. The embodiments
are applicable to any FET device.
[0030] In FIG. 4, the gate terminal GATE of power FET MN0 has
several circuits coupled to it. These circuits supply and modulate
the power FET gate current and voltage. These circuits make up gate
driver circuitry. The terminal GATE receives pull up bias current
from transistor M405, which is a P-FET transistor that is part of a
current mirror formed with transistor M407, and which is biased by
the current source "Ipullup_bias."
[0031] Current source Ipullup_bias provides the current used to
pull up the gate terminal GATE to the voltage supply Vpump in
response to an enable signal EN. When a signal is received on the
enable terminal EN, the charge pump 401 and the pullup bias current
source Ipullup_bias become active. Transistors M405 and M407 also
provide current scaling, here shown as 10:1, to increase the
current supplied to the GATE terminal of the FET MN0 from the
charge pump voltage Vpump by 10 times the current Ipullup_bias. By
using different sized transistors in the current mirrors, the
various transistors can provide current scaling. The scaling
feature allows smaller bias and reference current sources to be
used. Other scaling factors can be used to form additional
embodiments. The scaling factors in FIG. 4 are indicated by two
integers spaced by a colon such as: 1:1, 2:1, 1:5, 10:1.
[0032] In addition, the gate terminal GATE of the power FET MN0 is
coupled by transistor M403 and transistor M402, which form a
current mirror, to the voltage Vpump by transistor M409. Transistor
M413 forms a current mirror with M409 and mirrors a pull down
current labeled "Ilim_pulldn" to the gate terminal GATE.
[0033] A current sensing circuit is included in FIG. 4. A sense
resistor Rsense is coupled between the pad VBB_PAD for coupling a
battery supply voltage and the drain terminal of power FET MN0. The
FET drain voltage Vd is coupled to an operational amplifier 412 in
a voltage to current converter. The operational amplifier and
transistor M417, along with resistor Rvm, form the voltage to
current converter circuit. Because the operational amplifier 412 is
arranged in a feedback configuration with a virtual ground between
the negative and positive inputs, the circuit modulates the output
voltage to minimize the difference between the positive and
negative inputs, and the voltage at the gate terminal of transistor
M417 will be increased or decreased with the voltage Vd. The
current Isense flowing through resistor Rvm will be proportional to
the output current lout, providing a current sensing function.
[0034] The sensed current Isense is used to control the operation
of the power FET by comparing Isense to several other reference
current levels. Transistor M419 couples Isense to a current mirror
formed by transistors M421 and M423, and then to a comparator
formed using transistors M432, M433 and M434. A current source
labeled "Itrim" is supplied to the comparator by a current mirror
formed of transistor M429 and M431. The trim current Itrim can be
fixed or adjustable, for example Itrim can be provided by a digital
to analog converter (DAC) with a digital control word to set the
current for Itrim. Additional arrangements include trimmable
current sources or fixed current sources to provide the current
Itrim. A programmable current source using a writeable register to
selectively couple parallel current sources to M429 can be used to
for the trimmable current source Itrim. In another embodiment, the
current source Itrim can be set by the use of a digital bus
interface such as an I2C bus, and a settable register value. By
writing values to the register a system controller or CPU can
change the value of the current Itrim. In other embodiments, the
value can be set at manufacture time using trim resistors. In still
other embodiments, the value for the current source Itrim can be
fixed. The comparator formed by transistors M433, M432 is coupled
to a current mirror formed from transistors M434 and M435.
