Design implementation and comparison of various cmos charge pumps

Proceedings of the 2nd International Conference on Current Trends in Engineering and Management ICCTEM -2014
INTERNATIONAL JOURNAL OF ELECTRONICS AND
17 – 19, July 2014, Mysore, Karnataka, India
COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)
ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Volume 5, Issue 8, August (2014), pp. 97-106
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DESIGN, IMPLEMENTATION AND COMPARISON OF VARIOUS CMOS
CHARGE PUMPS
Stuti Sharon1, Vani V2, Shobha B N3, Archana H R4
1Student, Dept. of ECE, SCE, Bangalore, India
2Assistant Professor, Dept. of ECE, SCE, Bangalore, India
3PG Coordinator, Dept of ECE, SCE, Bangalore, India
4Assitant Professor, Dept. of ECE, BMSCE, Bangalore, India
97
ABSTRACT
A charge pump is a kind of DC to DC converter that uses capacitors as energy storage
elements to create a higher or lower voltage power source. Charge pumps make use of switching
devices for controlling the connection of voltage to the capacitor. The use of charge transfer switches
(CTSs) can improve the voltage pumping gain. Applying dynamic control to the CTSs can reduce
reverse currents. This paper includes voltage and power analysis of various charge pump circuits.
And a comparison is drawn between the three charge pumps analyzed.
Keywords: Charge Pump, Charge Transfer Switch, DC-DC Converter, Dickson Charge Pump,
Dynamic Charge Pump, Static Charge Pump.
I. INTRODUCTION
Charge Pump (CP) is an electronic circuit that converts the supply voltage Vin to a DC output
voltage VOUT that is several times higher than Vin (i.e., it is a DC-DC converter whose input voltage
is lower than the output one). Unlike the other traditional DC-DC converters, which employ
inductors, CPs are only made of capacitors and switches (or diodes), thereby allowing integration on
silicon [1]–[2]. CPs were originally used in smart power ICs[3]–[6] and nonvolatile memories [7]–
[13] and, given the continuous scaling down of ICs power supplies, they have also been employed in
a vast variety of integrated systems such as switched capacitor circuits, operational amplifiers,
voltage regulators, SRAMs, LCD drivers, piezoelectric actuators, RF antenna switch controllers, etc.
[14]–[23]. Charge pumps usually operate at high frequency level in order to increase their output
power within a optimum size of total capacitance used for charge transfer. This operating frequency
is adjusted by compensating for changes in the power requirements and saving the energy delivered
to the charge pump.

Here in this paper we analyze and compare three kinds of charge pumps Dickson Charge
Pump, Charge Pump using Static CTSs and Charge Pump using Dynamic CTSs. This paper
describes new charge pumps that are suitable for low-voltage operation and offer better voltage
pumping gains and higher output voltages than the Dickson charge pump. The various types of
Charge pumps implemented and their performances are provided in sections II to VI. The charge
pump utilizing the CTSs is described in Section V. A 4 stage prototype implemented in standard,
180nm CMOS technology is demonstrated. The circuit can operate with a 1.5V supply and generate
a boosted output of 4.2 V. In Section VI new charge pumps employing the dynamic CTSs to increase
the voltage Pumping gain are described. The performance improvement is verified by simulation
results.
II. COCKCROFT WALTON CHARGE PUMP
The first widely used voltage boosting circuit was the Cockcroft-Walton voltage multiplier.
This circuit, shown in fig. 1, uses diodes and serially connected capacitors and can boost to several
times the supply voltage. The Cockcroft-Walton charge pump provides efficient multiplication only
if the coupling capacitors are much larger than the stray capacitance in the circuit, making it
undesirable for use in integrated circuits.
Fig.1: 4-Stage Cockcroft-Walton Charge Pump
98
III. DICKSON CHARGE PUMP
Most MOS charge pumps are based on the circuit proposed by Dickson [5]–[7]. As shown in
Fig. 2, the MOST’s in the Dickson charge pump function as diodes, so that the charges can be
pushed only in one direction. The two pumping clocks clk1 and clk2 are out-of-phase and have a
voltage amplitude of Vclk. The value of Vclk is usually identical to the supply voltage Vin. Through
the coupling capacitors C1–C4, the two clocks push the charge voltage upward through the
transistors. Neglecting the boundary conditions, the voltage fluctuation V at each pumping node is
identical and can be expressed as:
V = Vclk .{[C/(C+ Cs)] – {I o /[f(C+Cs)]}} (1)
where C is the capacitance of C1–C4, Cs is the parasitic capacitance associated with each
pumping node, f is the frequency of the pumping clocks, Io and is the output current loading.
When clk1 goes from low to high and clk2 from high to low, the voltage at node 1 is settled
to V1 + V , and the voltage at node 2 is settled to V2, where V1 and V2 are the steady-state lower
voltage at node1 and node2 respectively. Both M1 and M3 are reverse biased, and the charges are
being pushed from node 1 to node 2 through M2. The final voltage difference between node 1 and 2

