U.S. patent application number 09/741951 was filed with the patent office on 2002-06-20 for dc/dc converter and method of operating a dc/dc converter.
Invention is credited to Bayer, Erich, Schmeller, Hans.
Application Number | 20020075705 09/741951 |
Document ID | / |
Family ID | 7934167 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020075705 |
Kind Code |
A1 |
Bayer, Erich ; et
al. |
June 20, 2002 |
DC/DC CONVERTER AND METHOD OF OPERATING A DC/DC CONVERTER
Abstract
The invention relates to a DC/DC converter including a charge
pump circuit comprising one or more capacitors and a plurality of
controllable switches connected thereto, the controllable switches
being controllable by a control circuit so that the capacitor(s)
is/are alternatingly switched in a charging and discharge phase; a
first current source set to a predetermined base current located
either in the discharge or charging path of the charge pump circuit
and a second current source connected in parallel thereto; and a
regulator circuit for generating a first control signal
representing the difference between a voltage characterizing the
output voltage and a first reference voltage and controlling the
second current source when the charge pump circuit is active so
that the controllable current is reduced or increased with an
increase and reduction respectively in the difference to track the
voltage characterizing the output voltage in accordance with the
first reference voltage; and for generating a second control signal
guided to the control circuit, this signal assuming a first status
when the voltage characterizing the output voltage exceeds a second
reference voltage at a predetermined level above the first
reference voltage, upon which the control circuit deactivates the
charge pump circuit, and assumes a second status when the voltage
characterizing the output voltage drops below the second reference
voltage, upon which the control circuit activates the charge pump
circuit. The invention relates further to a method of operating a
DC/DC converter.
Inventors: |
Bayer, Erich; (Thonhausen,
DE) ; Schmeller, Hans; (Falkenberg, DE) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
|
Family ID: |
7934167 |
Appl. No.: |
09/741951 |
Filed: |
December 20, 2000 |
Current U.S.
Class: |
363/59 |
Current CPC
Class: |
Y02B 70/10 20130101;
H02M 1/146 20130101; H02M 1/0025 20210501; H02M 1/0032 20210501;
H02M 3/073 20130101 |
Class at
Publication: |
363/59 |
International
Class: |
H02M 003/18 |
Claims
1. A DC/DC converter including a charge pump circuit comprising:
one or more charge pump capacitors and a plurality of controllable
switches connected thereto, said controllable switches being
controllable by a control circuit so that said charge pump
capacitor(s) is/are alternatingly switched in a charging and
discharge phase so that an output voltage deviating from the input
voltage of said converter is generated at the output of said
converter; a first current source set to a predetermined base
current located either in the discharge path of said charge pump
circuit via which in the discharge phase current is supplied to
said output of said converter, or in the charging path of said
charge pump circuit, via which said charge pump capacitor(s) is/are
charged in the charging phase of said charge pump circuit; and a
second current source connected in parallel to said first current
source, the current of said second current source being
controllable; and an output voltage regulator circuit for
generating a first control signal representing the difference
between a voltage characterizing said output voltage and a first
reference voltage and controlling said second current source when
said charge pump circuit is active so that said controllable
current is reduced or increased with an increase and reduction
respectively in the difference to track the voltage characterizing
said output voltage in accordance with said first reference
voltage; and for generating a second control signal guided to said
control circuit, this signal assuming a first status when said
voltage characterizing said output voltage exceeds a second
reference voltage at a predetermined level above said first
reference voltage, upon which said control circuit deactivates said
charge pump circuit, and assumes a second status when said voltage
characterizing said output voltage drops below said second
reference voltage, upon which said control circuit activates said
charge pump circuit.
2. The DC/DC converter as set forth in claim 1, at the output of
which a storage capacitor is provided.
3. The DC/DC converter as set forth in claim 1 or 2 wherein said
second current source is a voltage-controlled current source.
4. The DC/DC converter as set forth in claim 3 wherein said output
voltage regulator circuit comprises an operational amplifier
receiving at its inputs said voltage characterizing said output
voltage of said converter and said first reference voltage and
generates at its output a current which varies with the difference
between its two input voltages, its output being connected to a RC
pad, by the voltage of which said second current source is
controlled, and a comparator for comparing said voltage
characterizing said output voltage of said converter to said second
reference voltage and the output of which is connected to said
control circuit receiving said second control signal and activating
or deactivating said charge pump circuit as a function of the
status of said second control signal.
5. The DC/DC converter as set forth in any of the preceding claims
comprising in addition a reference voltage generator circuit for
generating said first reference voltage.
6. The DC/DC converter as set forth in claim 5 wherein said second
reference voltage is generated from said first reference
voltage.
7. The DC/DC converter as set forth in any of the preceding claims
wherein said control circuit comprises a clock, the clock signal of
which cycles said controllable switches of said charge pump circuit
ON/OFF.
8. The DC/DC converter as set forth in any of the preceding claims
wherein one or more of said controllable switches forms said first
current source and/or said controllable second current source.
9. The DC/DC converter as set forth in any of the preceding claims
wherein all of said controllable switches are MOSFETs.
10. The DC/DC converter as set forth in any of the preceding claims
wherein said charge pump circuit comprises a charge pump capacitor
and four controllable switches, one electrode of said charge pump
capacitor being connectable via a first of said four switches to
the input voltage of said converter and via a second of said four
switches to GND, and the other electrode of said capacitor being
connectable via the third of said four switches to said input
voltage and via the fourth of said four switches to the output of
said converter.
11. The DC/DC converter as set forth in any of the preceding claims
comprising a further charge pump circuit configured and circuited
corresponding to said first charge pump circuit, the controllable
switches of said further charge pump circuit being signalled by
said control signal opposite in phase to those of said first charge
pump circuit so that the ripple of said output voltage of said
DC/DC converter is reduced.
