question on low drop out regulator

[leonken]

Two kind of regulators: NMOS pass device and PMOS pass device.
Why the voltage drop across the pass deivce of PMOS is smaller than the NMOS?

Added after 37 minutes:

The figure

 

[pugongying]

voltage drop using NMOS is determined by VGS
while using PMOS it is determined by VDS
you can find the difference

 

[wwwww12345]

For NMOS, Vin >VG>VS(Vout)+VTN, so VDS=Vin-Vout>VTN, but for PMOS, no such formula is expected.

 

[leonken]

Because the Vdd(Vin) of the opamp is supplied by Vin. The output of opamp is less than Vdd(Vin).

 

[chenmy]

Hi,Leonken:

For NMOS , Vin is the supply of OPAMP,so Vin >VG>Vout+VTN should be met.
so VDS=Vin-Vout>VTN (about ~700mV in 0.35um process)

For PMOS,you only need to make VSD=Vin-Vout larger than Vdsat(usually about 100mV~300mV)

So the PMOS LDO headroom is larger than NMOS LDO. It is the NMOS Source follower that reduces your headroom.

Best regards!

Chenmy

 

[leo_o2]

Some LDO add a charge-pump and can also realize low drop-out with NMOS

 

[paley]

because the nmos' Vgs

 

[manissri]

if u apply pmos pass devic the it will give u gain and phase shift of 180.
when u use nmos as pass device then it will act as a source follower.
so unity gain and no phase shift.

 

[joe_chen]

one is source follower and the other is simply common source stage