Download A Charge Pump Circuit by using Voltage

INTERNATIONAL JOURNAL OF DESIGN, ANALYSIS AND TOOLS FOR CIRCUITS AND SYSTEMS, VOL. 1, NO. 1, JUNE 2011
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A Charge Pump Circuit by using Voltage-Doubler
as Clock Scheme
Wen Chang Huang, Jin Chang Cheng, and Po Chih Liou
Abstract—A new charge pump circuit with a clock that shows
an increased clock voltage as its stage is increased is proposed in
the paper. The charge pump circuit utilizes the cross-connected
NMOS, voltage doubler, as a pumping stage. Each stage of the
voltage-doubler provides a pair of complementary clock voltages.
The clock voltage also increases as the stage of voltage doubler
is increased. It shows that a voltage up to 37.85V was obtained
after eight-stage’s pumping of the circuit, through the simulation
of HSpice under 0.35 um process with 2V of supply voltage and
clock voltage.
Index Terms—Charge pump, high voltage clock generator,
voltage doubler.
Fig. 1.
Four-stage Dickson charge pump.
I. I NTRODUCTION
HARGE pumps are the circuits that used to generate dc
voltages those are higher than the normal power supply
voltage or lower than the ground voltage of the chip. Charge
pumps have been used in the nonvolatile memories, such as
EEPROM or flash memories, to write or to erase the floatinggate devices [1, 2]. They can also be used in the low-supplyvoltage circuits and switched-capacitor systems that require
high voltage to drive the analog switches [3]. Analog circuitry
also requires efficient charge pump to augment the internal
voltage supplies in order to achieve the increased dynamic
range and simplify the design [4].
The charge pump circuit reported by Dickson had been
widely used for generating high voltages [5, 6] and in some
circuit application. The structure of the circuit makes use of
capacitors, which are interconnected by diodes and coupled
in parallel with two non-overlapping clocks. Diodes in the
Dickson circuit can be replaced by NMOS, which will result
a more practical implementation [7]. Fig. 1 shows a four-stage
Dickson charge pump circuit. However its performance is
limited due to the threshold voltage drop of the NMOS devices
and the reverse charge-sharing phenomenon. Moreover, for
high output generated voltages, the increase in the threshold
voltage due to the body effect can significantly reduce the
pumping efficiency.
In order to overcome the problems mentioned above in
the Dickson charge pump, a charge pump, called NCP-2 [8],
is reported which utilizes the charge transfer switches (MSi
transistors). Each of the MSi transistors is controlled by the
pass transistors MNi (nMOS) and MPi (pMOS). In that way
the charge transfer switches can be turned off completely when
required, preventing the reverse charge flow. Also they can be
C
W. C. Huang and P. C. Liou are with the Department of Electronic
Engineering, Kun Shan University, Tainan, Taiwan R.O.C.
J. C. Cheng is with the Department of Accounting and Information System,
Chang Jung Christian University, Tainan, Taiwan R.O.C.
turned more effectively by the high voltage generated in the
next stage. More complicated circuit scheme for charge pump
have been applied to increase the voltage gain, such as all
PMOS charge pump for low voltage operation [9], CMOS
charge pump [10], charge transfer switches in combination
with pumping the output stage clock of enhanced voltage
amplitude . In our previous studied, we proposed a voltagedoubler charge pump circuit (VDCP) by cascading multistages of voltage-doubler [11], and the charge pump circuit
by using multi-staged voltage-doubler as the clock scheme
(MVDCP) [12] as shown in Fig. 2. Both these two charge
pump circuits showed high efficiency of pumping voltage.
The basic structure of the MVDCP is based on a chained
of MOS-diode which combined with pumping capacitor. And
each stage of the capacitor is pumped by a serial of clock
voltage. The pumping clock of a multi-staged voltage-doubler
is designed to replace the traditional clock. The supply voltage
is constant at VDD . The input clock voltage of the first stage
is also constant at the same as the supply voltage, VDD . The
MVDCP shows high pumping efficiency while it also shows
the deficiency of large transistor counts.