12. A method for operating a DC/DC converter including a charge
pump circuit comprising one or more charge pump capacitors and a
plurality of controllable switches connected thereto comprising the
steps cycling said charge pump capacitor(s) by said controllable
switches in a charging and discharge phase during operation of said
charge pump circuit so that an output voltage deviating from the
input voltage of said converter is generated at the output of said
converter; setting a controllable current flowing parallel to a
predetermined base current with said charge pump circuit active in
the discharge or charging path of said charge pump circuit as a
function of the difference between a voltage characterizing said
output voltage and a first reference voltage so that said
controllable current is reduced or increased with an increase and
reduction respectively in the difference to track said voltage
characterizing said output voltage in accordance with said first
reference voltage; and deactivating said charge pump circuit when
said voltage characterizing said output voltage exceeds a second
reference voltage at a predetermined level above said first
reference voltage and activating said charge pump circuit when said
voltage characterizing said output voltage drops below said second
reference voltage.
Description
[0001] The invention relates to a DC/DC converter including a
charge pump circuit and a method of operating one such DC/DC
converter.
[0002] In addition to the supply voltage many electronic circuits
require further voltages, the levels of which are sometimes higher
than that of the supply voltage. One cost-effective, simple
and--especially as compared to coil converters highly compact
solution to furnishing these further voltages are DC/DC converters
operating on the charge pump principle. Such converters are
described e.g. in the text book "The Art of Electronics" by Paul
Horowitz, 2nd Edition, Cambridge University Press, New York 1991 on
pages 377 to 379 thereof.
[0003] Horowitz also describes a simple DC/DC converter operating
on the charge pump principle with which an output voltage
corresponding maximally to roughly twice the input voltage is
achievable. The basic circuit of the converter consists
substantially of a charge pump capacitor and four controllable
switches (e.g. MOSFETs) whereby one electrode of the charge pump
capacitor is connectable via a first switch to the input voltage
terminal of the converter and via a second switch to GND, and the
other electrode of the capacitor is connectable via the third
switch to the input voltage terminal and via the fourth switch to
the output voltage terminal of the converter. The converter
comprises furthermore a control circuit including a clock which
clocks the switches so that in a first phase of a clock cycle, the
so-called charging phase, the second switch and the third switch
are ON whilst the other switches are OFF, so that the charge pump
capacitor is charged to the input voltage, and in the second phase
of a clock cycle, the so-called discharge phase, the first switch
and the fourth switch are ON whils the other switches are OFF, so
that then the charged charge pump capacitor is connected in series
to the input voltage which outputs a voltage value to the smoothing
and storage capacitor located at the output of the circuit, this
voltage value corresponding to roughly twice the input voltage.
[0004] Correspondingly, charge pumps are conceivable which produce
an optimum multiple of the input voltage, which invert or reduce
the input voltage.
[0005] However, in the DC/DC converter operating on the charge pump
principle as described above the output voltage drops off
undesirably so even for small load currents. Since in the majority
of applications the output voltage which e.g. in digital electronic
circuits amounts often to 3.3 or 5 V, is fixedly defined and is
only allowed to fluctuate in a tight range, regulated converters
have been developed which set the output voltage to a fixed desired
voltage value.
[0006] These DC/DC converter regulators comprise as a rule a
comparator which compares the actual output voltage or a voltage
proportional to the actual output voltage (which may be derived
from the output voltage e.g. across a voltage divider) to a defined
reference voltage representing the design output voltage, and then
when a deviation is sensed, outputs a control signal, with the aid
of which the actual output voltage is adapted to the defined design
output voltage value.
[0007] Described in U.S. Pat. No. 5,680,300 are two types of
regulators used with DC/DC converters operating on the charge pump
principle, the so-called linear regulator and the so-called
skip-mode regulator.
[0008] In the linear regulator the control signal of the comparator
changes, e.g. via a gate of one of the MOSFET switches, the ON
resistance of the MOSFET so that the drop in voltage across the
switch is increased or decreased resulting in a reduction or
increase in the actual output voltage of the converter. The linear
regulator has, however, the disadvantage that the energy losses
resulting from switching the switches of the charge pump are
relatively high since the charge pump is always in operation in the
case of the linear regulator. These energy losses result from the
currents required to charge the gates of the MOS power transistors
at a constant frequency even when no current flows at the output of
the converter.
[0009] These disadvantage do not occur in the skip-mode regulator
which makes use of the control signal of the comparator to cycle
the charge pump ON/OFF depending on the output current requirement
and the resulting actual output voltage of the DC/DC converter so
that a charge is pumped to the smoothing and storage capacitor
located at the output of the circuit only if the voltage across the
capacitor has dropped below the design output voltage level. The
skip-mode regulator thus operates particularly energy-saving and is
particularly suitable for applications in which small load currents
alternate with large load currents, i.e. it guaranteeing a minor
quiescent current of the converter. The disadvantage of the
skip-mode regulator is, however, that the ON/OFF switching times of
the charge pump depend on the average output current in each case
to be furnished by the DC/DC converter, i.e. the frequency spectrum
resulting at the output of the converter is totally undefined. In
addition to this the ripple of the output voltage is relatively
heavy since the flow of output current is not continual in the
skip-mode regulator.
[0010] The object of the present invention is thus to provide a
DC/DC converter operating on the charge pump principle which is
superior to the DC/DC converters regulated hitherto either by the
skip mode or linear regulator principle and obviates the
disadvantages as described above at least in part. In addition, the
intention is to provide a corresponding method of operating a DC/DC
converter which is superior to the method hitherto.
