How to calculate the value of a capacitor to maintain VCCOUT

[snoop835]

Capacitor

Hi all,

As you can see in the attachment file, I tried to main the VCCOUT dc level when the input VCC droop to 0.5V and stay for few us and rise again to 1.8V. I used diode connected NMOS and capacitor so the VCCOUT = VCC - Vth. Can somebody tell me what is the equation to be used to calculate the value of the capacitor in order to maintain VCCOUT even when VCC has a droop?

Is there any other circuit that can perform the same function if not better since my circuit will have voltage drop because of the diode connected transistor.

I hope you guys can throw me some light on this.

appreciate any comments!

thanks

-snoop835-

 

[alenhan]

Capacitor

Hi,
I think the drop is due to the charge sharing to the parasitic cap. from the diode connected NMOS, in
one clock cycle
C*(1.8-0.5)=(C+Cpar)*VCCOUT_min
so if the C is much larger than Cpar, the VCCOUT
can be more close to VCC-Vthn.

 

[Syukri]

Re: Capacitor

try to use tansmission gate raher than nMOS switched

 

[snoop835]

Re: Capacitor

Hi syukri,

Do you mean replacing the diode with transmission gate...How to use transmission gate to replace the nmos diode connected? Pls elaborate....

appreciate your help.

thanks