[0035] In operation, when the current Isense is greater than the
trim current Itrim, the comparator outputs a pull down current
ILim_pulldn. This pull down current is mirrored by the P type FET
devices M413, M409 to the gate of the FET transistor MN0, and with
mirror formed by transistors M402, M403, is used to reduce the
current flowing into the gate terminal GATE of the power FET MN0,
and thereby controls the output current Iout. When the current
limit set by the value for Itrim is reached, the pull down current
can be used to keep the output current Tout at the desired current
limit. Note that various transistor and current mirror devices in
the example embodiment 400 have device scaling used to increase the
currents. For example, the current mirror formed from M402, M403
has a 5:1 size ratio which increases the pull down current at the
gate of FET MN0. Similarly, the current mirror formed by
transistors M413, M409 has a 2:1 ratio. These scaling factors can
be modified to form alternative embodiments as needed for a
particular application. Use of transistor scaling allows the sense
current Isense to be lower than the output current Tout, saving
power in the driver control circuitry.
[0036] When the circuit 400 is to be turned on, an enable signal EN
is received and turns on the charge pump 401, outputting the power
supply voltage Vpump. The pull up current Ipullup_bias is supplied
through the current mirror formed by transistors M407, M405 to the
gate of the FET MN0. Note in this example embodiment, transistors
M405, M407 provide a 10:1 scaling to increase the pull up current
at the gate terminal GATE of FET MN0. When the FET is initially
turned off, the gate capacitor between the gate and source
terminals Cgs is fully discharged. As the voltage Vpump and the
pullup current from the bias current source Ipullup bias is
supplied into the gate of the FET, the source of FET MN0 follows
the increasing voltage at the gate in a source follower
configuration until the gate capacitance Cgs of the power FET MN0
is charged, the gate terminal GATE exceeds a threshold voltage for
the power FET MN0, and the FET MN0 turns on. The rise time of the
gate terminal GATE of the FET MN0 is set by the bias current
Ipullup_bias. This pull up bias current value can be fixed prior to
manufacture, after manufacture, and can be made adjustable or
trimmable.
[0037] Once the FET MN0 turns on, the gate current and voltage can
be used to control the current Tout. The FET MN0 is operated in the
linear mode for most situations. Although not shown for clarity of
the illustrations, additional circuitry for pulling the gate
terminal GATE down in a FET turn off situation can be used to
discharge the gate rapidly using transistors M402, M403 along with
additional control circuitry to couple the gate to VOUT_PAD.
[0038] In an operation supplying current to a load at the terminal
VOUT_PAD, the resistors Rsense and Rvm operate in a ratio to form
current Isense that is less than, but proportional to, the output
current Tout, according to Equations 1 and 2:
Rsense*Iout=Isense*Rvm (1)
Isense=Iout*(Rsense/Rvm) (2)
[0039] When the sensed current Isense is greater than or equal to
the adjustable current Itrim, a current comparator formed of
transistors M432, M433, M434, M435 will begin to output the pull
down current Ilim_pulldn. The comparator circuits form the
difference (Isense-Itrim) as shown in FIG. 4, which is mirrored to
become ILim_pulldn. The relationship of the output current Tout to
the trim and pull down currents when the current limit is reached
is given by Equation 3:
Iout(limit)=(Rvm/Rsense)*(Itrim+Ilim_pulldn) (3)
[0040] By arranging the value of the trim current Itrim, the
current limit for the output current Tout can be controlled and
set. Note that the sizes of transistors in the circuitry can be
modified to ratio various currents in the circuit. For example,
transistor M413 and M409 have a two to one ratio, while transistor
M403 and M402 have a five to one ratio, so that the output current
Tout can be much larger than the other currents in the circuit.
[0041] In addition to the current limiting circuitry, the
embodiment in FIG. 4 also features a current time derivative
control circuit 410. In the embodiments, the power FET MN0 can have
reduced or eliminated current overshoot during a current limited
operation. The output current overshoot that is observed in
conventional current limiting circuitry is eliminated or greatly
reduced by use of the embodiments. This is achieved by first
detecting a rapid rate increase in the sensed current prior to the
current flowing in the power FET reaching the current limit, and by
reducing the rate of the output current increase. As the output
current lout reaches the current limit, the rate of increase in the
output current is reduced. The reduction in current rate increase
as the current limit is approached allows the output current to
settle at the current limit, but either with no overshoot or
without significant overshoot. That is, by sensing the rate of
increase in the output current and by modulating the rate of
current increase through the power FET even as the current limit is
approached, the output current can safely reach the current limit
and settle at the current limit without the large current
overshoots of conventional current limit circuitry. In this manner,
the FET can be operated within the SOA conditions, protecting the
FET from damage or reduced lifetime.