[0011] This object is achieved by a DC/DC converter including a
charge pump circuit comprising:
[0012] one or more charge pump capacitors and a plurality of
controllable switches connected thereto, the controllable switches
being controllable by a control circuit so that the charge pump
capacitor(s) is/are alternatingly switched in a charging and
discharge phase so that an output voltage deviating from the input
voltage of the converter is generated at the output of the
converter;
[0013] a first current source set to a predetermined base current
located either in the discharge path of the charge pump circuit via
which in the discharge phase current is supplied to the output of
the converter, or in the charging path of the charge pump circuit,
via which the charge pump capacitor(s) is/are charged in the
charging phase of the charge pump circuit; and
[0014] a second current source connected in parallel to the first
current source, the current of the second current source being
controllable; and
[0015] an output voltage regulator circuit
[0016] for generating a first control signal representing the
difference between a voltage characterizing the output voltage and
a first reference voltage and controlling the second current source
when the charge pump circuit is active so that the controllable
current is reduced or increased with an increase and reduction
respectively in the difference to track the voltage characterizing
the output voltage in accordance with the first reference voltage;
and for generating a second control signal guided to the control
circuit, this signal assuming a first status when the voltage
characterizing the output voltage exceeds a second reference
voltage at a predetermined level above the first reference voltage,
upon which the control circuit deactivates the charge pump circuit,
and assumes a second status when the voltage characterizing the
output voltage drops below the second reference voltage, upon which
the control circuit activates the charge pump circuit.
[0017] In addition this object is achieved by a method for
operating a DC/DC converter including a charge pump circuit
comprising one or more charge pump capacitors and a plurality of
controllable switches connected thereto comprising the steps
[0018] cycling the charge pump capacitor(s) by the controllable
switches in a charging and discharge phase during operation of the
charge pump circuit so that an output voltage deviating from the
input voltage of the converter is generated at the output of the
converter;
[0019] setting a controllable current flowing parallel to a
predetermined base current with the charge pump circuit active in
the discharge or charging path of the charge pump circuit as a
function of the difference between a voltage characterizing the
output voltage and a first reference voltage so that the
controllable current is reduced or increased with an increase and
reduction respectively in the difference to track the voltage
characterizing the output voltage in accordance with the first
reference voltage; and deactivating the charge pump circuit when
the voltage characterizing the output voltage exceeds a second
reference voltage at a predetermined level above the first
reference voltage and
[0020] activating the charge pump circuit when the voltage
characterizing the output voltage drops below the second reference
voltage at a predetermined level above the first reference
voltage.
[0021] The DC/DC converter in accordance with the invention makes
clever use of the advantages afforded by the two differingly
regulated converters by it being skip-mode regulated when the
converter output current is low and linearly regulated when the
converter output current is higher, selecting the one or other
regulating mode being done automatically and simply achievable. The
DC/DC converter in accordance with the invention comprises both a
high efficiency at low output currents and a defined output
frequency spectrum at high output currents.
[0022] Advantageous further embodiments of the invention are
characterized in the sub-claims.
[0023] The invention will now be detailled by way of example
embodiments as shown in the drawings in which:
[0024] FIG. 1 is a circuit diagram of a first embodiment of the
DC/DC converter in accordance with the invention;
[0025] FIG. 2 is a circuit diagram of a second embodiment of the
DC/DC converter in accordance with the invention;
[0026] Referring now to FIG. 1 there is illustrated a circuit
diagram of a first embodiment of the DC/DC converter in accordance
with the invention, the configuration of which will first be
described.
[0027] The DC/DC converter in accordance with the invention as
shown in FIG. 1 comprises substantially a charge pump circuit and a
regulator circuit which regulates the output voltage of the DC/DC
converter to a design value.
[0028] The charge pump circuit comprises a charge pump capacitor
Cpump and four controllable switches S1, S2, S3, S4 preferably
consisting of MOSFETs as shown in FIG. 1. The one electrode of the
charge pump capacitor Cpump is connectable via a first controllable
switch S1 to the input 1 of the DC/DC converter and via the second
controllable switch S2 to GND, and the other electrode of the
charge pump capacitor Cpump is connectable via the third
controllable switch S3 to the input 1 of the DC/DC converter and
via the fourth controllable switch to the output 2 of the DC/DC
converter.
[0029] The charge pump circuit comprises in addition a first
current source 3 located between the input 1 of the DC/DC converter
and the first controllable switch S1 and furnishing a predetermined
and constant basic current Ib, and a second current source 4
connected in parallel to the first current source 3 and furnishing
an additional current Ir, the amperage of which is controllable.
Located at the output 2 of the DC/DC converter, as is usual for
DC/DC converters, is a storage capacitor Cout.
[0030] The output voltage regulator circuit comprises an
operational amplifier 5 receiving at its non-inverting input a
reference voltage Vref which e.g. may originate from a reference
voltage generator circuit (not shown in detail). At its inverting
input the operational amplifier receives a voltage proportional to
the output voltage Vout of the DC/DC converter, this proportional
voltage being tapped from the resistor R2 of the voltage divider
comprising the two resistors R1 and R2 at which the output voltage
Vout of the DC/DC converter is connected. The operational amplifier
5 is a so-called transconductance amplifier (VC-OPV) and furnishes
at its output a current Ic as a function of the difference between
the voltages applied to its inputs. It is this current that results
in a voltage Vk being generated at the output RC compensation pad
comprising the resistor Rk and the capacitor Ck, the voltage Vk
being proportional to the difference between the voltage
(R2/(R1+R2))*Vout characterizing the output voltage, and the
reference voltage. It is this voltage Vk that is used to control
the current of the controllable current source 4 as explained
below.
[0031] The output voltage regulator circuit comprises in addition a
comparator 6 receiving at its inverting input the voltage
(R2/(R1+R2))*Vout proportional to the output voltage Vout and at
its non-inverting input a second reference voltage Voff
corresponding to the first reference voltage Vref plus a small
offset voltage .DELTA.Voff generated by the voltage source 7:
Voff=Vref+.DELTA.Voff (1)
[0032] The comparator 6 outputs a control signal to the control
circuit 8 which has one of two possible output statuses; a first
output status when the voltage characterizing the output voltage is
smaller than the reference voltage Voff, and a second output status
when the voltage characterizing the output voltage is larger than
the second reference voltage Voff.