At the aim of reducing the transistor counts, we present
another version of MVDCP which called MVDCP-2. The
clock, multi-staged voltage-doubler, shows the characteristic of
out of phase in its two outputs. So, each stage of the clock can
provide clock voltage for two stages of the transfer transistor.
The number of transistors of MVDCP-2 is greatly reduced as
it was compared with the MVDCP-1. The operation principle
and the simulation results will be discussed in the following
sections.
INTERNATIONAL JOURNAL OF DESIGN, ANALYSIS AND TOOLS FOR CIRCUITS AND SYSTEMS, VOL. 1, NO. 1, JUNE 2011
II. P ROPOSED N EW C HARGE P UMP C IRCUIT :
M ULTI - STAGED VOLTAGE -D OUBLER C HARGE P UMP
C IRCUIT-2 (MVDCP-2)
The proposed charge pump circuit is shown in Fig. 3. A
serial of MOS transfer diode with pumping capacitors are
used to transfer charge. The multi-staged voltage-doubler is
designed to be the pumping clock. Fig. 3 shows a fourstaged connection of the multi-staged voltage-doubled charge
pump circuit (MVDCP-2). Compared with the MVDCP-1,
each stage of the voltage-doubler provides two clock-voltages
for two stages of the MOS-diode. While the structure of
MVDCP-1, each of voltage doubler only providing one clockvoltage to one stage of MOS-diode. The operation of the
high-voltage clock generator and the concept of multi-staged
voltage doubler are discussed in section II-A. The simulation
results and discussion of the proposed MVDCP-2 is stated in
section II-B.
Table 1 shows a comparison of the number transistor,
pumping capacitor and load capacitor of the two circuits. Both
the two circuits are four stages of pumping. It shows the
number of transistor and capacitor of MVDCP-2 is greatly
reduced. The fewer number of transistors and capacitors of
MVDCP-2 will result in lesser area of chip size as it compared
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with MVDCP-1.
TABLE I
C OMPONENTS C OUNTS O F A F OUR S TAGES C HARGE P UMP C IRCUIT
Circuit
MVDVP-1
MVDCP-2
Number of components
NMOS/PMOS/Int. Cap./Load Cap.
22 / 9 / 12 / 1
12 / 5 / 8 / 1
Comments
One clock output
Two clock outputs
A. Multi-staged voltage-doubler clock generator
The unit cell of the multi-staged voltage-doubler clock
generator is a clock voltage doubler[8] as shown in Fig. 4.
In the circuit, the amplitude of input clock voltage, VCLK ,
is oscillated between 0 to VDD . The power supply voltage is
constant at the voltage value of VDD . As the clock is at high
voltage value (VDD ), the nMOSFET(M8) will be turn on and
the output voltage of the inverter will be discharged to 0V. As
the clock goes to low voltage value (0V), this will turn on the
pMOSFET (M7), because its gate voltage is 0V and its source
voltage is 2VDD . The source voltage (2VDD ) of the transistor
M7 will charge the output capacitor of the inverter. Eventually,
the output node of the inverter becomes 2VDD . So, the value
of Vout4 oscillated between 0V to 2VDD during the action of
clock. The left hand side of the circuit shows similar operation
principle with opposite polarity.
Fig. 2. The charge pump circuit by using multi-staged voltage-doubler as
the clock scheme (MVDCP-1)[12].
Fig. 4.
Fig. 3. The proposed charge pump circuit by using multi-staged voltagedoubler as clock scheme (MVDCP-2).
The circuit diagram of the clock voltage doubler.
There are two purposes of the produced output clock voltage
of each stage in the MVDCP-2 circuit. Firstly, it is to be the
pumping clock of the nMOS diode in the chained of charge
pump structure. Secondly, it provides clock scheme for the
following stage of clock voltage-doubler to produce higher
amplitude of clock voltage. The concept is realized by the
circuit of MVDCP-2, as shown in Fig. 3. Now we discussed
the multi-staged voltage doubler clock scheme first. In the
clock scheme, voltage doubler is connected stage by stage. As
the input clock goes from 0 to VDD , the left output of the
first stage will go from 0 to 2VDD , the right output of the
first stage will go from 2VDD to 0. The left output of the
second stage will go from 3VDD to 0 and the right output of
the second stage will go from 0 to 3VDD at steady state. By
suitable connection, the above four output clock voltages are
INTERNATIONAL JOURNAL OF DESIGN, ANALYSIS AND TOOLS FOR CIRCUITS AND SYSTEMS, VOL. 1, NO. 1, JUNE 2011
Fig. 5.
