U.S. patent application number 11/219026 was filed with the patent office on 2007-03-08 for three-phase low noise charge pump and method.
This patent application is currently assigned to Texas Instruments Incorporated. Invention is credited to Sergey Alenin.
Application Number | 20070053216 11/219026 |
Document ID | / |
Family ID | 37809615 |
Filed Date | 2007-03-08 |
United States Patent
Application |
20070053216 |
Kind Code |
A1 |
Alenin; Sergey |
March 8, 2007 |
THREE-PHASE LOW NOISE CHARGE PUMP AND METHOD
Abstract
A low noise charge pump circuit includes a first terminal of a
first flying capacitor selectively coupled to a first voltage
during a first recharging phase and a second terminal of the first
flying capacitor selectively coupled to a second voltage during the
first recharging phase. The second terminal of the first flying
capacitor is coupled to a precharge control circuit during a first
parasitic capacitance precharging phase that occurs after the first
recharging phase to cause the voltage of the first terminal of the
first flying capacitor to equal an output voltage. The first
terminal of the first flying capacitor is coupled to an output
conductor conducting the output voltage during a first discharging
phase that occurs after the first parasitic capacitance precharging
phase. The second terminal of the first flying capacitor is coupled
to a discharge control circuit which increases the voltage of the
second terminal of the first flying capacitor during the first
discharging phase until the output voltage is equal to a regulated
value.
Inventors: |
Alenin; Sergey; (Tucson,
AZ) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
US
|
Assignee: |
Texas Instruments
Incorporated
|
Family ID: |
37809615 |
Appl. No.: |
11/219026 |
Filed: |
September 2, 2005 |
Current U.S.
Class: |
363/60 |
Current CPC
Class: |
H02M 3/07 20130101; H02M
3/077 20210501 |
Class at
Publication: |
363/060 |
International
Class: |
H02M 3/18 20060101
H02M003/18 |
Claims
1. A three-phase charge pump circuit for producing a low noise
output voltage on an output conductor, the three-phase charge pump
comprising: (a) a first flying capacitor; (b) a first amplifier
circuit having an output coupled to control a first current source
to produce a first controlled current in a first conductor in
response to the output voltage, a first input coupled to a first
supply voltage, and a second input coupled to the output conductor;
(c) a second amplifier circuit having an output coupled to control
a second current source to produce a second controlled current in a
second conductor in response to a precharge voltage, a first input
coupled to the first supply voltage, and a second input coupled to
receive the precharge voltage; (d) a first switching circuit for
selectively coupling a first terminal of the first flying capacitor
to the first supply voltage during a first recharging phase and to
the output voltage during a first discharging phase; (e) a second
switching circuit for selectively coupling a second terminal of the
first flying capacitor to a second supply voltage during the first
recharging phase and to the first conductor during the first
discharging phase; and (f) the second switching circuit coupling
the second terminal of the first flying capacitor to the second
conductor during a first parasitic capacitance precharging phase
that occurs between the first recharging phase and the first
discharging phase so as to cause the voltage of the first terminal
of the first flying capacitor to have a value that avoids noise
spikes on the output conductor due to charge redistribution when
the first terminal of the first flying capacitor is coupled to the
output conductor.
2. The three-phase charge pump circuit of claim 1 including a
reservoir capacitor coupled to the output conductor.
3. The three-phase charge pump circuit of claim 1 including i. a
second flying capacitor; ii. a third switching circuit for
selectively coupling a first terminal of the second flying
capacitor to the output voltage during a second discharging phase;
iii. a fourth switching circuit for selectively coupling a second
terminal of the second flying capacitor to the second supply
voltage during the second recharging phase and to the first
conductor during the second discharging phase; and iv. the fourth
switching circuit coupling the second terminal of the second flying
capacitor to the second conductor during a second parasitic
capacitance precharging phase that occurs between the second
recharging phase and the second discharging phase so as to cause
the voltage of the first terminal of the second flying capacitor to
equal the output voltage.
4. The three-phase charge pump circuit of claim 3 including i. a
third flying capacitor; ii. a fifth switching circuit for
selectively coupling a first terminal of the third flying capacitor
to the first supply voltage during a third recharging phase and to
the output voltage during the third discharging phase; iii. a sixth
switching circuit for selectively coupling a second terminal of the
third flying capacitor to the second supply voltage during the
third recharging phase and to the first conductor during the third
discharging phase; and iv. the sixth switching circuit coupling the
second terminal of the third flying capacitor to the second
conductor during a third parasitic capacitance precharging phase
that occurs between the third recharging phase and the third
discharging phase so as to cause the voltage of the first terminal
of the third flying capacitor to equal the output voltage.
5. The three-phase charge pump circuit of claim 4 wherein the first
recharging phase, the second discharging phase, and the third
parasitic capacitance precharging phase occur during a first time
interval.
6. The three-phase charge pump circuit of claim 5 wherein the first
parasitic capacitance precharging phase, the second recharging
phase, and the third discharging phase occur during a second time
interval that occurs after the first time interval.
7. The three-phase charge pump circuit of claim 6 wherein the first
discharging phase, the second parasitic capacitance precharging
phase, and the third precharging phase occur during a third time
interval that occurs after the second time interval.
8. The three-phase charge pump circuit of claim 1 wherein the first
switching circuit couples the first terminal of the first flying
capacitor to a third conductor conducting the precharge voltage
during the first parasitic capacitance precharging phase.
9. The three-phase charge pump circuit of claim 1 wherein the first
switching circuit couples the first terminal of the first flying
capacitor to an electrically floating conductor during the first
parasitic capacitance precharging phase.
