U.S. patent application number 14/923776 was filed with the patent office on 2017-04-27 for bus voltage correction circuit.
This patent application is currently assigned to STMICROELECTRONICS INTERNATIONAL N.V.. The applicant listed for this patent is STMicroelectronics International N.V.. Invention is credited to Jitendra Jain, Ranajay Mallik.
Application Number | 20170117799 14/923776 |
Document ID | / |
Family ID | 57637292 |
Filed Date | 2017-04-27 |
United States Patent
Application |
20170117799 |
Kind Code |
A1 |
Jain; Jitendra ; et
al. |
April 27, 2017 |
BUS VOLTAGE CORRECTION CIRCUIT
Abstract
A tunable voltage regulator has an output generating a variable
voltage and an input that receives a trimming signal for
controlling the output variable voltage. A current regulating
circuit operates to regulate a current flowing through a load in
response to the variable voltage. A control circuit senses the
variable voltage and a drop voltage of the current regulating
circuit, and determines whether the current regulating circuit has
failed to regulate the current flowing through the load because the
variable voltage is too low. In response thereto, the control
circuit generates the trimming signal to set the variable voltage
to a value sufficient for the current regulating circuit to
successfully regulate the current flowing through the load.
Inventors: |
Jain; Jitendra; (Firozabad,
IN) ; Mallik; Ranajay; (Ghaziabad, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
STMicroelectronics International N.V. |
Amsterdam |
|
NL |
|
|
Assignee: |
STMICROELECTRONICS INTERNATIONAL
N.V.
Amsterdam
NL
|
Family ID: |
57637292 |
Appl. No.: |
14/923776 |
Filed: |
October 27, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H05B 45/37 20200101;
H05B 45/10 20200101; H05B 45/50 20200101; Y02B 20/30 20130101; Y02B
20/347 20130101; H02M 3/04 20130101; H05B 45/46 20200101 |
International
Class: |
H02M 3/04 20060101
H02M003/04; H05B 33/08 20060101 H05B033/08 |
Claims
1. A circuit, comprising: a load coupled between a first node and a
second node; a current regulating circuit coupled to the second
node and configured to regulate a current flowing through the load;
a tunable voltage regulator having an output configured to generate
a variable voltage applied to the first node, said tunable voltage
regulator having input configured to receive a trimming signal for
controlling the output variable voltage; a control circuit having a
first input configured to sense the variable voltage at the first
node, a second input configured to sense a drop voltage at the
second node, and having an output configured to generate the
trimming signal, said control circuit configured to operate to:
vary the trimming signal to cause the variable voltage to decrease;
determine a difference between the sensed decreasing variable
voltage and the sensed drop voltage; identify that the current
regulating circuit is failing to regulate the current flowing
through the load if the determined difference exceeds a threshold;
and output the trimming signal to set the variable voltage to a
value in excess of a decreased variable voltage where the current
regulating circuit failed to regulate the current flowing through
the load.
2. The circuit of claim 1, wherein the load comprises at least one
string of light emitting diodes.
3. The circuit of claim 1, wherein the load comprises a plurality
of strings of light emitting diodes.
4. The circuit of claim 1, wherein the trimming signal is a
filtered pulse width modulated signal.
5. A method for setting a variable voltage output by a tunable
voltage regulator for application to a load that passes a current
regulated by a current regulator circuit, comprising: controlling
the tunable voltage regulator to output a decreasing variable
voltage; sensing the decreasing variable voltage; sensing a drop
voltage at the current regulator circuit; determining a difference
between the sensed decreasing variable voltage and the sensed drop
voltage; identifying that the current regulating circuit is failing
to regulate the current passing through the load if the determined
difference exceeds a threshold; and setting the variable voltage
output to a value in excess of a decreased variable voltage where
it was determined that the current regulating circuit failed to
regulate the current passing through the load.
6. The method of claim 5, wherein controlling the tunable voltage
regulator comprises incrementally decreasing the variable voltage
in steps.
7. The method of claim 6, wherein determining comprises determining
at each step.
8. The method of claim 7, wherein setting comprises setting the
value of the variable voltage to a previous value of the variable
voltage associated with a previous step.
9. The method of claim 5, as implemented in a calibration mode of
operation.
