power transistor should always work in satur in ldo design?

[lxcpku]

ldo triode region

hello,all
i wonder in a ldo design, the power pmos transistor (that is pass transistor) should be always work in saturation region ? or even triode would also be OK ? some times, simulations is good (since gain still meets the need,and current and output voltage is also good) while mos is not in saturation region, will it be ok ?

thanks a lot

 

[iamxo]

Re: power transistor should always work in satur in ldo desi

in heavy load, the pass transistor should be in saturation region, offcourse, to reduce dropout voltage.

while, there is zero load, the pass transistor can be in subthreshold region, but should also keep the output voltage.

 

[DenisMark]

Re: power transistor should always work in satur in ldo desi

Some LDO circuitries allow pass transistor working in triode region. But this citcuits are utilized more complex frequency compensation. See reference for example.

 

[lxcpku]

ok. thx a lot

 

[hung_wai_ming@hotmail.com]

To save space, you may consider triode region.
However, triode region means you lose PSRR easily

 

[WindRay]

Re: power transistor should always work in satur in ldo desi

If the pass transistor is in triode region, the output voltage response to load step change is poor I think.

 

[bicave]

Re: power transistor should always work in satur in ldo desi

hello,all
i wonder in a ldo design, the power pmos transistor (that is pass transistor) should be always work in saturation region ? or even triode would also be OK ? some times, simulations is good (since gain still meets the need,and current and output voltage is also good) while mos is not in saturation region, will it be ok ?

thanks a lot

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

If PMOS in saturation PSRR is better.

 

[lxcpku]

Re: power transistor should always work in satur in ldo desi

hello,all
i wonder in a ldo design, the power pmos transistor (that is pass transistor) should be always work in saturation region ? or even triode would also be OK ? some times, simulations is good (since gain still meets the need,and current and output voltage is also good) while mos is not in saturation region, will it be ok ?

thanks a lot

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

If PMOS in saturation PSRR is better.

got it ,thanks

 

[shaq]

In triode can make the on-resistance (Ron) smaller.

 

[allennlowaton]

Re: power transistor should always work in satur in ldo desi

hi guys..i try to confirm this:

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

in the .lis file of HSPICE
there appears Vds and Vdsat...i'm wondering what to use...
because, if i'm going Vdsat...my result don't follow the equation given above..

thank you...