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conventional the influence of the VTH on the functionality

conventional
four-phase charge pump is presented. Based on multi-step charging and the
charge sharing concept, the charge pump is able to reduce the overall power
consumption by 18.27% compared to a conventional
four- phase charge pump and by compared to a charge sharing charge pump with 11V output voltage.

 Index
terms – Charge pump, EEPROM, NVMs

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I. INTRODUCTION

Charge
pump circuits charges upward to produce voltages higher than the regular supply
voltage 1. They are usually applied to the non volatile memories (NVMs), such
as Electrically erasable Programmable Read-only memory (EEPROM) or flash memory
to erase or to write the floating gate devices 2.

Classical
charge pumps are based on Cockcroft and Walton voltage multiplier 3. But, In
Cockcroft Walton multiplier ,chip monolithic integration is hard to reach.
Therefore, to overcome this limitation, Dickson voltage multiplier is used
which is based on a diode chain is proposed in 4.It has poor conversion

 

 

 

 

 

 

 

 

 

efficiency
because body bias effect 5 on threshold voltage (VTH).

Many
configurations have been developed to reduce the influence of the VTH on
the functionality of the charge pump by canceling the input of the body effect.

 

In
6, the charge pump using a four–phase clock and boost capacitors on the
switch transistor eliminates the influence of VTH. The circuit of a
conventional four-phase boosted charge pump is described in fig 1. CLK3 and CLK4
are two overlapped clock phases used to charge the pump capacitors. CLK2 and
CLK4 are two non-overlapped clocks used to boost the gate voltage of the charge
transfer device during charge transfer.

 

However,
all of these configurations do not completely optimize power consumption.
Several studies have been led to reduce the power consumption.

 In 7, the charge sharing technique applied
to a conventional four-phase charge pump to reduce the power consumption.

 

 

 

The
paper is organized as follows, Section II presents the charge sharing concept.
Section III presents the multi-step concept and its association with the charge
sharing concept. In Section IV, simulation results are presented .

 

II. CHARGE SHARING CHARGE PUMP

Capacitor
in each stage of the conventional four-phase charge pump circuit charged to VDD
during one-half clock cycle. After that, charge transfer is made with capacitor
of next stage finally at end of the process, remaining charges are lost because
discharging to the ground is applied. Energy needed to charge that capacitor
will be reduced because remaining charge are reuse to charge that capacitor in
the circuit. This technique is known as charge sharing or charge reuse. As
shown in fig 2, CLK2 and CLK4 are two non-overlapped clocks used to create the
two new clock signals Sh_CLK2 and Sh_CLK4 for charging and discharging in two
steps the pumping capacitor of each charge pump stage as shown in fig 3.

In
fig 2, Initially the capacitors C1 and C2 are discharged also their potential
is set to GND. When CLK2 is high and CLK4 is low, the pass gate is OFF and
capacitor C1 is charged to VDD whereas C2 remains grounded. After that, when
both CLK2 and CLK4 are low, Pass gate turns ON and Chg_Sh goes high. There is
charge sharing between C1 and C2. Finally when CLK4 goes high while CLK2 is
still low, the pass gate is OFF and C1 is discharged to GND and C2 is charged
to VDD. Now the similarity between the conventional four-phase charge pump and
the charge sharing charge pump is that they have same architecture but the only
difference is that charging sequence. In conventional charge pump, capacitor
charges in one step, but in charge sharing charge pump, capacitor charges in
two steps.

During
charge sharing operation, no loss of energy by the source. Only the step from
VDD/2 to VDD is realized by the voltage source and the energy supplied is
1/2C.VDD2 . Also the charging from VDD/2 to VDD is done using one
step.Therfore, charging is done by Mintermediate voltage steps,the total
supplied energy can be reduced. So, it is more efficient when multi-step charging
technique is combined with charge sharing technique.

III. MULTI STEP CHARGING

If
the voltage difference before and after charging a capacitor C from V0 to VDD
is then the energy delivered by the voltage source is C.V². However, the energy
stored by the capacitor C is ½.C.VDD². So there is half of the energy loss in
the switch resistance R. Using multistep charging is one of the best way to
save energy.

Using
M voltage steps for charging C to VDD, the total energy delivered by the
voltage source is given by M+1/2.M .C.V² and the total dissipated energy is
equal to1/M .C.V², so that the total energy saving, in percentage, is equal to
M-1/2.M.Therefore, the higher the number of steps the higher the energy saving.
Practically, it is difficult to design a circuit with a large number of voltage
steps because of all voltage sources needed and the corresponding control
switches leading to more power losses. For this reason, three steps are chosen
(M = 3). In this case, the total energy saving is 34% compared to one step charging.
In this context, if the three steps charging technique is applied to the charge
sharing concept, the resulting charge pump system will be more energy efficient
compared to the classical charge sharing charge pump.

 

 

The
circuit shown in Fig. 4 describes the generation of the two clocks used to
charge and discharge pump capacitors with three steps charging combined with charge
sharing technique.

Its operation is
similar to the circuit shown in Fig. 2 , but additional signals are required to
perform the three step charging. These are 

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