10. A circuit, comprising: a tunable voltage regulator having an
output configured to generate a variable voltage and having an
input configured to receive a trimming signal for controlling the
output variable voltage; a current regulating circuit configured to
regulate a current flowing through a load in response to the
variable voltage; a control circuit configured to sense the
variable voltage and a drop voltage of the current regulating
circuit and in response thereto determine a difference between the
sensed variable voltage and the sensed drop voltage and identify
that the current regulating circuit has failed to regulate the
current flowing through the load if the determined difference
exceeds a threshold, the control circuit further configured to
respond to said identification by generating the trimming signal to
set the variable voltage to a value sufficient for the current
regulating circuit to successfully regulate the current flowing
through the load.
11. The circuit of claim 10, wherein the control circuit operates
in a calibration mode to control the tunable voltage regulator
through the trimming signal to incrementally decrease the variable
voltage until the identification is made that the current
regulating circuit has failed to regulate the current flowing
through the load.
12. The circuit of claim 11, wherein the control circuit further
operates to control the tunable voltage regulator to set a value of
the variable voltage at level in excess of the incrementally
decreased value associated with the identification that the current
regulating circuit has failed to regulate the current flowing
through the load.
13. The circuit of claim 10, further comprising the load connected
between the tunable voltage regulator and the current regulating
circuit.
14. The circuit of claim 13, wherein the load comprises at least
one string of light emitting diodes.
15. The circuit of claim 10, wherein the trimming signal is a
filtered pulse width modulated signal.
Description
TECHNICAL FIELD
[0001] The present invention relates to power supply circuits and,
in particular, to a power supply circuit implementing a bus voltage
correction so as to supply a minimum bus voltage required for load
circuit operation.
BACKGROUND
[0002] Reference is made FIG. 1 showing a light emitting diode
(LED) circuit 10. The circuit 10 includes a load circuit 12 formed
by a string of series connected light emitting diodes (LEDs) 14.
The string is connected between a node 16 and node 18. Node 16 may
correspond to a supply bus in an implementation of the circuit 10
including plural strings of LEDs. A voltage regulator 20 is coupled
to receive an unregulated supply voltage Vsup and operates to
generate a fixed regulated bus voltage Vbus that is applied to the
supply bus at node 16. A current sink driver circuit 22 is
connected to node 18 and operates to regulate a constant load
current I.sub.L through the load circuit 12 formed by the string of
LEDs.
[0003] Those skilled in the art understand that there is a minimum
voltage Vmin for the bus voltage Vbus that is necessary in order to
be able to regulate the constant current I.sub.L in the string.
[0004] That minimum voltage Vmin is set by the following
equation:
Vmin=V.sub.L+V.sub.min-drop
where: V.sub.L is the voltage drop across the load formed by the
string of LEDs from node 16 to node 18, and V.sub.min-drop is
minimum required "drop-out" voltage across the channel of the
MOSFET device within the current sink driver circuit 22 needed to
maintain a regulated output for the load current I.sub.L.
[0005] As the circuit 10 ages, the values for V.sub.L and
V.sub.min-drop increase. This results in a corresponding increase
in the value for Vmin. Accordingly, circuit designers will choose a
value for the fixed regulated bus voltage Vbus that is higher than
the anticipated range of Vmin over the life of the circuit 10. A
side effect of making this choice for the regulated bus voltage
Vbus value is an excess power dissipation (equal to
(Vbus-Vmin)*I.sub.L). In higher power load situations, the loss
associated with this excess power dissipation can be excessive.
[0006] A need accordingly exists in the art for circuitry to
dynamically maintain the regulated bus voltage generated by the
voltage regulator at a value which substantially equals (or
slightly exceeds) the instantaneous minimum voltage Vmin necessary
to achieve a regulated constant current I.sub.L in the string.
SUMMARY
[0007] In an embodiment, a circuit comprises: a load coupled
between a first node and a second node; a current regulating
circuit coupled to the second node and configured to regulate a
current flowing through the load; a tunable voltage regulator
having an output configured to generate a variable voltage applied
to the first node, said tunable voltage regulator having input
configured to receive a trimming signal for controlling the output
variable voltage; a control circuit having a first input configured
to sense the variable voltage at the first node, a second input
configured to sense a drop voltage at the second node, and having
an output configured to generate the trimming signal, said control
circuit configured to operate to: vary the trimming signal to
decrease the variable voltage; determine from the sensed variable
voltage and sensed drop voltage a failure of the current regulating
circuit to regulate the current flowing through the load; and
output the trimming signal to set the variable voltage to a value
in excess of the decreased variable voltage where the current
regulating circuit failed to regulate the current flowing through
the load.