[0042] In FIG. 4, the current time derivative or "di/dt" function
410 receives the sense current Isense from a current mirror
including transistor M421 and transistor M427. The current time
derivative function 410 determines the rate of increase of the
sense current Isense, which is proportional to the rate of increase
of the output current Tout. A current mirror formed with
transistors M430, M438 couples an output current Isense_Rate to the
pull up bias current circuitry using transistors M415, M411. The
current Isense_Rate flows through M438 to ground and reduces the
pullup current Ipullup_bias flowing into the GATE terminal of the
power FET MN0.
[0043] In operation, when the rate of increase of the sensed
current Isense exceeds a threshold, the current time derivative
circuit 410 will output current Isense_Rate and cause the pull up
current supplied to the gate of FET MN0 to be reduced. Reducing the
current supplied to the gate of FET MN0 reduces the slew rate of
the FET gate terminal GATE, and slows down the rate of increase of
the output current Tout. Once the current limit is reached, the
rate of change of the output current will again fall (as the
current Tout is fixed at the current limit and no longer
increasing) and the current time derivative circuit 410 will no
longer output Isense_rate, no longer affecting the output current.
The current time derivative circuit 410 will not detect any
increase in the sensed current rate once the output current Tout is
fixed or is approximately fixed at the limit current.
[0044] The current time derivative circuit 410 can be implemented
utilizing a variety of circuits that are useful for current-time or
di/dt sensing. Resistive capacitive sensing using a current to
voltage conversion of Isense, along with a capacitor that is then
periodically sampled, can be used. Hall effect sensors, coils,
current loops and other field sensing approaches can be used. The
current rate threshold can be fixed, programmable, or can be a
one-time programmable value programmed during a calibration
operation after manufacture is complete. Trimmable resistors can be
used to set the current rate threshold. A register accessible by a
controller or processor over a bus, such as an I2C interface bus,
can provide a programmable value for the current increase rate
threshold.
[0045] In the embodiment shown in FIG. 4, the current time
derivative circuit 410 is always active. While this arrangement
provides the desired reduced overshoot in the output current lout,
the current time derivative circuit 410 will also reduce the slew
rate of FET MN0 throughout the rise time, and thus the FET turn on
time will be increased. In some applications this effect can be
undesirable, as it may limit inrush current to a load in an initial
power up operation, or it may slow system start up.
[0046] FIG. 5 depicts a circuit diagram for an alternative
embodiment 500. In FIG. 5, many of the components are similar to
and function in the same manner as for the embodiment of FIG. 4.
For these components, the reference labels are similar to those
shown in FIG. 4, but with the leading numerals changed from "4" to
"5", for convenience.
[0047] In FIG. 5, power FET MN0 is coupled between a supply voltage
at a power pad VBB_PAD and an output terminal VOUT_PAD. The power
FET MN0 has supply and bias circuitry couple to a gate terminal
GATE to provide gate current to control turning on and turning off
the power FET MN0 in the same manner as described above with
respect to FIG. 4. The charge pump 501 outputs a supply voltage
Vpump when it receives an enable signal EN. The pullup bias current
supply Ipullup_bias is also enabled by signal EN.
[0048] In addition to the current sense circuitry including the op
amp 512 arranged and operating as described above with respect to
FIG. 4, in FIG. 5, the time derivative circuit 510 is coupled to
the pull up bias circuitry and the pull up current source
Ipullup_bias by an enable switch, transistor M539. An additional
enabling circuit is provided that outputs the control signal
EN_di_dt_FB. This control signal is used to cause switch M539 to
couple the Isense_Rate output of the current time derivative
circuit 510, which is a di/dt sensing circuit, to the pull up bias
supply circuit of transistors M505, M507 only when the current
Isense (and correspondingly, Tout) has increased to a selected
percentage of the current limit while it is approaching the current
limit.