The pumping voltage of various stages of MVDCP-2, where
VDD =2V, VCLK =2V and f=25MHz in the circuit.
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Fig. 6.
The pumping voltages of a four-staged MVDCP-2 at various
simulation frequencies (@ VDD =2V, VCLK =2V).
used to be the pumping clock of the chained of the nMOS
diode. The receiving of the pumping voltage of capacitor C1 ,
C2 , C3 and C4 will be 0V, 2VDD , 0V and 3VDD , respectively
as the input clock is 0V. The receiving of the pumping voltage
of capacitor C1 , C2 , C3 and C4 will be 2VDD , 0V, 3VDD and
0V, respectively as the input clock is VDD . The circuit scheme
of MVDCP-2 shows smaller chip area as it compared with the
circuit scheme of MVDCP.
B. Simulation results of the multi-staged voltage-doubler
charge pump circuit (MVDCP-2)
A four-staged multi-staged voltage-doubled charge pump
circuit (MVDCP-2) is shown in Fig. 3. The simulation result of
the output voltage of each stage of the multi-staged voltagedoubler charge pump circuit (MVDCP-2) is shown in Fig.
5. Both the supply voltage VDD and the input clock voltage
VCLK are 2V, and the operation frequency is 25MHz. This
figure shows the output voltage vs. operation time of different
output stages. The output voltage can reach to 4.21V after one
stage of pumping. As the number of the stages is increased,
the output voltage is increased steadily. For an eight-staged
MVDCP-2, its output voltage can be pumped up to 37.85V.
Fig. 6 shows the pumping voltage of a four-staged MVDCP2 at various operating frequencies. The operating frequency
is varied from 25, 50, 100, 200 to 500MHz, respectively. A
higher output voltage is obtained at low frequency operation.
The operation frequency of 25, 50 and 100MHz of the circuit
can pump similar output voltage. The output voltage degraded
serious as the operation frequency increased to 200MHz. The
pumping capacitor can be charged to a more saturated situation
under low frequency operation. On the other hand, as the
operating frequency is increased the charging of the pumping
capacitor is reduced, so it resulted in a lower pumping voltage.
A comparison of the pumping voltage of various charge pump
circuits is discussed. The MVPCP-2 is compared with some
other charge pump circuits, the MVDCP-1, the VDCP, the
Dickson charge pump circuit [5] and NCP-2 [8] charge pump
circuit. The output voltage vs. number of stages of various
charge pump circuits, Dickson charge pump, NCP-2, VDCP,
Fig. 7. A comparison of the pumping voltage of MVDCP-2, MVDCP-1,
VDCP, NCP-2 and Dickson charge pump at different number of pumping
stages.
MVDCP-1 and MVDCP-2 is shown in Fig. 7. The results
are the simulation of HSpice under CMOS 0.35 um process
with VCLK =VDD =2V, the pump capacitor is 20pF, the load
capacitor is 10pF and at the operating frequency of 25MHz.
At each stage, the MVDCP-2 shows a lower output voltage
than that of MVDVP-1 while much higher than that of another
circuits. In the MVDCP-1, the pumping voltage is equal to
4.31V after one stage pumping, is equal to 31.09V after five
stages pumping and is equal to 40.37V after seven stages
pumping. The MVDCP-2 shows lower pumping voltages at
the same simulation condition. The pumping voltage is equal
to 4.21V after one stage pumping, is equal to 21.31V after
five stages pumping and is equal to 34.28V after seven stages
pumping. While, these results are much higher than that of the
VDCP, the Dickson charge pump and the NCP-2. In this figure
we also find that the Dickson charge pump and NCP-2 will
saturate after many stages of pumping while the MVDCP-2
did not show any tendency for saturation. The pumping voltage
can be increased more as the stages of the MVDCP-2 are
increased. Fig. 8 compares the simulated output voltages of the
Dickson, NCP-2, VDCP, and the new proposed charge pump
INTERNATIONAL JOURNAL OF DESIGN, ANALYSIS AND TOOLS FOR CIRCUITS AND SYSTEMS, VOL. 1, NO. 1, JUNE 2011
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Fig. 9. The output voltage vs. output current of the MVDCP-2, MVDCP-1,
VDCP, NCP-2 and Dickson charge pump.