[0008] In an embodiment, a method for setting a variable voltage
output by a tunable voltage regulator for application to a load
that passes a current regulated by a current regulator circuit
comprises: controlling the tunable voltage regulator to output a
decreasing variable voltage; sensing the decreasing variable
voltage; sensing a drop voltage at the current regulator circuit;
determining from the sensed decreasing variable voltage and the
sensed drop voltage a failure of the current regulating circuit to
regulate the current passing through the load; and setting the
variable voltage output to a value in excess of the decreased
variable voltage where it was determined that the current
regulating circuit failed to regulate the current passing through
the load.
[0009] In an embodiment, a circuit comprises: a tunable voltage
regulator having an output configured to generate a variable
voltage and having an input configured to receive a trimming signal
for controlling the output variable voltage; a current regulating
circuit configured to regulate a current flowing through a load in
response to the variable voltage; a control circuit configured to
sense the variable voltage and a drop voltage of the current
regulating circuit and in response thereto determine whether the
current regulating circuit has failed to regulate the current
flowing through the load because the variable voltage is too low,
the control circuit further configured to respond to said
determination by generating the trimming signal to set the variable
voltage to a value sufficient for the current regulating circuit to
successfully regulate the current flowing through the load.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] For a better understanding of the embodiments, reference
will now be made by way of example only to the accompanying figures
in which:
[0011] FIG. 1 is a circuit diagram for a conventional light
emitting diode circuit;
[0012] FIG. 2 is a circuit diagram for a light emitting diode
circuit with a regulated minimum bus supply voltage;
[0013] FIG. 3 illustrates regulated operation for the circuit of
FIG. 2;
[0014] FIG. 4 is a flow diagram for a process to configure a
variable voltage for the bus supply voltage;
[0015] FIG. 5 is a circuit diagram for a light emitting diode
circuit with a regulated minimum bus supply voltage; and
[0016] FIG. 6 is a practical circuit implementation of the circuits
of FIGS. 2 and 5.
DETAILED DESCRIPTION OF THE DRAWINGS
[0017] Reference is now made to FIG. 2 showing a light emitting
diode (LED) circuit 100. The circuit 100 includes a load circuit
112 formed by a string of series connected light emitting diodes
(LEDs) 114. The string is connected between a node 116 and node
118. Node 116 may correspond to a supply bus in an implementation
of the circuit 100 including plural strings of LEDs. A tunable
voltage regulator 120 is coupled to receive an unregulated supply
voltage Vsup and operates to generate a variable bus voltage Vbusv
that is applied to the supply bus at node 116. A current sink
driver circuit 122 is connected to node 118 and operates to
regulate a constant load current I.sub.L through the load circuit
112 formed by the string of LEDs.
[0018] The variable bus voltage Vbusv must equal or exceed a
minimum voltage Vmin in order for the current sink driver circuit
122 to be capable of operation to regulate a constant current
I.sub.L in the string. Consider the following Table which
illustrates operation of the circuit 100 in response to changes in
the variable bus voltage Vbusv:
TABLE-US-00001 Vbusv I.sub.L 11.50 V 304 mA 11.25 V 305 mA 11.00 V
305 mA 10.75 V 305 mA 10.50 V 305 mA 10.25 V 304 mA 10.00 V 290
mA
[0019] The Table shows an example operation of the circuit 100 with
a constant current IL set equal to about 304 mA (.+-.approximately
0.5%). When variable bus voltage Vbusv falls below about 10.25 V,
the current sink driver circuit 122 is no longer able to regulate
the current and the current drops significantly by about 5% to 290
mA. The foregoing relationship may be graphically presented as
shown in FIG. 3 with curve 126 that plots the voltage drop V.sub.L
across the load formed by the string of LEDs from node 16 to node
18 against the voltage drop V.sub.drop across the channel of the
MOSFET device within the current sink driver circuit 122 (it being
understood that V.sub.L and V.sub.drop vary dependent on changes in
the variable bus voltage Vbusv). The voltage drop V.sub.L across
the load remains substantially constant versus change in V.sub.drop
so long as the variable bus voltage Vbusv equals or exceeds the
minimum voltage Vmin. However, a knee point 128 is reached in the
operation curve 126 when the variable bus voltage Vbusv falls below
the minimum voltage Vmin. This corresponds to the point where
V.sub.min-drop (the minimum required "drop-out" voltage across the
channel of the MOSFET device within the current sink driver circuit
22 needed to maintain a regulated output for the load current
I.sub.L) is located. The voltage drop V.sub.L across the load is no
longer constant for changes in V.sub.drop below V.sub.min-drop, and
in this condition the load current I.sub.L is no longer
regulated.