10. The three-phase charge pump circuit of claim 1 wherein the
capacitance associated with the output conductor is substantially
less than the capacitance of the first flying capacitor.
11. The three-phase charge pump circuit of claim 2 wherein the
capacitance of a reservoir capacitor coupled to the output
conductor is substantially greater than the capacitance of the
first flying capacitor.
12. The three-phase charge pump circuit of claim 1 wherein the
first amplifying circuit includes a voltage source circuit which
determines a regulated value of the output voltage.
13. The three-phase charge pump circuit of claim 12 wherein at
least one of the first and second amplifier circuits includes a
first transistor having a gate which functions as an inverting
amplifier input and is coupled to the first supply voltage, a
source which functions as a non-inverting amplifier input and is
coupled to a gate and drain of a diode-connected second transistor,
a source of the second transistor being coupled to the output
conductor, the second transistor functioning as the voltage source
circuit, a drain of the first transistor being coupled to a current
source and functioning as the output of the first amplifier
circuit.
14. The three-phase charge pump circuit of claim 13 wherein the
first controlled current source includes a third transistor having
a source coupled to the first supply voltage, a gate coupled to the
drain of the first transistor, and a drain coupled to the first
conductor.
15. A method of operating a charge pump circuit to produce a low
noise output voltage, comprising: (a) selectively coupling a first
terminal of a first flying capacitor to a first voltage during a
first recharging phase and selectively coupling a second terminal
of the first flying capacitor to a second voltage during the first
recharging phase; (b) coupling the second terminal of the first
flying capacitor to a precharge control circuit during a first
parasitic capacitance precharging phase that occurs after the first
precharging phase to cause the voltage of the first terminal of the
first flying capacitor to have a value that avoids noise spikes on
the output conductor due to charge redistribution when the first
terminal of the first flying capacitor is coupled to the output
conductor; and (c) coupling the first terminal of the first flying
capacitor to an output conductor conducting the output voltage
during a first discharging phase that occurs after the first
parasitic capacitance precharging phase and coupling the second
terminal of the first flying capacitor to a discharge control
circuit which increases the voltage of the second terminal of the
first flying capacitor during the first discharging phase until the
output voltage is equal to a regulated value.
16. The method of claim 15 wherein step (b) includes coupling the
first terminal of the first flying capacitor to a precharge voltage
during the first parasitic capacitance precharging phase.
17. The method of claim 15 wherein step (b) includes coupling the
first terminal of the first flying capacitor to an electrically
floating conductor during the first parasitic capacitance
precharging phase.
18. The method of claim 15 including, during step (a), coupling a
first terminal of a second flying capacitor to the output conductor
during a second discharging phase and coupling a second terminal of
the second flying capacitor to the discharge control circuit to
increase the voltage of the second terminal of the second flying
capacitor during the second discharging phase until the output
voltage is equal to the regulated value, and coupling a second
terminal of a third flying capacitor to the precharge control
circuit during a third parasitic capacitance precharging phase to
cause the voltage of a first terminal of the third flying capacitor
to have a value that avoids noise spikes on the output conductor
due to charge redistribution when the first terminal of the third
flying capacitor is coupled to the output conductor; during step
(b), selectively coupling the first terminal of the second flying
capacitor to the first voltage during a second recharging phase and
selectively coupling the second terminal of the second flying
capacitor to the second voltage during the second recharging phase,
and coupling a first terminal of the third flying capacitor to the
output conductor during a third discharging phase and coupling the
second terminal of the third flying capacitor to the discharge
control circuit to increase the voltage of the first terminal of
the third flying capacitor during the third discharging phase until
the output voltage is equal to the regulated value; and during step
(c), coupling the second terminal of the second flying capacitor to
the precharge control circuit during a second parasitic capacitance
precharging phase to cause the voltage of the first terminal of the
second flying capacitor to have a value that avoids noise spikes on
the output conductor due to charge redistribution when the first
terminal of the second flying capacitor is coupled to the output
conductor, and selectively coupling the first terminal of the third
flying capacitor to the first voltage during a third recharging
phase and selectively coupling the second terminal of the third
flying capacitor to the second voltage during the third recharging
phase.
19. A three-phase charge pump circuit for producing a low noise
output voltage on an output conductor, comprising: (a) first means
for selectively coupling a first terminal of a first flying
capacitor to a first voltage during a first recharging phase and
selectively coupling a second terminal of the first flying
capacitor to a second voltage during the first recharging phase;
(b) second means for coupling the second terminal of the first
flying capacitor to a precharge control circuit during a first
parasitic capacitance precharging phase that occurs after the first
precharging phase to cause the voltage of the first terminal of the
first flying capacitor to have a value that avoids noise spikes on
the output conductor due to charge redistribution when the first
terminal of the first flying capacitor is coupled to the output
conductor; and (c) third means for coupling the first terminal of
the first flying capacitor to the output conductor during a first
discharging phase that occurs after the first parasitic capacitance
precharging phase and coupling the second terminal of the first
flying capacitor to a discharge control circuit which increases the
voltage of the second terminal of the first flying capacitor during
the first discharging phase until the output voltage is equal to a
regulated value.