[0033] The control circuit 8 which serves to control the
controllable switches S1, S2, S3 and S4 of the charge pump circuit
comprises conventionally as the central element a clock from which
the signals "CLK" and "NCLK" are derived which are applied to the
gates of the controllable switches S2, S3 or S1, S4. In this
arrangement the "VCLK" signal is opposite in phase to that of the
"NCLK" signal.
[0034] The functioning of the DC/DC converter as shown in FIG. 1
will now be detailled:
[0035] The charge pump circuit comprising the charge pump capacitor
Cpump and four controllable switches S1, S2, S3, S4 cooperates with
the control circuit 8 conventionally, i.e. the MOSFETs S2, S3 and
MOSFETs S1, S4 are cycled ON by the clock signals "CLK" and "NCLK"
so that the corresponding other MOSFETs in each case are OFF. Thus,
when the MOSFETs S2, S3 are ON and the MOSFETs S1, S4 are OFF
(charging phase) the charge pump capacitor Cpump is charged to the
input voltage Vin, whereas when the MOSFETs S1, S4 are ON and the
MOSFETs S2, S3 are OFF (discharge phase) the output capacitor Cout
is charged by the input voltage source furnishing the voltage Vin
and the charge pump capacitor Cpump. Maximally twice the input
voltage Vin is attainable across the output capacitor Cout.
Charging phase and discharge phase cycle as controlled by the
control circuit 8.
[0036] The current flowing in the discharge phase representing the
output current of the DC/DC converter is dictated by the sum of the
currents from the first current source 3 and the controllable
second current source 4. In this arrangement the average output
current Iout is given by the following equation:
Iout=(Ir+Ib)/2 (2)
[0037] where Ib is the constant basic current furnished by the
first current source 3 and Ir is the controllable current furnished
by the second current source 4. The factor {fraction (1/2)} results
from the fact that the charge pump circuit furnishes a current only
during the discharge phase (assuming that charging and discharge
phase are equally long). As long as the output voltage Vout is
smaller than the second reference voltage Voff regulation of the
output voltage Vout is handled by the operational amplifier 5 in
conjunction with the RC compensation pad Rk, Ck and the
controllable second current source 4. When e.g. a change in the
load occurs at the output of the DC/DC converter and thus the
current flowing at the output of the DC/DC converter required as an
average drops, then the output voltage Vout will increase until at
some time the voltage (R2/(R1+R2))*Vout characterizing the output
voltage will exceed the value of the first reference voltage Vref.
This increase is counteracted by the "linear regulation mechanism"
involving the operational amplifier 5, the RC pad Rk, Ck and the
controllable second current source 4 due to the difference between
the reference voltage Vref and the voltage characterizing the
output voltage at the inputs of the operational amplifier 5
generating a current Ic corresponding to this difference at the
output of the operational amplifier which produces across the RC
pad a corresponding voltage Vk with which the voltage-controlled
current source Ir is then controlled so that the current Ir is
reduced. As a result of this the output capacitor Cout receives
less current in the discharge phase of the charge pump cycle,
resulting in the output voltage Vout finally reattaining the
desired design voltage value Vref* (R1+R2)/R2. The linear
regulation runs correspondingly inverse when the current required
as an average at the output 2 of the DC/DC converter increases, the
controllable second current source Ir then being controlled so that
the current Ir is increased.
[0038] As soon as the current flowing at the output of the DC/DC
converter attains on a time average the value Ib/2, the "linear
regulation mechanism" as described above signals the second current
source 4 OFF so completely that no current Ir flows any more at
all, i.e. any further regulation of the output voltage Vout with a
further drop in the average output current then no longer being
possible by the operational amplifier 5 and the RC pad Rk, Ck.
[0039] When the average output current of the DC/DC converter drops
below the value Ib/2 more charge is furnished to the output 2 of
the DC/DC converter by the charge pump circuit per unit of time
than is needed in this case, resulting in the output voltage Vout
of the converter increasing. As soon as the output voltage Vout
exceeds the value of the second reference voltage Voff, the
comparator 6--now operating on the skip-mode principle--handles
regulation of the output voltage Vout of the DC/DC converter, by a
control signal having the first status (e.g. HI) being output to
the control circuit 8 advising it that it is now required to
deactivate the charge pump circuit 8 (the control signal output by
the comparator 6 then assuming the above-mentioned second status
(e.g. a LO) when the output voltage Vout is smaller than the second
reference voltage Voff, differing from the first status). The
control circuit 8 then deactivates the charge pump circuit (e.g. by
deactivating the clock) as a result of which the output capacitor
Cout no longer receives a charge for some time and the output
voltage Vout across the output capacitor Cout descreases until at
some point in time it drops below the value of the second reference
voltage Voff. The comparator 6 then outputs the second control
signal to the control circuit 8 as a result of which it activates
the charge pump circuit 8 by it again commencing to cycle the
MOSFETs S1, S4 and S2, S3 ON/OFF respectively in the manner as
described above.
[0040] As compared to DC/DC converters operating hitherto in
accordance with the charge pump principle the DC/DC converter in
accordance with the invention has a number of advantages. When the
output currents of the DC/DC converter are larger on an average, at
which it operates in the linear regulation mode, it furnishes a
defined frequency spectrum due to the known switching frequency of
the control circuit clock. When the output currents of the DC/DC
converter are smaller on an average, at which it operates in the
skipmode, it has a high efficiency since the charge pump circuit is
only activated when energy is actually required at the output. As
compared to existing solutions the ripple of the output voltage in
the skip-mode is greatly reduced. In this arrangement the output
voltage ripple in the skip-mode is limited by the output current
peaks limited by the predetermined constant basic current Ib. The
basic current Ib furished during skip-mode regulation is precisely
defined thus assuring that the selection between the two regulation
modes is always made at the same output current. In addition to
this the maximum output current of the charge pump circuit is
limited by the arrangement of the two current sources to a value of
(Ib+Irmax)/2.