[0049] In FIG. 5, a comparator circuit is formed by the reference
current Iref, the transistors M546, M547, M548, M549, the limit
current Itrim, and the sensed current Isense. Transistor M528
mirrors the sensed current Isense to a comparator including a
current mirror formed by transistors M547, M546. The trim current
Itrim is mirrored and scaled by the respective sizes of transistors
M547 and M546 as indicated by the scaling factors "X" and "Y" shown
in FIG. 5. This scaled trim current is subtracted from the sensed
current Isense. This difference current is then mirrored by
transistors M548 and M549 to the input terminal of the Schmitt
trigger device 551, where it is compared to a reference current
Iref. When the difference current between Isense and the scaled
product of (Y/X)*Itrim is greater than Iref, the input to the
Schmitt trigger 551 falls, and the output EN_di_dt_FB goes high, as
the Schmitt trigger 551 is an inverter. The Schmitt trigger device
551 outputs a pulse only when the difference between the sense
current Isense and the scaled trim current (Y/X)*Itrim is greater
than or equal to Iref.
[0050] The enable signal EN_di_dt_FB will go high when the
inequality of Equation 4 is met:
Isense-(Y/X)*Itrim>=Iref (4)
[0051] The enable signal EN_di_dt_FB output from the Schmitt
trigger 551 is coupled to the transistor M539. Transistor M539
operates to selectively couple the output of the current time
derivative circuit 510, the current Isense_rate, to the pull up
bias circuitry. In this manner the output of the current time
derivative circuit 510 is only used for that portion of the rise
time of the FET MN0 gate terminal GATE when the difference between
the sensed current Isense and the scaled trim current (Y/X)*Itrim
meets or exceeds the reference current Iref. For operations where
the sensed current does not rise to this threshold, the time
derivative circuit 510 does not affect the current of the power FET
MN0. In a case where the output current is rising and the
difference between the sensed current Isense and the scaled trim
current (Y/X)*Itrim does equal or exceed the reference current
Iref, the time derivative circuit 510 is coupled to the pull up
bias circuitry. The current Isense_rate then reduces the rate of
increase in the current lout as the current rises towards the
current limit.
[0052] The enable signal EN_di_dt_Fb can be adjusted to become
active at a predetermined percentage of the current limit. As
described above, the current limit is set by adjusting the current
source Itrim and the percentage of the current limit for enabling
the current time derivative circuit can be set using the transistor
scaling ratio Y:X by selecting the relative sizes of the
transistors M546, M547. In addition, in alternative arrangements
the scaling can be accomplished by providing a writeable storage
circuit such as addressable register coupled to a DAC circuit. An
interface to an external processor such as an I2C serial interface
can be arranged to allow the scaling factor to be stored. Further
the operation of the enable signal can be adjusted by adjusting the
current sources Iref, and Itrim.
[0053] FIGS. 6A-6D depict in four graphs the operation of example
embodiments such as are shown in FIGS. 4 and FIG. 5. In all of the
graphs FIG. 6A-6D, the horizontal axis plots time in milliseconds.
In FIG. 6A, the enable signal voltage EN is plotted on the vertical
axis. Data line 601 plots voltage at the enable terminal. At time
20.0 milliseconds, enable signal EN rises from an approximately
zero voltage to a voltage sufficient to cause the pull up bias
circuits and the charge pump to turn on the power FET in the above
circuit embodiments of FIGS. 4 and 5.