20. The three-phase charge pump circuit of claim 19 including means
for performing, during the coupling performed by the first means,
the function of coupling a first terminal of a second flying
capacitor to the output conductor during a second discharging phase
and coupling a second terminal of the second flying capacitor to
the discharge control circuit to increase the voltage of the second
terminal of the second flying capacitor during the second
discharging phase until the output voltage is equal to the
regulated value, and coupling a second terminal of a third flying
capacitor to a precharge control circuit during a third parasitic
capacitance precharging phase to cause the voltage of the first
terminal of the third flying capacitor to have a value that avoids
noise spikes on the output conductor due to charge redistribution
when the first terminal of the third flying capacitor is coupled to
the output conductor; means for performing, during the coupling
performed by the second means, the function of selectively coupling
a first terminal of the second flying capacitor to the first
voltage during a second recharging phase and selectively coupling
the second terminal of the second flying capacitor to the second
voltage during the second recharging phase, and coupling a first
terminal of the third flying capacitor to the output conductor
during a third discharging phase and coupling the second terminal
of the third flying capacitor to the discharge control circuit to
increase the voltage of the first terminal of the third flying
capacitor during the third discharging phase until the output
voltage is equal to the regulated value; and means for performing,
during the coupling performed by the third means, the function of
coupling the second terminal of the second flying capacitor to the
precharge control circuit during a second parasitic capacitance
precharging phase to cause the voltage of the first terminal of the
second flying capacitor to have a value that avoids noise spikes on
the output conductor due to charge redistribution when the first
terminal of the second flying capacitor is coupled to the output
conductor, and selectively coupling the first terminal of a third
flying capacitor to the first voltage during a third recharging
phase and selectively coupling the second terminal of the third
flying capacitor to the second voltage during the third recharging
phase.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to charge pumps, and
more particularly to an improvement which provides substantially
reduced output noise compared to prior charge pumps.
[0002] On-chip generation of an internal supply voltage at a value
greater than the power supply voltage rail VCC has been one
approach to providing rail-to-rail operation of an operational
amplifier. However, generation of such an internal supply by means
of a charge pump circuit has been problematic due to the large
amount of output noise (at the charge-pump clock frequency)
produced by known charge pumps.
[0003] A standard charge pump circuit is a two-phase circuit
including two "flying capacitors" and one "reservoir capacitor"
which operate to store and maintain the output voltage of the
charge pump circuit. FIG. 1A shows a standard charge pump circuit
1, which includes an amplifying circuit 2 having an output 3
connected to a control terminal of a controlled current source 4.
Controlled current source 4 produces a current 10. The (-) input of
amplifier 2 is connected to VCC. The (+) input of amplifier 2 is
connected to the (-) terminal of a voltage source circuit 1 1, the
(+) terminal of which is connected to a conductor 10 which conducts
the output signal Vout produced by prior art charge pump 1. The
upper terminal of controlled current source 4 is connected to VCC,
and its lower terminal is connected to conductor 5. (It should be
appreciated that amplifier 2 and controlled current source 4 can be
implemented in various ways. For example, amplifier 2 can be
implemented by means of a single P-channel transistor having its
gate and source connected to the (-) input and (+input),
respectively of amplifier 2, and its drain connected to the output
3 of amplifier 2. The implementation of amplifier 2 can be simple
or moderately complex, depending on how accurate and/or fast it
needs to be. Controlled current source 4 can be implemented by
means of a single P-channel transistor having its source connected
to VCC, its gate connected to the output 3, and its drain connected
to conductor 5. Voltage source circuit 11 can be implemented by
means of one or more diode-connected transistors and associated
circuitry to achieve a desired voltage drop.)
[0004] Charge pump circuit 1 includes a first "flying" capacitor C1
having its upper plate connected by conductor 8 to one terminal of
a switch 9 that controllably connects conductor 8 to either VCC or
Vout conductor 10. The lower plate of capacitor C1 is connected by
conductor 7 to one terminal of a switch 6 that controllably
connects conductor 7 to either ground or conductor 5 of controlled
current source 4. Charge pump circuit 1 also includes a second
flying capacitor C2 having its upper plate connected by conductor
17 to one terminal of a switch 20 that controllably connects
conductor 17 to either VCC or Vout conductor 10. The lower plate of
capacitor C2 is connected by conductor 16 to one terminal of a
switch 15 that controllably connects conductor 16 to either ground
or conductor 5 of controlled current source 4. The lower plates of
capacitors C1 and C2 are connected by conductors 7 and 16 to
parasitic capacitors C1p and C2p, respectively. A relatively large
"reservoir" or "output" capacitor Cout is connected between Vout
conductor 10 and VCC. A load 13 is connected between Vout conductor
10 and ground.
[0005] Each of the two flying capacitors has a recharge phase or
"phase 1" (PH1) for charging a flying capacitor to VCC, and also
has a subsequent discharge phase or "phase 2" (PH2) for discharging
it through Vout conductor 10 into reservoir capacitor Cout or load
13. Discharge through controlled current source 4 is controlled to
achieve regulation of Vout.
[0006] A drawback of prior art two-phase charge pump circuit 1 is
that it has a fast, noise-producing transient process between its
above mentioned first and second phases, during which the top plate
of one of the flying capacitors is connected to reservoir capacitor
Cout at the same time the voltage across the associated parasitic
capacitor connected between the bottom plate of that flying
capacitor and ground (i.e., the integrated circuit substrate) is
still at 0 volts. This causes partial charge redistribution from
the reservoir capacitor to the parasitic capacitor thereby
producing negative voltage spikes on Vout conductor 10 which
constitute a large amount of undesirable noise in the output
voltage signal, as illustrated with respect to subsequently
described FIG. 2.