[0041] Referring now to FIG. 2 there is illustrated a circuit
diagram of a second embodiment of the DC/DC converter in accordance
with the invention. The embodiment as evident from FIG. 2 differs
from that as shown in FIG. 1 merely by, in this case, the second
charge pump circuit (Cpump2, S5, S6, S7, S8, first current source
10 (Ib2), where Ib1=Ib2, controllable second current source 11
(Ir2) likewise controlled from the operational amplifier 5 in
conjunction with the RC pad Rk,Ck) being provided which is
configured and circuited the same as the first charge pump circuit
(Cpump1, S1, S2, S3, S4, first current source 3 (Ib1), controllable
second current source 4 (Ir1)). The second charge pump circuit
comprising the charge pump capacitor Cpump2 and the four switches
S5, S6, S7, S8 is connected in parallel to the first charge pump
circuit, it being controlled by the control circuit 8 opposite in
phase to the first charge pump circuit, so that when the first
charge pump circuit (Cpump1, S1-S4) is in the discharge phase (S1,
S4 ON; S2, S3 OFF) then the second charge pump circuit (Cpump2,
S5-S8) is in the charge phase (S6, S7 ON; S5, S8 OFF) and
vice-versa. This results in a continual flow of current to the
output 2 of the DC/DC converter when the charge pump circuits are
active, thus reducing the ripple in the output voltage Vout as
compared to the embodiment as shown in FIG. 1. The opposite phase
control is evident in FIG. 2 from the differing distribution of the
two switching signals "CLK" and "NCLK" at the gates of the MOSFETs
of the two charge pump circuits.
[0042] The various embodiments of the DC/DC converter in accordance
with the invention are fabricated preferably in the form of an
integrated circuit.
[0043] In addition, the circuit may be simplified also so that the
MOSFETs are made use of as the first and second current source (3,
4 and 10, 11 resp.) as already provided in the discharge path, via
which current flows in the discharge phase of the charge pump
circuit to the output of the DC/DC converter, as controllable
switches of the charge pump circuit. Thus, in the embodiment as
shown in FIG. 2 the MOSFETs S1, S4, S5 or S8 may be used as current
sources which, when the circuit is fabricated integrated, results
in a reduction in the chip surface area required.
[0044] The person skilled in the art will readily appreciate that
that the circuits selected as example embodiments may be modified
in many different ways without departing from the scope of
protection afforded by the attached claims. Thus, for instance, the
two current sources may be arranged e.g. in the charge pump circuit
at a location other than that as shown in in the embodiments, e.g.
in the charging path of the charge pump circuit via which in the
charging phase of the charge pump circuit the charge pump
capacitor(s) is/are charged. Accordingly, even in a variation in
the concrete configuration of the charge pump circuit in many
different ways--whereby the circuit may, of course, comprise
several charge pump capacitors and more or fewer controllable
switches than is the case in the selected embodiments--this is
still within the gist of the invention, the same as in making use
of charge pump circuits which increase, invert or reduce the input
voltage of the DC/DC converter.
DC/DC Converter and Method of Operating a DC/DC Converter
[0045] The invention relates to a DC/DC converter including a
charge pump circuit and a method of operating one such DC/DC
converter.
[0046] In addition to the supply voltage many electronic circuits
require further voltages, the levels of which are sometimes higher
than that of the supply voltage. One cost-effective, simple
and--especially as compared to coil converters--highly compact
solution to furnishing these further voltages are DC/DC converters
operating on the charge pump principle. Such converters are
described e.g. in the text book "The Art of Electronics" by Paul
Horowitz, 2nd Edition, Cambridge University Press, New York 1991 on
pages 377 to 379 thereof.
[0047] Horowitz also describes a simple DC/DC converter operating
on the charge pump principle with which an output voltage
corresponding maximally to roughly twice the input voltage is
achievable. The basic circuit of the converter consists
substantially of a charge pump capacitor and four controllable
switches (e.g. MOSFETS) whereby one electrode of the charge pump
capacitor is connectable via a first switch to the input voltage
terminal of the converter and via a second switch to GND, and the
other electrode of the capacitor is connectable via the third
switch to the input voltage terminal and via the fourth switch to
the output voltage terminal of the converter. The converter
comprises furthermore a control circuit including a clock which
clocks the switches so that in a first phase of a clock cycle, the
so-called charging phase, the second switch and the third switch
are ON whilst the other switches are OFF, so that the charge pump
capacitor is charged to the input voltage, and in the second phase
of a clock cycle, the so-called discharge phase, the first switch
and the fourth switch are ON whilst the other switches are OFF, so
that then the charged charge pump capacitor is connected in series
to the input voltage which outputs a voltage value to the smoothing
and storage capacitor located at the output of the circuit, this
voltage value corresponding to roughly twice the input voltage.
[0048] Correspondingly, charge pumps are conceivable which produce
an optimum multiple of the input voltage, which invert or reduce
the input voltage.
[0049] However, in the DC/DC converter operating on the charge pump
principle as described above the output voltage drops off
undesirably so even for small load currents. Since in the majority
of applications the output voltage which e.g. in digital electronic
circuits amounts often to 3.3 or 5 V, is fixedly defined and is
only allowed to fluctuate in a tight range, regulated converters
have been developed which set the output voltage to a fixed desired
voltage value.