is the threshold voltage of M2. The necessary condition for the charge pump to function is that V
must be larger than the MOST’s threshold voltage Vtn , i.e.:
V Vtn (2)
The voltage pumping gain for the second pumping stage Gv2 is defined as the voltage
difference between and, which can be expressed as (3),
Gv2 = V2 - V1 = V - Vtn (V2) (3)
Where Vtn(V2) is the threshold voltage of M2, modified by the body effect due to the source
voltage V2. The geometric dimension of the MOST’s has no effect on the voltage pumping gain.
However, if the W/L ratio of the MOST diodes is too small so that the transient response of the
pumping operation cannot settle within the period when the corresponding clock is high, then the
resulting voltage pumping gain will be smaller than that predicted by (3).
As the supply voltage decreases, both Vclk and V are decreased accordingly, and the voltage
pumping gain per stage is reduced. Furthermore, if V is not much larger than the MOST’s threshold
voltage, then the influence of the MOST’s body effect cannot be neglected. Especially at the high
voltage nodes near the output, the increase in the threshold voltage can lower the voltage pumping
gain significantly. In fact, the circuit’s output voltage reaches its maximum when the body effect
causes the threshold voltage to be equal to V [6]. It is possible to use floating-well devices to
eliminate the body effect [8]. However, the resulting charge pumps may generate substrate currents
and the voltage pumping gain per stage is still reduced by the threshold voltage.
Fig.2: 4-Stage Dickson Charge Pump
Fig.3: 4-Stage Schematic of Dickson Charge Pump
99

Fig.4. Output Waveform of 4-Stage Dickson Charge Pump
The drawback of the Dickson charge pump circuit is that the boosting ratio is degraded by the
threshold drops across the diodes. The body effect makes this problem even worse at higher voltages.
IV. CHARGE PUMPS (NCP 1) USING STATIC CTSS
Instead of using the diodes to direct the flow of charges in pumping operation, the MOST
switches with proper on/off cycles, referred to as CTSs, have been used to realize the charge pumps
and show better voltage pumping gain than the diodes [9]–[13]. Fig. 5 shows a new charge pump
(NCP-1) using the MOST CTSs with static backward control [3]. MD1–MD4 are diodes for setting
up the initial voltage at each pumping node. They are not involved in the pumping operation. MS1–
MS4 are the CTSs. The idea is using the already established high voltage to control the CTS of the
previous stage. If the switches can be turned on and turned off at the designated clock phases, they
can also allow the charge to be pushed in only one direction. Then for each pumping stage, the upper
voltage of the input is equal to the lower voltage of output, since the MOST switch is turned on at
this moment. The voltage pumping gain per stage can be expressed as:
Gv = Gv2 = V2 - V1 = V (4)
Comparing with (3), the NCP-1 is expected to have better charge pumping performance. In
Fig. 4, when clk1 is high and clk2 is low, the voltage at node 1 is pushed from V1 to V2 , the voltage
at node 2 is V2, the voltage at node 3 is V3 + V. For nominal operation, the MS2 switch must be
turned on by the voltage at node 3. The gate-to-source voltage of MS2 is 2V which must be larger
than the threshold voltage Vtn modified by the source voltage V2, i.e:
2V Vtn (V2) (5)
Comparing with (2), the NCP-1 is more suitable for low-voltage operation.
On the other hand, when is clk1 low and clk2 is high, the voltage at node 1 is V1, the voltage
at node 2 is V3, and the voltage at node 3 is also V3. For ideal operation, MS2 is to be turned off.
Therefore, the gate-to-source voltage of MS2, which is 2V, must be smaller than the threshold
voltage modified by the source voltage V1, i.e.:
2V Vtn (V1) (6)
100

Fig.5: 4-Stage Static Charge Pump
Fig.6: 4-Stage Schematic of Static Charge
Fig.7: Output Waveform of 4-Stage Static Charge Pump
Since (5) must always be true, the requirement of (6) can never be met. Therefore, MS2
cannot be completely turned off, and reverse charge sharing between node 2 and node 1 can occur. In
such cases, the operation of the charge pump becomes complicated. The voltage fluctuations at the
pumping nodes are different and smaller than that predicted by (1). As a result, the overall voltage
101

