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

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allennlowaton

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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...
 

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

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
 
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?
 

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