[0054] In FIGS. 6B-6D, current through the power FET is plotted on
the vertical axis in amperes (A). Data line 603 in FIG. 6B
illustrates the FET drain current response of a power FET with a
pull down limit current circuit such as shown in FIGS. 4 and 5, but
without the current time derivative circuitry being enabled. In
FIG. 6B, at timeM15: 20.16404, an overshoot is seen in the response
in data line 603 with the current rising to over 108 Amperes. An
oscillation continues to overshoot the desired current limit of
about 90 Amperes for several cycles until the oscillation dies away
at time 21.0, the current lout finally settles at time M16 21.22289
at about 91 Amperes, near the desired current limit of 90
Amperes.
[0055] In FIG. 6B, the output current lout is shown for the
embodiment of FIG. 4 with the current time derivative circuit 410
always operating. Data line 605 represents the drain current of the
power FET. At time 20.05, the output current begins rising in
response to the enable signal, but the rate of increase (slope) is
limited by the current time derivative circuit, so that the final
current at the limit of 91.39 Amperes, which is just above the
limit of about 90 Amperes, is reached at about time M17: 20.485.
Note that in contrast to data line 603 in FIG. 6A, there is no
visible overshoot in the current output data line 605 in FIG. 6B.
The current time derivative or di/dt circuit has eliminated the
current overshoot, however the rise time for the output current
through the power FET has also been substantially increased.
[0056] In FIG. 6D, data line 607 indicates the drain current for
the embodiment of FIG. 5, where the current time derivative circuit
is enabled only after the sensed current reaches a percentage
threshold of the current limit value. The threshold can be a
percentage of the maximum current or limit current, such as 80%. In
alternatives, other percentage thresholds can be utilized. In FIG.
6D, the current quickly starting at time 20.05 and by time 20.5 the
current has reached a maximum current limit of about 90 Amperes. In
data line 605, at time M18: 20.15187 a slight overshoot is seen to
about 93 Amperes, but this overshoot immediately settles out to
near the desired limit of 90 Amperes, settling at 91.39 Amperes,
the same settling current as for the embodiment of FIG. 4, plotted
in FIG. 6C. The turn on time for the power FET is much shorter in
the plot of FIG. 6D. Depending on the requirements of a particular
application, the appropriate embodiment of FIGS. 4 and 5 can be
utilized. The current limit, the percentage threshold, and the
current rate increase threshold can all be modified to form
additional embodiments as appropriate for a particular design
requirement.
[0057] FIG. 7 depicts in another circuit diagram an embodiment for
a low side driver application. In FIG. 7, elements that correspond
to similar elements in FIGS. 4 and 5 are given similar reference
numerals, except that the first digit of the reference numeral is
now a "7". Circuit 700 can be implemented as a single integrated
circuit, using discrete components, on a circuit board or module,
using a multi-chip module, or using multiple dies in a single
package to form a highly integrated device. In one arrangement, the
power FET MN0 and the sense resistor Rsense may be formed in a
single integrated circuit, while the remaining devices are formed
in a second, separate integrated circuit to provide a driver IC for
the FET. Other combinations of the devices in single or in multiple
integrated circuits are also possible.
[0058] In FIG. 7, the power FET MN0 has a drain terminal coupled to
an input pad labeled "DRAIN_PAD". In an application this terminal
can be coupled to one end of a load. The power FET MN0 has its
current conduction path, from drain to source, coupled between the
load and a ground potential at the pad "GND_PAD", that is, FIG. 7
illustrates a low side driver arrangement. The supply voltage for
the various voltage and current supplies to the gate of the power
FET MN0 is labeled "DRIVER_SUPPLY." As was described with respect
to FIG. 2 above, for a low side driver configuration, the gate of
the FET MN0 does not require a boosted voltage to turn on the FET.
A load (not shown in FIG. 7) can be coupled between a power supply
and the drain of the FET MN0 at the input DRAIN_PAD. The FET MN0
will allow current to flow through the load and to ground in
response to the enable signal EN.
