PMOS pass device in LDO needs to be in SATURATION always?

[allennlowaton]

hello guys...
i'm doing LDO now, I'm just wondering, do I need to operate the PMOS in saturation always during its operation? If not, why?

thank you very much...

 

[shaq]

No. Linear region is good for load transient in order to reduce recovery time and spike current.

 

[allennlowaton]

thank you for your reply shaq..
i had read something here in edaboard.
it says

PMOS not always in Saturations. Depend on VIN, VOUT. If
- Vout<VIN-Vdsat, it will be on sat
- Vout> Vin-Vdsat, it will be on triode

...i try to confirm this..but, it seemed wrong simulation result...
..any idea about this?

 

[rampat]

Re: PMOS pass device in LDO needs to be in SATURATION always

It depends if you are operating in dropout region PMOS will be in linear region and If vout-vin > drop out voltage ,PMOS will be in saturation region..

Some times PMOS is always operated in linear region to save chip area at the cost of PSRR..

-rampat

 

[allennlowaton]

thank you rampat...
i'm confused with the drop out voltage...
i try to find the value of drop out voltage in the .lis file of HSPICE.
is it the Vdsat or the Vds?