Fig. 8. A comparison of the pumping voltage of MVDCP-1, MVDCP-2,
VDCP, NCP-2 and Dickson charge pump at different supply voltage.
ACKNOWLEDGMENT
circuits under different power supply voltages (VDD ) without
output current loading. The output voltages are obtained after
three stages of pumping of these circuits, respectively. The
output voltages of these charge pump circuits are degraded
when the power supply voltage is decreased. However, the new
proposed charge pump circuit still has higher output voltages
under the lower power supply voltage because the proposed
charge pump circuit has better pumping efficiency. Thus, the
proposed charge pump circuit is more suitable in low-voltage
processes than the prior designs. Fig. 9 shows the simulated
output voltages of the MVDCP-2, MVDCP-1, VDCP, NCP2 and Dickson charge pump under different output currents.
When the output current is increased, the output voltages of
these charge pump circuit are decreased. The output voltage
is 7.75V, 5.95V, 4.7V, 3.7V and 2.4V, respectively, at the
output current of 30 µA. The output voltages of the proposed
charge pump circuit with different output currents are much
higher than those of other charge pump circuits. Especially,
with the higher output current of 50 µA, the proposed charge
pump circuit still has the better pumping performance than
others. In addition, the high pumping clock voltage in the
new proposed charge pump circuit could fully turned on
to transfer the charges, but all MOSFET switches in the
Dickson charge pump circuit and NCP-2 are diode-connected
transistors, which have the threshold voltage drop problem.
Thus, the proposed charge pump circuit has better pumping
performance.
III. C ONCLUSION
The new version (MVDCP-2) of charge pump circuit by using multi-staged voltage-doubler as clock scheme is proposed.
Although it shows lower pumping efficiency than the previous
version (MVDCP-1) while the area of chip size is greatly
reduced. The MVDCP-2 still shows much higher pumping
voltage than another charge circuits. No saturation of the
pumping voltage is found as the pumping stage increased. It
also shows suitable application for low power supply system.
The authors would like to thank the National Science
Council of the Republic of China, for financially supporting
the research under Contract No. NSC-97-2221-E-168-004. The
simulation software was support by CIC.
R EFERENCES
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INTERNATIONAL JOURNAL OF DESIGN, ANALYSIS AND TOOLS FOR CIRCUITS AND SYSTEMS, VOL. 1, NO. 1, JUNE 2011
Wen-Chang Huang was born on February 16, 1967.
He received the B.S. degree in electronics engineering from Tamkang University 1989 and the M. S.and
Ph.D. degrees in electronic engineering from Chiao
Tung University, Hsinchu, Taiwan in 1991 and 1996,
respectively.
From 1998 to 2003, he was an Assistant Professor
at the Department of Electronic engineering, Ta Hwa
Institute of Technology, Hsin Chu, Taiwan. From
2003 to 2005, he was an Assistant Professor at the
Department of Electronic engineering, Kun Shan
University, Tainan, Taiwan. He is currently an Associate Professor at the
Department of Electronic engineering, Kun Shan University, Tainan, Taiwan.
His current research is in the areas of thin films material, semiconductor
device and VLSI design.
Jin-Chang Cheng received the Ph.D. degree from
the Department of Electrical Engineering, State
University of New York at Stony Brook in 1986.
He is now with the Department of Accounting
and Information System, Chang Jung Christian
University, Tainan, Taiwan, R.O.C.. His research
interests include digital system design, digital signal
processing and image processing.
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Po-Chih Liu received the MS. degree from the
Department of Electronic Engineering, Kun Shan
University in 2007. He is currently an IC layout
engineer, Synerchip Co. Ltd.