[0007] Referring to FIG. 1A and the "Switches 6 & 9" and
"Switches 15 & 20" waveforms of FIG. 2, switches 6 and 9 are
closed during phase 1 of flying capacitor C1 to charge it up to
VCC, and alternately, preferably with a 50% duty cycle, switches 15
and 20 are closed to charge C2 up to VCC in such a manner as to
effectively maintain Vout at its desired voltage level while
supplying whatever current is needed by reservoir capacitor Cout
and load 13. The value of the desired regulated voltage level is
established by the voltage drop across voltage source circuit 11.
Thus, half of the time each flying capacitor is being recharged by
being connected between VCC and ground while the other flying
capacitor is being controllably discharged into Vout conductor 10
to supply to reservoir capacitor Cout and load 13 whatever amount
of current is needed maintain a regulated Vout at its desired
voltage. The roles of the two flying capacitors are reversed the
other half of the time. A switch control circuit 18 is coupled to
control terminals of switches 6, 9, 15 and 20 to control their
operation as described herein.
[0008] The configurations of switches 6, 9, 15 and 20 are
illustrated in FIGS. 1A and 1B for the first half ("Interval 1")
and the second half ("Interval of 2"), respectively, of each cycle
of operation of charge pump 1. Specifically, in FIG. 1A, capacitor
C1 is in its phase 1 (PHI), with upper switch 9 connected to VCC
and switch 6 connected to ground to recharge capacitor C1, and at
the same time capacitor C2 is in its phase 2 (PH2), with upper
switch 20 connected to Vout and lower switch 15 connected to
current source conductor 5 to cause capacitor C2 to be discharged
into Vout conductor 10. Similarly, in FIG. 1B capacitor C2 is in
its phase 1 (PH1), with upper switch 20 connected to VCC and switch
15 connected to ground to ground to recharge capacitor C2, and at
the same time capacitor C1 is in its phase 2 (PH2), with upper
switch 9 connected to Vout and lower switch 6 connected to current
source conductor 5 to cause capacitor C1 to be discharged into Vout
conductor 10.
[0009] Thus, as C1 is being recharged while it is connected between
VCC and ground, capacitor C2, which has just been charged up to VCC
volts, is being discharged into Vout conductor 10 by being
connected between the output of controlled current source 4 and
Vout conductor 10. At the instant when capacitor C2 is connected
between output conductor 10 and conductor 5, the connection to Vout
causes the voltage of top plate conductor 17 of capacitor C2 to
equal Vout, and the full charge voltage VCC across capacitor C2
causes the voltage of bottom plate conductor 16 to equal Vout-VCC.
Then controlled current source 4 begins supplying current 10
through conductor 5 to bottom plate conductor 16, charging up
parasitic capacitor C2p and increasing the voltage of bottom plate
conductor 16. This also increases the voltage of top plate
conductor 17 of capacitor C2 and causes capacitor C2 to discharge
through top plate conductor 17 into output conductor 10. Thus, the
top plate conductor 17 goes to Vout and the bottom plate conductor
16 goes to Vout-VCC volts. As the current 10 continues to be
supplied to bottom plate conductor 16 and increase its voltage, top
plate conductor 17 remains at Vout, causing capacitor Cout to
discharge a current equal to 10 into output conductor 10. More
specifically, amplifier 2 together with controlled current source 4
form a feedback loop which keeps Vout constant (as much as the loop
gain allows) and the amount of current 10 is determined by the load
current required by load 13 and reservoir capacitor Cout plus some
energy loss in the parasitic capacitive dividers.
[0010] Amplifier 2 continues to control current source 4 in
response to Vout so as to properly regulate Vout, and at the same
time, switch control circuit 18 operates according to a suitable
50% duty cycle such that just before the voltage on bottom plate
conductor 16 reaches VCC or just before controlled current source 4
saturates, switch control circuit 18 reverses the roles of flying
capacitors C1 and C2 so a freshly recharged flying capacitor is
available to supply the needed current to output conductor 10. FIG.
1B shows the configuration of switches 9A and 6A and switches 20A
and 1 5A during Interval 2 immediately after the roles of
capacitors C1 and C2 have been reversed.
[0011] The connection of either one of the flying capacitors, for
example capacitor C1, between Vout and conductor 5 causes the
above-mentioned noise on Vout conductor 10. At the instant when
capacitor C1 is connected between output conductor 10 and
controlled current source conductor 5, a capacitive divider circuit
is formed which includes parasitic capacitor C1p parasitic and
reservoir capacitor Cout. Therefore, some of the charge of
reservoir capacitor Cout is redistributed to parasitic capacitance
C1p, in accordance with the ratio between them and parasitic
capacitance C1p "discharges" or partially discharges reservoir
capacitor Cout. This causes a fast negative-going spike in Vout,
which constitutes the noise above mentioned noise. Current source 4
then operates to increase Vout from the bottom of that
negative-going spike back up to its proper regulated level.
[0012] Such negative-going noise spikes occur every time the roles
of flying capacitors C1 and C2 are reversed, i.e., the noise occurs
at the clock frequency of charge pump 1, as shown in the Vout
waveform of FIG. 2. The approximate typical amplitude of the
negative noise spikes can be calculated from Vout-Vcc (which can be
about 1.5-2.0 volts) and the ratio between reservoir capacitor Cout
and the parasitic capacitance C1p or C2p. The reservoir capacitance
Cout may be 3 to 5 times the capacitance of the flying capacitors,
and the capacitance of a flying capacitor can be roughly 4 to 10
times the associated parasitic capacitance. This can typically
result in negative noise spikes of roughly 70 millivolts.
[0013] Thus, there is an unmet need for an improved charge pump
circuit having substantially reduced output noise.