[0050] These DC/DC converter regulators comprise as a rule a
comparator which compares the actual output voltage or a voltage
proportional to the actual output voltage (which may be derived
from the output voltage e.g. across a voltage divider) to a defined
reference voltage representing the design output voltage, and then
when a deviation is sensed, outputs a control signal, with the aid
of which the actual output voltage is adapted to the defined design
output voltage value.
[0051] Described in U.S. Pat. No. 5,680,300 are two types of
regulators used with DC/DC converters operating on the charge pump
principle, the so-called linear regulator and the so-called
skip-mode regulator.
[0052] In the linear regulator the control signal of the comparator
changes, e.g. via a, gate of one of the MOSFET switches, the ON
resistance of the MOSFET so that the drop in voltage across the
switch is increased or decreased resulting in a reduction or
increase in the actual output voltage of the converter. The linear
regulator has, however, the disadvantage that the energy losses
resulting from switching the switches of the charge pump are
relatively high since the charge pump is always in operation in the
case of the linear regulator. These energy losses result from the
currents required to charge the gates of the MOS power transistors
at a constant frequency even when no current flows at the output of
the converter.
[0053] These disadvantage do not occur in the skip-mode regulator
which makes use of the control signal of the comparator to cycle
the charge pump ON/OFF depending on the output current requirement
and the resulting actual output voltage of the DC/DC converter so
that a charge is pumped to the smoothing and storage capacitor
located at the output of the circuit only if the voltage across the
capacitor has dropped below the design output voltage level. The
skip-mode regulator thus operates particularly energy-saving and is
particularly suitable for applications in which small load currents
alternate with large load currents, i.e. it guaranteeing a minor
quiescent current of the converter. The disadvantage of the
skip-mode regulator is, however, that the ON/OFF switching times of
the charge pump depend on the average output current in each case
to be furnished by the DC/DC converter, i.e. the frequency spectrum
resulting at the output of the converter is totally undefined.
[0054] In addition to this the ripple of the output voltage is
relatively heavy since the flow of output current is not continual
in the skip-mode regulator.
[0055] A general object of the present invention is thus to provide
a DC/DC converter operating on the charge pump principle which is
superior to the DC/DC converters regulated hitherto either by the
skip mode or linear regulator principle and obviates the
disadvantages as described above at least in part. In addition, the
intention is to provide a corresponding method of operating a DC/DC
converter which is superior to the method hitherto.
[0056] This and other objects and features are achieved in
accordance with one aspect of the invention by a DC/DC converter
including a charge pump circuit comprising:
[0057] one or more charge pump capacitors and a plurality of
controllable switches connected thereto, the controllable switches
being controllable by a control circuit so that the charge pump
capacitors is/are alternatingly switched in a charging and
discharge phase so that an output voltage deviating from the input
voltage of the converter is generated at the output of the
converter;
[0058] a first current source set to a predetermined base current
located either in the discharge path of the charge pump circuit via
which in the discharge phase current is supplied to the output of
the converter, or in the charging path of the charge pump circuit,
via which the charge pump capacitors is/are charged in the charging
phase of the charge pump circuit; and
[0059] a second current source connected in parallel to the first
current source, the current of the second current source being
controllable; and
[0060] an output voltage regulator circuit for generating a first
control signal representing the difference between a voltage
characterizing the output voltage and a first reference voltage and
controlling the second current source when the charge pump circuit
is active so that the controllable current is reduced or increased
with an increase and reduction respectively in the difference to
track the voltage characterizing the output voltage in accordance
with the first reference voltage; and for generating a second
control signal guided to the control circuit, this signal assuming
a first status when the voltage characterizing the output voltage
exceeds a second reference voltage at a predetermined level above
the first reference voltage, upon which the control circuit
deactivates the charge pump circuit, and assumes a second status
when the voltage characterizing the output voltage drops below the
second reference voltage, upon which the control circuit activates
the charge pump circuit.
[0061] Another aspect of the invention includes a method for
operating a DC/DC converter including a charge pump circuit
comprising one or more charge pump capacitors and a plurality of
controllable switches connected thereto comprising the steps
[0062] cycling the charge pump capacitors by the controllable
switches in a charging and discharge phase during operation of the
charge pump circuit so that an output voltage deviating from the
input voltage of the converter is generated at the output of the
converter;
[0063] setting a controllable current flowing parallel to a
predetermined base current with the charge pump circuit active in
the discharge or charging path of the charge pump circuit as a
function of the difference between a voltage characterizing the
output voltage and a first reference voltage so that the
controllable current is reduced or increased with an increase and
reduction respectively in the difference to track the voltage
characterizing the output voltage in accordance with the first
reference voltage; and deactivating the charge pump circuit when
the voltage characterizing the output voltage exceeds a second
reference voltage at a predetermined level above the first
reference voltage and activating the charge pump circuit when the
voltage characterizing the output voltage drops below the second
reference voltage at a predetermined level above the first
reference voltage.
[0064] The DC/DC converter in accordance with, the invention makes
clever use of the advantages afforded by the two differing
regulated converters by it being skip-mode regulated when the
converter output current is low and linearly regulated when the
converter output current is higher, selecting the one or other
regulating mode being done automatically and simply achievable. The
DC/DC converter in accordance with the invention comprises both a
high efficiency at low output currents and a defined output
frequency spectrum at high output currents.
[0065] Advantageous further embodiments of the invention are
characterized in the sub-claims.
[0066] The invention will now be detained by way of example
embodiments as shown in the drawings in which:
[0067] FIG. 1 is a circuit diagram of a first embodiment of the
DC/DC converter in accordance with the invention;
[0068] FIG. 2 is a circuit diagram of a second embodiment of the
DC/DC converter in accordance with the invention;
[0069] Referring now to FIG. 1 there is illustrated a circuit
diagram of a first embodiment of the DC/DC converter in accordance
with the invention, the configuration of which will first be
described.
[0070] The DC/DC converter in accordance with the invention as
shown in FIG. 1 comprises substantially a charge pump circuit and a
regulator circuit which regulates the output voltage of the DC/DC
converter to a design value.
