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cross couple charge pumps, waveform inquiry? (Circuit and waveforms are attached)

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bio_man

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cross couple charge pump, waveform inquiry? (circuit and waveforms are attached)

Hi,

I am using 6 modules from cross-coupled charge pumps, CCCP, ( circuit attached), to generate >1.8V out of 0.5V and I am using 1pF for all caps including output and minimum transistor sizes, nMOS (1.5um/0.6um) and pMOS (1.5um/0.6um)
These CCCP are simulated twice, first case with body terminals of nMOS connected to zero (there will be body effect). Second case with Vsb=0 (no body effect). The simulation waveforms are attached.

My questions about the body effect in these waveforms? I was expecting to see much difference for Vout voltage in steady-state because of higher Vth for nMOS devices in the circuit, but it was not the case! the difference is close to 0.1V
The only difference i can see is that with body effect, case 1, the settling time for Vout is much much higher than case 2.
 

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Re: cross couple charge pump, waveform inquiry? (circuit and waveforms are attached)

with body effect, case 1, the settling time for Vout is much much higher than case 2.

Because for case 1 (nMOS Vsb>0) Vt_eff is higher than Vt in case 2 (Vsb=0). So its path resistance (for same Vgs) is higher --> greater RC time constant.
 
Re: cross couple charge pump, waveform inquiry? (circuit and waveforms are attached)

So, it means the Body effect ( Vsb!=0) will affect only the transient?

Previously, I thought the body effect will lower the output voltage because the Vgs in case 1 (Vsb!=0) is not high enough to let the nMOS conduct. That's why I was thinking to increase the number of modules to compensate for this voltage loss.
 

Re: cross couple charge pump, waveform inquiry? (circuit and waveforms are attached)

... I thought the body effect will lower the output voltage because the Vgs in case 1 (Vsb!=0) is not high enough to let the nMOS conduct.

It still conducts, but as the difference Vgs-Vt_eff is lower than Vgs-Vt , it will allow just a lower Id current, so it will take longer to load the caps. Probably, the final CP output voltage will also be a little bit lower, about n*(Vt_eff - Vt). I think, your above diagram shows this, too.
 
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