SUMMARY OF THE INVENTION
[0014] It is an object of the invention to provide an improved
charge pump circuit having substantially reduced output noise.
[0015] It is another object of the invention to provide an improved
charge pump without having to provide a large reservoir capacitor
connected to the charge pump output conductor.
[0016] Briefly described, and in accordance with one embodiment,
the present invention provides a low noise charge pump circuit that
includes a first terminal (8) of a first flying capacitor (C1)
selectively coupled to a first voltage (VCC) during a first
recharging phase and a second terminal (7) of the first flying
capacitor (C1) selectively coupled to a second voltage (GND) during
the first recharging phase. The second terminal (7) of the first
flying capacitor (C1) is coupled to a precharge control circuit
(25,27) during a first parasitic capacitance precharging phase that
occurs after the first recharging phase to cause the voltage of the
first terminal (8) of the first flying capacitor (C1) to have a
value that avoids noise lights on the output conductor due to
charge redistribution when the first terminal (8) of the first
flying capacitor (C1) is coupled to the output conductor (10). The
first terminal (8) of the first flying capacitor (C1) is coupled to
an output conductor (10) conducting the output voltage (Vout)
during a first discharging phase that occurs after the first
parasitic capacitance precharging phase. The second terminal (7) of
the first flying capacitor (C1) is coupled to a discharge control
circuit (2,4) which increases the voltage of the second terminal
(7) of the first flying capacitor (C1) during the first discharging
phase until the output voltage (Vout) is equal to a regulated
value.
[0017] In a described embodiment, a three-phase charge pump circuit
for producing a low noise output voltage (Vout) on an output
conductor (10) includes a first flying capacitor (C1), a first
amplifier circuit (2) having an output (3) coupled to control a
first current source (4) to produce a first controlled current (10)
in a first conductor (5) in response to the output voltage (Vout),
a first input coupled to a first supply voltage (VCC), and a second
input coupled to the output conductor (10). A second amplifier
circuit (25) has an output (26) coupled to control a second current
source (27) to produce a second controlled current (I3) in a second
conductor (30) in response to a precharge voltage (Vprecharge), a
first input coupled to the first supply voltage (VCC), and a second
input coupled to receive the precharge voltage (Vprecharge). A
first switching circuit (9A) selectively couples a first terminal
(8) of the first flying capacitor (C1) to the first supply voltage
(VCC) during a first recharging phase and to the output voltage
(Vout) during a first discharging phase. A second switching circuit
(6A) selectively couples a second terminal (7) of the first flying
capacitor (C1) to a second supply voltage (GND) during the first
recharging phase and to the first conductor (5) during the first
discharging phase. The second switching circuit (6A) couples the
second terminal (7) of the first flying capacitor (C1) to the
second conductor (30) during a first parasitic capacitance
precharging phase that occurs between the first recharging phase
and the first discharging phase so as to cause the voltage of the
first terminal (8) of the first flying capacitor (C1) to have a
value that avoids noise spikes on the output conductor (10) due to
charge redistribution when the first terminal (8) of the first
flying capacitor (C1) is coupled to the output conductor (10). The
three-phase charge pump circuit of claim 1 may include a reservoir
capacitor (Cout) coupled to the output conductor (10).
[0018] In the described embodiments, the three-phase charge pump
circuit includes a second flying capacitor (C2), a third switching
circuit (20A) for selectively coupling a first terminal (17) of the
second flying capacitor (C2) to the first supply voltage (VCC)
during a second recharging phase and to the output voltage (Vout)
during a second discharging phase, a fourth switching circuit (15A)
for selectively coupling a second terminal (16) of the second
flying capacitor (C2) to the second supply voltage (GND) during the
second recharging phase and to the first conductor (5) during the
second discharging phase. A fourth switching circuit (20A) couples
the second terminal (16) of the second flying capacitor (C2) to the
second conductor (30) during a second parasitic capacitance
precharging phase that occurs between the second recharging phase
and the second discharging phase so as to cause the voltage of the
first terminal (17) of the second flying capacitor (C2) to have a
value that avoids noise spikes on the output conductor (10) due to
charge redistribution when the first terminal (17) of the second
flying capacitor (C2) is coupled to the output conductor (10). The
three-phase charge pump also includes a third flying capacitor
(C3), a fifth switching circuit (36) for selectively coupling a
first terminal (34) of the third flying capacitor (C3) to the first
supply voltage (VCC) during the third recharging phase and to the
output voltage (Vout) during a third discharging phase, a sixth
switching circuit (31) for selectively coupling a second terminal
(33) of the third flying capacitor (C3) to the second supply
voltage (GND) during the third recharging phase and to the first
conductor (5) during the third discharging phase. The sixth
switching circuit (31) couples the second terminal (33) of the
third flying capacitor (C3) to the second conductor (30) during a
third parasitic capacitance precharging phase that occurs between
the third recharging phase and the third discharging phase so as to
cause the voltage of the first terminal (34) of the third flying
capacitor (C3) to have a value that avoids noise spikes on the
output conductor (10) due to charge redistribution when the first
terminal (34) of the third flying capacitor (C3) is coupled to the
output conductor (10).
[0019] In one described embodiment, the first switching circuit
(9A) couples the first terminal (8) of the first flying capacitor
(C1) to a third conductor (40) conducting the precharge voltage
(Vprecharge) during the first parasitic capacitance precharging
phase. In another described embodiment, the first switching circuit
(9A) couples the first terminal (8) of the first flying capacitor
(C I) to an electrically floating conductor (40 of FIG. 6) during
the first parasitic capacitance precharging phase. The first
amplifying circuit (2) includes a voltage source circuit (11) which
determines a regulated value of the output voltage (Vout).