[0071] The charge pump circuit comprises a charge pump capacitor
Cpump and four controllable switches S1, S2, S3, S4 preferably
consisting of MOSFETs as shown in FIG. 1. The one electrode of the
charge pump capacitor Cpump is connectable via a first controllable
switch S1 to the input 1 of the DC/DC converter and via the second
controllable switch S2 to GND, and the other electrode of the
charge pump capacitor Cpump is connectable via the third
controllable switch S3 to the input 1 of the DC/DC converter and
via the fourth controllable switch to the output 2 of the DC/DC
converter.
[0072] The charge pump circuit comprises in addition a first
current source 3 located between the input 1 of the DC/DC converter
and the first controllable switch S1 and furnishing a predetermined
and constant basic current Ib, and a second current source 4
connected in parallel to the first current source 3 and furnishing
an additional current Ir, the amperage of which is controllable.
Located at the output 2 of the DC/DC converter, as is usual for
DC/DC converters, is a storage capacitor Cout.
[0073] The output voltage regulator circuit comprises an
operational amplifier 5 receiving at its non-inverting input a
reference voltage Vref which e.g. may originate from a reference
voltage generator circuit (not shown in detail). At its inverting
input the operational amplifier receives a voltage proportional to
the output voltage Vout of the DC/DC converter, this proportional
voltage being tapped from the resistor R2 of the voltage divider
comprising the two resistors R1 and R2 at which the output voltage
Vout of the DC/DC converter is connected. The operational amplifier
5 is a so-called transconductance amplifier (VC-OPV) and furnishes
at its output a current Ic as a function of the difference between
the voltages applied to its inputs. It is this current that results
in a voltage Vk being generated at the output RC compensation pad
comprising the resistor Rk and the capacitor Ck, the voltage Vk
being proportional to the difference between the voltage
(R2/(R1+R2))*Vout characterizing the output voltage, and the
reference voltage. It is this voltage Vk that is used to control
the current of the controllable current source 4 as explained
below.
[0074] The output voltage regulator circuit comprises in addition a
comparator 6 receiving at its inverting input the voltage
(R2/(R1+R2))*Vout proportional to the output voltage Vout and at
its non-inverting input a second reference voltage Voff
corresponding to the first reference voltage Vref plus a small
offset voltage AVoff generated by the voltage source 7:
Voff=Vref+.DELTA.Voff
[0075] The comparator 6 outputs a control signal to the control
circuit 8 which has one of two possible output statuses; a first
output status when the voltage characterizing the output voltage is
smaller than the reference voltage Voff, and a second output status
when the voltage characterizing the output voltage is larger than
the second reference voltage Voff.
[0076] The control circuit 8 which serves to control the
controllable switches S1, S2, S3 and S4 of the charge pump circuit
comprises conventionally as the central element a clock from which
the signals "CLK" and "NCLK" are derived which are applied to the
gates of the controllable switches S2, S3 or SI, S4. In this
arrangement the "CLK" signal is opposite in phase to that of the
"NCLK" signal.
[0077] The functioning of the DC/DC converter as shown in FIG. 1
will now be detained:
[0078] The charge pump circuit comprising the charge pump capacitor
Cpump and four controllable switches S1, S2, S3, S4 cooperates with
the control circuit 8 conventionally, i.e. the MOSFETs S2, S3 and
MOSFETs S1, S4 are cycled ON by the clock signals "CLK" and "NCLK"
so that the corresponding other MOSFETs in each case are OFF. Thus,
when the MOSFETs S2, S3 are ON and the MOSFETs S1, S4 are OFF
(charging phase) the charge pump capacitor Cpump is charged to the
input voltage Vin, whereas when the MOSFETs SI, S4 are ON and the
MOSFETs S2, S3 are OFF (discharge phase) the output capacitor Cout
is charged by the input voltage source furnishing the voltage Vin
and the charge pump capacitor Cpump. Maximally twice the input
voltage Vin is attainable across the output capacitor Cout.
Charging phase and discharge phase cycle as controlled by the
control circuit 8.
[0079] The current flowing in the discharge phase representing the
output current of the DC/DC converter is dictated by the sum of the
currents from the first current source 3 and the controllable
second current source 4. In this arrangement the average output
current lout is given by the following equation:
Iout=(Ir+Ib)/2 (2)
[0080] where Ib is the constant basic current furnished by the
first current source 3 and Ir is the controllable current furnished
by the second current source 4. The factor {fraction (1/2)} results
from the fact that the charge pump circuit furnishes a current only
during the discharge phase (assuming that charging and discharge
phase are equally long). As long as the output voltage Vout is
smaller than the second reference voltage Voff regulation of the
output voltage Vout is handled by the operational amplifier 5 in
conjunction with the RC compensation pad Rk, Ck and the
controllable second current source 4. When e.g. a change in the
load occurs at the output of the DC/DC converter and thus the
current flowing at the output of the DC/DC converter required as an
average drops, then the output voltage Vout will increase until at
some time the voltage (R2/(RI+R2))*Vout characterizing the output
voltage will exceed the value of the first reference voltage Vref.
This increase is counteracted by the "linear regulation mechanism"
involving the operational amplifier 5, the RC pad Rk, Ck and the
controllable second current source 4 due to the difference between
the reference voltage Vref and the voltage characterizing the
output voltage at the inputs of the operational amplifier 5
generating a current IC corresponding to this difference at the
output of the operational amplifier which produces across the RC
pad a corresponding voltage Vk with which the voltage-controlled
current source Ir is then controlled so that the current Ir is
reduced. As a result of this the output capacitor Cout receives
less current in the discharge phase of the charge pump cycle,
resulting in the output voltage Vout finally reattaining the
desired design voltage value Vref* (Rl+R2)/R2. The linear
regulation runs correspondingly inverse when the current required
as an average at the output 2 of the DC/DC converter increases, the
controllable second current source Ir then being controlled so that
the current Ir is increased.