[0020] In order to minimize power dissipation for operation of the
circuit 100 with the current sink driver circuit 122 operating to
regulate a constant current I.sub.L in the string, it is important
to set the value of the variable bus voltage Vbusv at a level which
places the voltage drop V.sub.drop across the channel of the MOSFET
device within the current sink driver circuit 122 slightly above
the knee point corresponding to the minimum required "drop-out"
voltage V.sub.min-drop. In other words, the variable bus voltage
Vbusv generated by the voltage regulator 120 should be set at a
value which substantially equals (or slightly exceeds) the
instantaneous minimum voltage Vmin necessary to achieve a regulated
constant current I.sub.L in the string (for example,
Vmin<Vbusv<Vmin+0.5%).
[0021] In the circuit 100, the generated variable bus voltage Vbusv
is tunable in response to a trimming signal (trim). A control
circuit 130, for example in the format of a microcontroller,
generates the trimming signal and effectuates changes in the trim
signal so as to set the value of the variable bus voltage Vbusv to
satisfy the operating condition mentioned in the preceding
paragraph. The control circuit 130 operates to sense the voltages
at the nodes 116 and 118, referred to herein as V116 and V118,
respectively, using voltage sensing inputs 132 and 134. For
example, the control circuit 130 may include one or more
analog-to-digital converters (ADC) coupled to convert the analog
voltage at each of nodes 116 and 118 to corresponding digital
signals indicative of the sensed voltage levels. The sensed
voltages at nodes 116 and 118 are monitored by the control circuit
130 while the control circuit 130 operates to change the variable
bus voltage Vbusv by changing the trimming signal applied to the
tunable voltage regulator 120. From the monitored voltages at nodes
116 and 118, the control circuit 130 identifies the value of the
variable bus voltage Vbusv associated with the knee point 128 where
the current sink driver circuit 122 no longer operates to regulate
a constant current I.sub.L. The control circuit 130 then sets the
value of the variable bus voltage Vbusv using the trimming signal
to a value which substantially equals (or slightly exceeds) the
instantaneous minimum voltage Vmin necessary to achieve a regulated
constant current I.sub.L in the string (for example,
Vmin<Vbusv<Vmin+0.5%). This will configure operation of the
circuit 100 with respect to curve 126 on the right side of the knee
point 128 and the minimum required "drop-out" voltage
V.sub.min-drop.
[0022] Reference is now made to FIG. 4 showing a flow diagram for
operation of the control circuit 130 for setting the variable bus
voltage Vbusv. This process may, for example, be executed by the
control circuit 130 in a calibration mode of operation for the
circuit 100. In step 400, calibration mode is entered. In step 402,
the control circuit 130 controls the trimming signal to cause the
tunable voltage regulator 120 to output a variable bus voltage
Vbusv which is in excess of the minimum voltage Vmin. The control
circuit 130 next senses the voltage at node 116 (step 404); senses
the voltage at node 118 (step 406); calculates the voltage
V.sub.L=V116-V118 (step 408); and saves the voltage V.sub.L (step
410) as a reference voltage V.sub.Lref. A control loop 412 is then
entered where the control circuit 130, with each iteration through
the loop 412: incrementally decreases (step 414) the variable bus
voltage Vbusv output by the tunable voltage regulator 120; senses
the voltage at node 116 (step 416); senses the voltage at node 118
(step 418); calculates a current voltage V.sub.Lcur=V116-V118 (step
420); and compares that current voltage V.sub.Lcur to the reference
voltage V.sub.Lref (step 422). In step 424, the control circuit 130
determines whether a difference voltage Vdiff=V.sub.Lref-V.sub.Lcur
exceeds a threshold voltage V.sub.th. If no, the process returns
(step 426) to repeat another iteration through the control loop
412. If yes, the knee point 128 has been reached (and perhaps
exceeded), and the control circuit 130 sets the variable bus
voltage Vbusv to a value associated with one of the previous
iterations through the control loop 412 (step 428).