[0020] In the described embodiments, a first terminal (17) of a
second flying capacitor (C2) is coupled to the output conductor
(10) during a second discharging phase and a second terminal (16)
of the second flying capacitor (C2) coupled to the discharge
control circuit (2,4) to increase the voltage of the second
terminal (17) of the second flying capacitor (C2) during the second
discharging phase until the output voltage (Vout) is equal to the
regulated value. A second terminal (33) of a third flying capacitor
(C3) is coupled to a precharge control circuit (25,27) during a
third parasitic capacitance precharging phase to cause the voltage
of the first terminal (34) of the third flying capacitor (C3) to
have a value that avoids noise spikes on the output conductor (10)
due to charge redistribution when the first terminal (34) of the
third flying capacitor (C3) is coupled to the output conductor
(10). The first terminal (17) of the second flying capacitor (C2)
is coupled to the first voltage (VCC) during a second recharging
phase and the second terminal (16) of the second flying capacitor
(C2) is selectively coupled to the second voltage (GND) during the
second recharging phase. The first terminal (34) of the third
flying capacitor (C3) is coupled to the output conductor (10)
during the third discharging phase and the second terminal (33) of
the third flying capacitor (C3) is coupled to the discharge control
circuit (2,4) to increase the voltage of the second terminal (34)
of the third flying capacitor (C3) during the third discharging
phase until the output voltage (Vout) is equal to the regulated
value. The second terminal (16) of the second flying capacitor (C2)
is coupled to the precharge control circuit (25,27) during a second
parasitic capacitance precharging phase to cause the voltage of the
first terminal (17) of the second flying capacitor (C2) to have a
value that avoids noise spikes on the output conductor (10) due to
charge redistribution when the first terminal (17) of the second
flying capacitor (C2) is coupled to the output conductor (10). The
first terminal (34) of a third flying capacitor (C3) is selectively
coupled to the first voltage (VCC) during a third recharging phase
and the second terminal (33) of the third flying capacitor (C3) is
coupled to the second voltage (GND) during the third recharging
phase.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1A is a schematic diagram of a prior art charge pump
with its switches in a first configuration.
[0022] FIG. 1B is a diagram of the charge pump of FIG. 1A with its
switches in a second configuration.
[0023] FIG. 2 is a diagram including several waveforms useful in
explaining the operation of the prior art charge pump of FIG.
1A.
[0024] FIG. 3A is a schematic diagram of a charge pump of the
present invention with a feed forward implementation of its
precharging circuitry, with its switches in a first
configuration.
[0025] FIG. 3B is a schematic diagram of the charge pump of FIG. 3A
with its switches in a second configuration.
[0026] FIG. 3C is a schematic diagram of the charge pump of FIG. 3A
with its switches in a third configuration.
[0027] FIG. 4 is a diagram including several waveforms useful in
explaining the operation of the charge pump of FIG. 3A so as to
greatly reduce the amount of noise generated on Vout conductor
10.
[0028] FIG. 5 is a schematic diagram illustrating a simplified
implementation of amplifier 2 and controlled current source 4 of
FIG. 3A.
[0029] FIG. 6 is a schematic of a charge pump of the present
invention with a feed-forward implementation of its precharging
circuitry.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] The method and structure of the present invention are
utilized to provide a very low noise charge pump circuit. This is
accomplished by providing a three-phase charge pump circuit 100
shown in FIG. 3A having three flying capacitors, each having three
phases of operation. An additional "phase 3" (PH3) of operation of
each flying capacitor and associated circuitry occurs between the
previously described "phase 1" (PH1) and "phase 2" (PH2)operations
of previously described prior art charge pump 1 of FIG. 1A.
[0031] FIG. 3A shows a charge pump circuit 100, which includes an
amplifying circuit 2 having an output 3 connected to a control
terminal of a controlled current source 4. Controlled current
source 4 produces a controlled current 10, wherein the amount of
controlled current 10 is determined by load demand of reservoir
capacitor Cout and load 13 and also by the feedback loop being
controlled through operation of amplifier 2. The (-) input of
amplifier 2 is connected to VCC. The (+) input of amplifier 2 is
connected to the (-) terminal of a voltage source circuit 11, the
(+) terminal of which is connected to a conductor 10 which conducts
the output signal Vout produced by charge pump 100. The upper
terminal of controlled current source 4 is connected to VCC, and
its lower terminal is connected to conductor 5. Amplifier 2 and
controlled current source 4 can be implemented in various ways,
from simple to moderately complex, depending on how accurate and/or
fast amplifier 2 needs to be. A simple implementation is shown in
FIG. 7.
[0032] Charge pump circuit 100 also includes an additional
amplifying circuit 25 having an output 26 connected to a control
terminal of a controlled current source 27. Controlled current
source 27 produces a current 13. The (-) input of amplifier 25 is
connected to VCC. The (+) input of amplifier 25 is connected to the
(-) terminal of a voltage source circuit 11A, the (+) terminal of
which is connected to a conductor 40 which conducts the precharge
signal Vprecharge.
[0033] Charge pump circuit 100 includes a first flying capacitor C1
having its upper plate connected by conductor 8 to one terminal of
a switch 9A that selectively connects conductor 8 to one of VCC,
Vout conductor 10, or Vprecharge conductor 40. The lower plate of
flying capacitor C1 is connected by conductor 7 to one terminal of
a switch 6A that selectively connects conductor 7 to one of ground,
conductor 5 of controlled current source 4, or conductor 30 of
controlled current source 27.