pumping gain is reduced. Note that the maximum voltage pumping gain between node 1 and node 3
is determined only by the threshold voltage of MS2, i.e.:
Max (Gv2 + Gv3) = Max(V3 - V1) = Vtn (V1) (7)
A four-stage NCP-1 is fabricated using standard 180nm CMOS technology. The prototype
is similar to the circuit shown in Fig. 5, except that MD5 and MDO are merged into one device, the
pumping capacitor C5 is connected to the source node of MDO, and the output voltage is smoothed
by an RC low-pass filter [3]. The threshold voltage of the nMOST’s is 0.7 V. The gate geometric
size is 1.4μm by 1μm for all devices. All pumping capacitors are 20 pF. The circuit is used in a 1.5V
switched-capacitor system for driving MOST analog switches. Fig. 7 shows the measured output
voltage of the prototype while a 1.5V supply is applied. The prototype can maintain a 2.7V output
voltage. For comparison, the output voltage of a five-stage Dickson charge pump operating under
identical condition is 2.2 V.
V. CHARGE PUMP (NCP 2) USING DYNAMIC CTSs
If the reverse charge sharing phenomenon inherent in the NCP-1 circuit can be eliminated,
better pumping performance can be obtained. In other charge pump designs [9]–[11], each CTS is
accompanied by an auxiliary pass transistor so that the CTSs can be turned off completely in the
designated period. However the CTSs in those charge pumps are difficult to turn on in the low
voltage environment. Techniques such as putting CTSs in individual wells to eliminate the body
effect and applying boosted clocks to drive the CTSs can be used [11]. The four-phase clock scheme
can be used to pre-charge the CTSs so that they become easier to turn on [9], [11]. However,
additional concern is that the auxiliary pass transistors must be turned on during the pre charging
phase.
Fig. 8 shows a new charge pump (NCP-2) that can assign the control inputs for the CTSs
dynamically by adding pass transistor NMOS and PMOS to the NCP-1 circuit [14]. The CTSs in
NCP-2 can be turned off completely when required and still can be turned on easily by the backward
control as in the NCP-1 case. The expression for the single-stage voltage pumping gain is the same
as (4).
The operation of the dynamically controlled CTSs is explained as follows. When clk1 is high
and clk2 is low, both the voltages at node 1 and node 2 are V2, and the voltage at node 3 is 2V
above. If:
2V Vtp and 2V Vtn(V2) (8)
where Vtp is the threshold voltage of pMOST’s, then MP2 is turned on, causing MS2 being
turned on by the voltage at node 3. In this period, MN2 is always off since its gate-to-source voltage
is zero.On the other hand, when is clk1 low and clk2 is high, the voltage at node 1 is V1, both the
voltages at node 2 and node3 are 2V above. If:
2V Vtn(V1) (9)
then MN2 can be turned on and MS2 can be turned off completely. In this period, MP2 is
also off, disengaging MS2 from the control of node 3. Unlike the NCP-1 case, the two necessary
conditions of (8) and (9) can be satisfied simultaneously. Note that, for each pMOST in Fig.8, each
individual well is connected to the device’s drain node. During the short period of transition when
clk1 goes from high to low, it is possible for the charges at the CTSs gate node to be injected into the
well.
102

All coupling capacitors are 2pF. The frequency of the pumping clocks is 25MHz. The NCP-2
exhibits the best charge pumping performance among the three circuits. For the NCP-1 charge pump,
the effect of the reverse charge sharing become more apparent as the supply voltage increases. In
case of 1.5V supply voltage the NCP-1 performs no better than the Dickson charge pump. Fig. 10
shows the measured output voltage at 1.5Vsupply voltage is 4.1V. Under the same condition, the
output voltage is 2.2 V for the Dickson charge pump and is 2.7 V for the NCP-1.
Fig.8: 4-Stage Dynamic Charge Pump
Fig.9: 4-Stage Schematic of Dynamic Charge Pump
Fig.10: Output Waveform of 4-Stage Dynamic Charge Pump
103

104
VI. RESULT
Table 1: Output voltage
Dickson NCP-1 NCP-2
Vout 2.2V 2.7V 4.2V
Pd 17.66pW 34.32pW 38.79pW
Parameters:
1. Vin = 1.5V
2. Pumping Capacitors- 2pf
3. Technology used-180nm
4. Width of NMOS-1.4micron
5. Clock Frequency-25MHz
Fig11: Simulated output voltages of various charge pumps. Vin= 1.5V
VII. CONCLUSION
The NCP-2 exhibits the best charge pumping performance among the three circuits. For the
NCP-1 charge pump, the effect of the reverse charge sharing become more apparent as the supply
voltage increases. In case of 1.5V supply voltage the NCP-1 performs no better than the Dickson
charge pump.
Charge pumps utilizing CTSs to direct charge flow can provide better voltage pumping gain
than those using MOST diodes. Using the internal boosted voltage to backward control the CTS of
the previous stage yields charge pumps that are suitable for low-voltage operation. The resulting
charge pumps (NCP-1) can operate under a supply voltage below 1.2 V and still offer good pumping
performance.
The reverse charge sharing phenomenon inherent in the NCP-1 circuits can be eliminated by
applying dynamic control to the CTSs. The NCP-2 charge pumps use two additional MOST’s per
stage to implement the dynamic CTSs. The performance improvement of the NCP-2 over the NCP-1
is more significant at higher supply voltages.

105
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