[0059] The current limiting circuitry in FIG. 7 is similar to and
operates in the same fashion as that shown in FIG. 5 for the high
side driver arrangement. In FIG. 7, the enable signal EN will turn
on the bias current source Ipullup_bias. The current mirror formed
of transistors M707 and M705 will supply this bias current to the
gate of the power FET MN0. The transistors M705, M707 are arranged
to scale the bias current by a 10:1 factor, so that the current at
the transistor gate terminal GATE is larger than the current source
Ipullup_bias. The voltage at the terminal DRIVER_SUPPLY is also
coupled to the gate terminal labeled GATE.
[0060] When the drain to source current is flowing in the power FET
MN0, a voltage proportional to the FET current appears across the
sense resistor Rsense. The operational amplifier, transistor M717,
and resistor Rvm are arranged to output a sense current that is
proportional to the load current Iload. The resistor ratio and
operation of the current sense circuit are the same as described
above with respect to FIG. 4. The sense current Isense is compared
to trimmable current source Itrim by a comparator that is formed by
the transistors M729, M731, M733, M732, M734 and M735, and the
difference current ILim_pulldn that is output is then used to
control the current into the gate terminal GATE after the current
limit is reached. The current ILim_pulldn is the difference between
the current Isense and the trimmable current Itrim, and can be set
by changing the value of the trimmable current source Itrim.
Transistors M713, M709 and M702, M703 form two current mirrors that
couple the limit current to pull down from the GATE terminal.
Scaling of the transistors is used so that the current at the gate
of the transistor MN0 from transistor M703 is larger than the
current ILim_pulldn. By using transistor scaling, smaller current
sources such as Itrim can be used to save power and area.
[0061] The embodiment of FIG. 7 also includes the current time
derivative circuitry to reduce the rate of increase in the current
through the power FET as it rises towards the current limit, and to
prevent overshoot current, as described above with respect to FIG.
5. The comparator circuitry formed by the transistors M747, M746,
M748, M749 and the reference current Iref will cause the Schmitt
trigger 751 to output an enable signal EN_di_dt_FB to the enable
switch M739 when the sensed current minus the trim current Itrim
multiplied by the transistor scaling ratio Y/X is greater than the
reference current Iref. By setting the reference current Iref and
the scaling factor Y/X, a percentage of the limit current can be
set as a threshold, for example 80%. Only after the sensed current
Isense reaches this threshold is the current time derivative
circuit 710 coupled to reduce the current to the gate of the power
FET MN0.
[0062] The current time derivative circuit 710 outputs a current
Isense_Rate that is coupled to reduce the pull_up bias current that
is input to the gate terminal GATE. Isense_rate is output when the
di/dt circuit 710 detects a rise in the sensed current Isense that
is greater than a threshold. In this manner, the rate of increase
of the current through the FET MN0 is controlled until the current
limit is reached, preventing overshoot. The current sources Iref,
Itrim, Ipullup_bias, and the scaling factors X and Y can be fixed
or adjustable. Programmable current sources formed using DAC
circuits and writeable registers can be used. Nonvolatile storage
registers can be programmed at manufacture or during use. An
interface bus such as an I2C interface can be used to write the
programmable registers. Trimmable resistors or fuses can also be
used.
[0063] FIG. 8 depicts in a flow diagram 800 an example method
embodiment. In the method illustrated in FIG. 8, a time derivative
current circuit or di/dt circuit is always active. In FIG. 8, the
method begins at step 810, Idle. At step 812 an enable signal is
received. The method then transitions to step 814 and a voltage and
bias current is coupled to the gate terminal of the power FET. The
power FET begins turning on in response to the bias current. After
sufficient current is input to the gate terminal to charge the gate
capacitance, the power FET enters the linear mode of operation.
[0064] At step 816, the current flowing through the power FET is
sensed. The sensed current is compared to a current limit. If the
comparison at step 816 is true, the method transitions to step 822.
If the comparison at step 816 is false, the method transitions to
step 818.