[0034] Charge pump circuit 100 also includes a second flying
capacitor C2 having its upper plate connected by conductor 17 to
one terminal of a switch 20A that selectively connects conductor 17
to one of VCC, Vout conductor 10, or Vprecharge conductor 40. The
lower plate of flying capacitor C2 is connected by conductor 16A to
one terminal of a switch 15 that controllably connects conductor 16
to one of ground, conductor 5 of controlled current source 4, or
conductor 30 of controlled current source 27.
[0035] Charge pump circuit 100 also includes a third flying
capacitor C3 having its upper plate connected by conductor 34 to
one terminal of a switch 36 that controllably connects conductor 34
to one of VCC, Vout conductor 10, or Vprecharge conductor 40. The
lower plate of flying capacitor C3 is connected by conductor 33 to
one terminal of a switch 31 that controllably connects conductor 33
to one of ground, conductor 5 of controlled current source 4, or
conductor 30 of controlled current source 27.
[0036] It should be appreciated that during the recharge phases PH
1, when the upper plate and lower plate of the flying capacitor are
coupled to VCC and ground, respectively, by the upper switches and
lower switches, noise injection into the VCC and ground power
supply rails can be reduced or minimized by providing current
limiting devices between the VCC rail and the upper switch (e.g.,
switch 9A) and/or between the GND rail and the lower switch (e.g.,
switch 6A), because the power supply impedance is never as low as
zero. FIG. 3A illustrates such current limiting devices 24
connected between the VCC supply rail and the VCC terminal of each
of upper switches 9A, 20A, and 36, and also illustrates current
limiting devices 23 connected between the GND supply rail and the
GND terminal of each of lower switches 6A, 1 5A, and 31. The
current limiting devices 23 and 24 can be low value resistors or
controlled current sources.
[0037] The lower plates of flying capacitors C1, C2, and C3 are
connected by conductors 7, 16 and 33 to parasitic capacitors C1p,
C2p, and C3p, respectively. A relatively large reservoir capacitor
Cout can be connected between Vout conductor 10 and VCC. A load 13
is connected between Vout conductor 10 and ground.
[0038] It should be understood that in some cases 3-phase charge
pump 100 can operate without a large reservoir capacitance Cout,
because of the much smoother, noise-free nature of the output
signal Vout being produced on conductor 10.
[0039] Each of the three flying capacitors C1, C2, and a C3 has a
charging "phase 1" (PH1) for charging that particular flying
capacitor to VCC, followed by a precharging "phase 3" (PH3) and a
subsequent discharge "phase 2" (PH2) for discharging it through
Vout conductor 10 into reservoir capacitor Cout and load 13. During
the precharging phase 3, the bottom plate parasitic capacitor of
the particular flying capacitor which has just been recharged up to
VCC volts then is pre-charged so that the voltage on its top plate
is equal to Vout immediately before it is directly connected to
Vout.
[0040] This substantially eliminates the previously mentioned
charge redistribution and the resulting negative-going noise spikes
which occur at the output of the prior art 2 phase charge pump 1 of
FIG. 1A. Discharge through controlled current source 4 is
controlled as previously explained to achieve regulation of Vout.
Discharge through controlled current source 27 is controlled
similarly to achieve proper precharging of parasitic capacitors
C1p, C2p, and C3p so as to greatly reduce output noise.
[0041] A suitable switch control circuit 18A is coupled to control
terminals of switches 6A, 9A, 15A, 20A, 31, and 33 to control the
operation of the switches in the manner described herein and as
illustrated in FIGS. 3A-C and in the waveforms of FIG. 4.
[0042] The phase sequence for one full cycle (including Interval 1
followed by Interval 2 followed by Interval 3) for flying capacitor
C1 is PH1-PH3-PH2 is illustrated for capacitor C1 in FIGS. 3A-B.
Similarly, the phase sequences for the same full cycle for the
flying capacitors C2 and C3 are PH2-PH1-PH3 and PH3-PH2-PH1,
respectively. These sequences of phases are illustrated for each of
flying capacitors C1, C2 and C3 in FIGS. 3A-C, and the waveforms of
FIG. 4.
[0043] Specifically, for capacitor C 1, switches 6A and 9A are
connected to ground and VCC, respectively, as shown in FIG. 3A to
provide phase PH 1, switches 6A and 9A are connected to precharge
current source conductor 30 and Vprecharge conductor 40,
respectively, as shown in FIG. 3B to provide phase PH3, and
switches 6A and 9A are connected to current source conductor 5 and
Vout conductor 10, respectively, as shown in FIG. 3C to provide
phase PH2. The various arrowheads shown in FIGS. 3A-C illustrate
the paths of the current flow for each of the three phases
PH1-3.
[0044] Similarly, for capacitor C2, switches 15A and 20A are
connected to current source conductor 5 and Vout conductor 10,
respectively, as shown in FIG. 3A to provide phase PH2, switches
15A and 20A are connected to ground and VCC, respectively, as shown
in FIG. 3B to provide phase PH1, and switches 15A and 20A are
connected to precharge current source conductor 30 and Vprecharge
conductor 40, respectively, as shown in FIG. 3C to provide phase
PH3.
