LDO with NMOS as pass transistor..

nmos pass transistor

I need help in designing the LDO with NMOS as passtransistor..

The gate voltage of the NMOS should be higher than the input voltage..
I need to use a chargepump to get this high voltage..
Anybody has any experience in this..
Any material is appreciated..
Thank u

ldo nmos

Isn't better to use PMOS instead?

ldo dropout voltage nmos

yes, you need the charge pump if you want a really low drop out voltage, but why you want to use an NMOS as pass-transistor?

pass transistor ldo

reasons to use NMOS as passtransistor..
Good stability and need not to worry about the ESR values of the load capacitor to compensate..
Smaller size..
Very low dropout voltage..

ldo nmos output

May I ask why the zero created by the ESR of the load cap does not need to worry?

nmos output capacitance

the output impedance of source fallower is very less and the zero from the ESR is pushed far..

nmos pass characteristic

The advantages of a NMOS transistor (source follower output) in LDO is that the output capacitance can be very small (few pF) and because of the inherent low impedance at the output, the output need not be a dominant pole.In PMOS pass transistor LDO's, the output has to be generally a dominant pole thus needing a large load cap for being stable.

The disadvantage is that the dropout voltage is more and thus needs to have a boosted up gate voltage for driving the NMOS pass transistor.

If the output branch is not regulated (like in replica bias NMOS output LDO's), the load regulation is not good as compared to the PMOS pass transistor LDO's.It is suitable for supplying digital circuits.

Regards.

Added after 2 minutes:

The NMOS pass transistor LDO is has a single pole roll-off characteristics.The gate of the NMOS pass transistor needs to be decoupled with capacitance to reduce the glitches in the output voltage while the load switches.

ldo pass nmos

Do u have any material on this..

ldo nmos transistor

Embedded 5V to 3.3V voltage regulator for supplying ics in 3.3v CMOS technology.

IEEE JSSC Vol. 33 No. 7 July 1998.

This is a paper on NMOS pass trans. voltage regulator with replica bias.

nmos ldo

The ESR zero stays the same because it is 1/(ESR*C), but the pole is pushed further because of the NMOS output.

pass transistor

naisare said:

reasons to use NMOS as passtransistor..
Good stability and need not to worry about the ESR values of the load capacitor to compensate..
Smaller size..
Very low dropout voltage..

Click to expand...

NMOS means High DropOut Voltage Not Low

regulated pass transistor

NMOS means High DropOut Voltage Not Low

Click to expand...

yes, i think that!s the source that you need the charge pump...