[0023] Consider the following Table which illustrates operation of
the circuit 100 using the process of FIG. 4:
TABLE-US-00002 Vbusv Vdiff Vth I.sub.L 11.50 V 2 mV 40 mV 304 mA In
regulation 11.25 V 2 mV 40 mV 305 mA 11.00 V 2 mV 40 mV 305 mA
10.75 V 4 mV 40 mV 305 mA 10.50 V 15 mV 40 mV 305 mA 10.25 V 23 mV
40 mV 304 mA 10.00 V 71 mV 40 mV 290 mA Out of regulation 10.25 V
Set value for operation
[0024] The Table shows the iterative process of progressively
incrementally decreasing the variable bus voltage Vbusv and
determining the difference voltage Vdiff. When the difference
voltage Vdiff is less than the threshold voltage V.sub.th, the
determination of step 424 is not satisfied and the process returns
through step 426 to repeat with an incrementally reduced value of
the variable bus voltage Vbusv. However, when the difference
voltage Vdiff exceeds the threshold voltage V.sub.th, the
determination of step 424 is satisfied and the variable bus voltage
Vbusv is increased and set in step 428 to a value of a previous
iteration where the circuit is operating with current regulation.
This set variable bus voltage Vbusv value, however, reduces power
waste because the circuit is configured for operation with a
variable bus voltage Vbusv just slightly above the minimum voltage
Vmin.
[0025] Reference is now made to FIG. 5 showing a light emitting
diode (LED) circuit 100'. The circuit 100' is substantially
identical to the circuit 100 of FIG. 3 except that circuit 100'
includes plural strings of LEDs for the load 112. In this
configuration, each individual string of LEDs has its own current
sink driver circuit 122. An effort may be made to ensure that the
current sink driver circuits 122 are matching, but this cannot be
assured to maintain over time. The circuit 100' accordingly
utilizes a multiplexer 150 to select the V118 voltage from each
string of LEDs for processing in accordance with FIG. 4. In an
embodiment, the multiplexer 150 may be a component part of the
control circuit 130. The control circuit 130 will operate to set
the variable bus voltage Vbusv value based on the worst case
scenario of circuit 100' operation. The worst case scenario
represents the worst possible voltage V.sub.L across a string of
LEDs and the worst possible voltage drop V.sub.drop across the
channel of the MOSFET device within one of the current sink driver
circuits 122. The process of FIG. 4 will perform the iterative
process with respect to determining the knee point 128 for each
string of LEDs and then set the variable bus voltage Vbusv value to
ensure proper operation of all strings of LEDs.
[0026] Reference is now made to FIG. 6 showing a practical circuit
implementation of the circuits 100 and 100'. The tunable voltage
regulator 120 may be implemented using an ST1S40 integrated
circuit. The resistive voltage divider 70 generates the feedback
voltage for regulating the output voltage Vbusv. The trimming
signal (trim) is also applied to the feedback input of the
regulator 120. The trimming signal is an analog voltage generated
by a low pass filter circuit 72 from a pulse width modulated (PWM)
signal output from the control circuit 130. The control circuit may
be implemented by an STM32F0 integrated circuit programmed to
implement the process of FIG. 4. A resistive voltage divider 74
senses the voltage at node 116 and applies a divided version of the
V116 voltage to an input of the STM32F0 integrated circuit. An
analog-to-digital converter within the STM32F0 integrated circuit
converts the sensed voltage at node 116 to a digital value for
processing in accordance with the process of FIG. 4. A resistive
voltage divider 76 senses the voltage at node 118 of each string of
LEDs through the multiplexer 150 and applies a divided version of
the V118 voltage to an input of the STM32F0 integrated circuit. An
analog-to-digital converter within the STM32F0 integrated circuit
converts the sensed voltage at node 118 to a digital value for
processing in accordance with the process of FIG. 4. The control
circuit 130 control selection of signals made by the multiplexer
150 through a control signal 78. The current sink driver circuit
122 may be implemented using an STP04CM05 integrated circuit.
Control signals for controlling operation of the STP04CM05
integrated circuit are generated by the control circuit 130 and
applied to appropriate inputs of the current sink driver circuit
122 on bus interface 80.
[0027] The foregoing description has been provided by way of
exemplary and non-limiting examples of a full and informative
description of the exemplary embodiment of this invention. However,
various modifications and adaptations may become apparent to those
skilled in the relevant arts in view of the foregoing description,
when read in conjunction with the accompanying drawings and the
appended claims. However, all such and similar modifications of the
teachings of this invention will still fall within the scope of
this invention as defined in the appended claims.
* * * * *