[0065] At step 818, the time derivative of the sensed current, or
di/dt, is compared to a threshold. If the rate of increase of the
sensed current is greater than a threshold, then the method
transitions to step 820. If the comparison is false, the method
transitions back to step 814 and the method continues.
[0066] At step 820, since the comparison at step 818 was true, the
rate of increase in the current flowing through the power FET is
reduced. This can be done by reducing the pull up bias current as
described above. The method then transitions back to step 814 and
continues.
[0067] Step 822 is entered when the comparison at step 816 is true,
that is when the current limit has been reached. At step 822, the
current flowing through the power FET is limited by pulling current
down from the gate terminal to the Vout terminal. By discharging
the gate capacitance, the power FET will have a reduced gate
voltage, and a reduced current flowing through the power FET. The
method will continue operating with the current limited to the
maximum limit.
[0068] Note that once the sensed current reaches the maximum limit,
the rate of increase in the current di/dt will become zero, so that
when the current is limited by the pull down current, the
comparison at step 818 will always be false.
[0069] The method steps shown in FIG. 8 illustrate one possible
order of steps. However, the order of steps can be modified to form
an alternative embodiment. For example, the comparisons at steps
816, 818 can be done in a different order or can be done
simultaneously instead of in the order shown in FIG. 8. These
modifications form additional embodiments that are within the scope
of the appended claims.
[0070] FIG. 9 depicts in another flow diagram an alternative
method. In FIG. 9, the method 900 includes selectively enabling a
current time derivative function. At step 910, the method begins in
an Idle step. At step 912, an enable signal is received. At step
914, power is supplied to a power FET and a voltage and a bias
current are applied to the gate terminal of the power FET, pulling
up the gate terminal and turning on the power FET.
[0071] At step 916, a current sensor senses the current flowing
through the power FET. If the sensed current exceeds a current
limit, the method transitions to step 922. If the comparison is
false, the method continues to step 917.
[0072] At step 917, the sensed current is compared to a reference
current. The reference current can be adjusted or set to a current
that is a percentage of the current limit, for example, in one
experiment the reference current was set to about 80% of the
current limit.
[0073] If the comparison as step 917, the method transitions to
step 918. At step 918, the rate of increase in the sensed current
is compared to a threshold in a current time derivative operation.
If the rate of change in the sensed current is increasing at a rate
in excess of a threshold, the method transitions to step 920. At
step 920, the pull up current is reduced, reducing the rate of
increase in the current flowing in the power FET. The method then
continues to step 914 and continues.
[0074] If the comparison at step 918 is false, the method
transitions to step 914 and continues. If the comparison at step
916 is true, the current limit has been reached, and the method
transitions to step 922. At step 922, a pull down current is
increased and the current flowing through the power FET is reduced
to maintain the output current at an output terminal limited to a
maximum current.
[0075] The method steps shown in FIG. 9 illustrate one possible
order of steps. However, the order of steps can be modified to form
alternative embodiments. For example, the comparisons at steps 916
and 917 can be done in a different order or simultaneously instead
of in the order shown in FIG. 9. These modifications are within the
scope of the appended claims.
[0076] Example embodiments and arrangements form power FET circuits
with continuously limited current. In described examples, current
sense circuitry coupled to a FET device supplies a sense current
corresponding to the current flowing through the FET. The sense
current is coupled to a comparator. A predetermined limit current
is compared to the sense current. A pull down current is applied to
limit the current flowing through the FET when the comparator
indicates the predetermined limit current has been reached. The
sense current is also supplied to a current time derivative
circuit. The rate of increase in the sense current is compared to a
threshold rate. When the rate of increase in the sense current
exceeds the threshold rate, a bias current to the FET is reduced to
reduce the rate of increase in the output current.
[0077] In a further example, an enable circuit is coupled between
the current time derivative circuit and a bias current supply, and
a current rate comparator circuit is coupled to the sense current
and a reference current, and outputs an enable signal to the enable
circuit.
[0078] In an additional example, an output is coupled from a
current limit comparator to enable the pull down current
circuit.