[0081] As soon as the current f lowing at the output of the DC/DC
converter attains on a time average the value Ib/2, the "linear
regulation mechanism" as described above signals the second current
source 4 OFF so completely that no current Ir flows any more at
all, i.e. any further regulation of the output voltage Vout with a
further drop in the average output current then no longer being
possible by the operational amplifier 5 and the RC pad Rk, Ck.
[0082] When the average output current of the DC/DC converter drops
below the value Ib/2 more charge is furnished to the output 2 of
the DC/DC converter by the charge pump circuit per unit of time
than is needed in this case, resulting in the output voltage Vout
of the converter increasing. As soon as the output voltage Vout
exceeds the value of the second reference voltage Voff, the
comparator 6--now operating on the skip-mode principle--handles
regulation of the output voltage Vout of the DC/DC converter, by a
control signal having the first status (e.g. HI) being output to
the control circuit 8 advising it that it is now required to
deactivate the charge pump circuit 8 (the control signal output by
the comparator 6 then assuming the above-mentioned second status
(e.g. a LO) when the output voltage Vout is smaller than the second
reference voltage Voff, differing from the first status). The
control circuit 8 then deactivates the charge pump circuit (e.g. by
deactivating the clock) as a result of which the output capacitor
Cout no longer receives a charge for some time and the output
voltage Vout across the output capacitor Cout decreases until at
some point in time it drops below the value of the second reference
voltage Voff. The comparator 6 then outputs the second control
signal to the control circuit 8 as a result of which it activates
the charge pump circuit 8 by it again commencing to cycle the
MOSFETs S1, S4 and S2, S3 ON/OFF respectively in the manner as
described above.
[0083] As compared to DC/DC converters operating hitherto in
accordance with the charge pump principle the DC/DC converter in
accordance with the invention has a number of advantages. When the
output currents of the DC/DC converter are larger on an average, at
which it operates in the linear regulation mode, it furnishes a
defined frequency spectrum due to the known switching frequency of
the control circuit clock. When the output currents of the DC/DC
converter are smaller on an average, at which it operates in the
skip-mode, it has a high efficiency since the charge pump circuit
is only activated when energy is actually required at the output.
As compared to existing solutions the ripple of the output voltage
in the skip-mode is greatly reduced. In this arrangement the output
voltage ripple in the skip-mode is limited by the output current
peaks limited by the predetermined constant basic current Ib. The
basic current Ib furnished during skip-mode regulation is precisely
defined thus assuring that the selection between the two regulation
modes is always made at the same output current. In addition to
this the maximum output current of the charge pump circuit is
limited by the arrangement of the two current sources to a value of
(Ib+Irmax)/2.
[0084] Referring now to FIG. 2 there is illustrated a circuit
diagram of a second embodiment of the DC/DC converter in accordance
with the invention. The embodiment as evident from FIG. 2 differs
from that as shown in FIG. 1 merely by, in this case, the second
charge pump circuit (Cpump2, SS, S6, S7, SS, first current source
10 (Ib2), where Ib1=Ib2, controllable second current source 11
(Ir2) likewise controlled from the operational amplifier 5 in
conjunction with the RC pad Rk, Ck) being provided which is
configured and circuited the same as the first charge pump circuit
(Cpumpl, S1, S2, S3, S4, first current source 3 (Ib1), controllable
second current source 4 (Ir1)). The second charge pump circuit
comprising the charge pump capacitor Cpump2 and the four switches
SS, S6, S7, SS is connected in parallel to the first charge pump
circuit, it being controlled by the control circuit 8 opposite in
phase to the first charge pump circuit, so that when the first
charge pump circuit (Cpumpl, S1-S4) is in the discharge phase (S1,
S4 ON; S2, S3 OFF) then the second charge pump circuit (Cpump2,
SS-SS) is in the charge phase (S6, S7 ON; SS, SS OFF) and
vice-versa. This results in a continual flow of current to the
output 2 of the DC/DC converter when the charge pump circuits are
active, thus reducing the ripple in the output voltage Vout as
compared to the embodiment as shown in FIG. 1. The opposite phase
control is evident in FIG. 2 from the differing distribution of the
two switching signals "CLK" and "NCLK" at the gates of the MOSFETs
of the two charge pump circuits.
[0085] The various embodiments of the DC/DC converter in accordance
with the invention are fabricated preferably in the form of an
integrated circuit.
[0086] In addition, the circuit may be simplified also so that the
MOSFETs are made use of as the first and second current source (3,
4 and 10, 11 resp.) as already provided in the discharge path, via
which current flows in the discharge phase of the charge pump
circuit to the output of the DC/DC converter, as controllable
switches of the charge pump circuit. Thus, in the embodiment as
shown in FIG. 2 the MOSFETs SI, S4, S5 or SB may be used as current
sources which, when the circuit is fabricated integrated, results
in a reduction in the chip surface area required.
[0087] The person skilled in the art will readily appreciate that
that the circuits selected as example embodiments may be modified
in many different ways without departing from the scope of
protection afforded by the attached claims. Thus, for instance, the
two current sources may be arranged e.g. in the charge pump circuit
at a location other than that as shown in the embodiments, e.g. in
the charging path of the charge pump circuit via which in the
charging phase of the charge pump circuit the charge pump
capacitor(s) is/are charged. Accordingly, even in a variation in
the concrete configuration of the charge pump circuit in many
different ways--whereby the circuit may, of course, comprise
several charge pump capacitors and more or fewer controllable
switches than is the case in the selected embodiments--this is
still within the gist of the invention, the same as in making use
of charge pump circuits which increase, invert or reduce the input
voltage of the DC/DC converter.
* * * * *