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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.

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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 external supplies V1and V2, T1clk2, and T2clk2
(active only when CLK2 is high) and T1clk4 and T2clk4 (active only when CLK4 is
high). As shown in Fig.5 CLK_DLY and CLK_DLY2 are delay signals
generated from an input clock signal CLK.external supplies V1and V2, T1clk2, and T2clk2
(active only when CLK2 is high) and T1clk4 and T2clk4 (active only when CLK4 is
high). As shown in Fig.5 CLK_DLY and CLK_DLY2 are delay signals
generated from an input clock signal CLK.

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