[0045] Finally, for capacitor C3, switches 31 and 36 are connected
to precharge current source conductor 30 and Vprecharge conductor
40, respectively, as shown in FIG. 3A to provide phase PH3,
switches 31 and 36 are connected to current source conductor 5 and
Vout conductor 10, respectively, as shown in FIG. 3B to provide
phase PH2, and switches 31 and 36 are connected to ground and VCC,
respectively, as shown in FIG. 3C to provide phase PH1.
[0046] In order to greatly reduce the noise generated by prior art
charge pump 1 of FIG. 1A associated with the parasitic capacitances
C1p and C2p as a result of the above described charge
redistribution between the reservoir capacitor Cout and the
above-mentioned parasitic capacitors, the third flying capacitor C3
and third phase PH3 are provided in the process of operating the
flying capacitors C1, C2 and C3.
[0047] The third phase PH3 is dedicated to equalizing a top plate
potential of each flying capacitor before it is actually connected
to Vout and reservoir capacitor Cout. According to the present
invention, the particular flying capacitor is not connected to Vout
immediately after being precharged to VCC volts. Instead, the
additional phase PH3 is provided during which the bottom plate
parasitic capacitance of that particular flying capacitor is
pre-charged such that the top plate of that particular flying
capacitor is equal to Vout before being connected directly to Vout,
in such a way as to avoid any appreciable charging or discharging
of that flying capacitor. Then there is no redistribution of the
charge of reservoir capacitor Cout when the upper plate of a flying
capacitor is connected to it, because the charge which otherwise
would be redistributed onto the parasitic bottom plate capacitance
has already been placed on it by the operation of amplifier 25 and
current source 27 through conductor 30 and switch 6A, 15A, or 31
without any charging or discharging of the flying capacitor.
[0048] The Vout waveform of FIG. 4 illustrates how the precharging
process described above avoids the large negative-going noise
signals produced by the prior art charge pumps.
[0049] The above described precharging of the parasitic capacitors
as illustrated in FIGS. 3A-C to eliminate the negative noise spikes
of the prior art is accomplished by feedback wherein operational
amplifier 27 is the same as operational amplifier 2, charging the
bottom plate of C1, C2 or C3, depending on which of the three
phases a particular flying capacitor is undergoing, and turns
current source 27 off when Vprecharge is equal to Vout.
[0050] A "feed forward" approach to precharging the parasitic
capacitances associated with the lower plates of the flying
capacitors to cause the voltages of the upper plates of the flying
capacitors to be essentially equal to Vout before directly
connecting them to Vout is shown in FIG. 6. In FIG. 6, the (+)
input of operational amplifier 27 is connected by means of
conductor 30 and one of lower switches 6A, 15A or 31 to the bottom
plate of one of the flying capacitors C1, C2 or C3 (rather than the
top plate thereof as in the feedback implementation of FIG. 3A) to
receive a voltage Vprecharge from that bottom plate. Conductor 40
is open-circuited, so that the upper plate of a flying capacitor
electrically floats during the precharging phases. Vout is known,
since it is established by circuit design parameters, and can be
equal, to VCC plus, for example, 1 volt. The precharge circuitry
including amplifier 25, voltage source 11A, and controlled current
source 27 in FIG. 6 operates to charge up the parasitic capacitance
C1p, C2p, or C3p to 1 volt (or other suitable voltage), which
causes the top plate of the corresponding flying capacitor to be at
precisely VCC+1 volt immediately before the top plate is connected
directly to Vout by means of one of the upper switches. The VCC+1
volt value is established by voltage source 11A. For this example,
the 1 volt value is the value of voltage source 11A and the VCC
value is the voltage across the flying capacitor. Note that this
feed forward technique does not require shunting of a minute amount
of current from the top plate of the flying capacitor in order for
voltage source 11A to function. This can be significant when
maximum efficiency (approaching 90-95% efficiency) and minimum
current loss in the charge pump is necessary in that every
micrompere of shunted current can be accounted for.
[0051] The described embodiments of the three-phase charge pump of
the present invention provide greatly reduced output noise compared
to the two-phase charge pumps of the prior art. The improvement can
reduce the output noise of 3-phase charge pump 100 by 1.5 or 2
orders of magnitude, which is quite significant.
[0052] FIG. 5 illustrates a very simple implementation of amplifier
2 and controlled current source 4 of FIG. 2A. Amplifier 2 can be
implemented by means of a single P-channel 2A, with its source
functioning as the (+) input of the amplifier and its gate
functioning as the (-) input of the amplifier. The source of
transistor 2A is connected by conductor 12 to the gate and a drain
of a P-channel diode-connected transistor 11B, which functions as
voltage source 11. The source of diode-connected transistor 11B is
connected to Vout conductor 10. The drain of transistor 2A is
connected to the output 3 of amplifier 2. A current source 22 is
connected between the drain of transistor 2A and ground or the
negative rail. Ordinarily, the current through current source 22
should be as low as possible because it constitutes an additional
load on the charge pump, but the current through current source 22
must be sufficiently large to meet the speed requirements of
amplifier 2. (The implementation of amplifier 2 can be simple or
somewhat complex, depending on how accurate and/or fast it needs to
be.) Controlled current source 4 also can be implemented by means
of a single P-channel transistor 4A having its source connected to
VCC, its gate connected to conductor 3, and its drain connected to
conductor 5.
[0053] While the invention has been described with reference to
several particular embodiments thereof, those skilled in the art
will be able to make various modifications to the described
embodiments of the invention without departing from its true spirit
and scope. It is intended that all elements or steps which are
insubstantially different from those recited in the claims but
perform substantially the same functions, respectively, in
substantially the same way to achieve the same result as what is
claimed are within the scope of the invention.
* * * * *