[0079] In at least one alternative example, the apparatus further
includes circuitry to reduce the bias current to the gate terminal
of the FET responsive to the sense rate current.
[0080] In another example, the apparatus includes a programmable
predetermined limit current. In a further example, the
predetermined limit current is stored in a writable storage
circuit. In a further example, the current time derivative circuit
includes a rate detection circuit for outputting the sense rate
current when the increase rate of the sensed current exceeds a
predetermined threshold.
[0081] In a further example, the apparatus further includes an
enable circuit coupled between the sense rate current and the bias
current supply; and a current rate comparator circuit coupled to
the sense current and to a reference current, and outputting an
enable signal to the enable circuit. In still another example, the
current rate comparator circuit includes a scaling factor and the
current rate comparator circuit compares a difference between the
sense current and a multiple of the scaling factor and the
predetermined limit current to the reference current.
[0082] In another example, the scaling factor is programmable. In
another additional example, a programmable storage circuit stored
the scaling factor. In still another example, an external bus
interface is coupled to address the programmable storage
circuit.
[0083] An example method includes: receiving an enable signal;
coupling a current conduction path of a FET between a supply
voltage terminal and an output terminal for supplying current;
coupling a voltage and a pull up bias current to a gate terminal of
the FET; sensing current flowing through the FET; comparing the
sensed current flowing through the FET to a current limit; reducing
the current at the gate terminal of the FET, responsive to
comparing the current flowing through the FET. The method further
includes comparing a rate of increase of current flowing through
the FET to a rate increase threshold; and reducing the pull up
current to the gate terminal of the FET responsive to the comparing
the rate of increase of current flowing through the FET.
[0084] In at least one example, the method further includes:
comparing the current flowing through the FET to a threshold that
is a predetermined percentage of a current limit; and enabling
reducing the pull up current to the gate terminal of the FET
responsive to the comparing. In another example, the method further
includes after reducing the current at the gate terminal of the
FET, operating the FET to supply current at the output terminal at
the current limit.
[0085] In another example, the method includes, after reducing the
current at the gate terminal of the FET, operating the FET to
supply current at the output terminal at the predetermined current
limit.
[0086] In another example, the method of coupling the current
conduction path of a FET between the supply voltage terminal and an
output terminal for supplying current further includes coupling a
drain terminal of the FET to a voltage supply and coupling a source
terminal of the FET to the output terminal.
[0087] In yet another example, the method of coupling the current
conduction path of a FET between the supply voltage and an output
terminal for supplying current further includes coupling the drain
terminal to a terminal for supplying current and coupling the
source terminal to a terminal for receiving a ground potential.
[0088] In a further example, the method further includes supplying
an increased voltage that is greater than a supply voltage. In
still another example, the method of comparing the sensed current
flowing through the FET to a predetermined current limit includes
comparing the sensed current to an adjustable current limit that is
stored in a writeable storage circuit.
[0089] In yet another example, an integrated circuit includes: a
terminal for driving a gate of a power FET having a current
conduction path coupled between a voltage supply and a load; an
voltage input terminal for coupling a supply voltage to the
integrated circuit; a pull up bias circuit coupled to the terminal
for driving the gate terminal of the power FET and to the input
terminal for coupling the increased power supply; a current sensing
circuit coupled to terminals to receive a voltage proportional to
the current flowing in the power FET, and having a sensed current
output; a current limit comparator comparing the sensed current to
a limit current; a pull down current circuit coupled to the current
limit comparator and to the terminal for driving the gate of the
power FET; a current time derivative circuit coupled to the sensed
current; and a pull-up current reduction circuit coupled to the
current time derivative circuit and to the pull up bias
circuit.
[0090] In still another example, the integrated circuit further
includes the limit current set by an adjustable trim current
supply.
[0091] Modifications are possible in the described embodiments, and
other embodiments are possible within the scope of the claims.
* * * * *