How to improve high frequency PSRR

[sjamil02]

Hi All,

I have designed LDO with NMOS pass transistor with 35mV dropout (1V supply regulated down to 0.965V). Since the dropout voltage is small then my PSRR is bad (-38dB @dc and 0db @10MHz). Furthermore the NMOS pass transistor is sized in triode region (to get smaller device and small dropout). To improve the psrr @dc, I have increase the gain of the error amplifier (I'm using folded cascode structure with Vdd=2.5V). It works and the psrr is slightly improved from -38dB --> -44dB. But this is not enough since i am targeting psrr = -69dB @dc. If I further increased the error amplifier gain, I'm afraid the bandwidth will be too low. Is there any other way to improve the dc psrr? Secondly, how to improve psrr at high frequency (10MHz)? I do not have luxury to increase the output capacitance. My target is psrr > -30dB @10MHz.

Please help.

Thanks in advanced
Sj

 

[ajit4u]

u can increase by decreasing the difference of the voltage or by increasing the supplay voltage

 

[DenisMark]

Hi, you can improve psrr by only using pass element in saturation region (with sufficient Rout).
Definitely, it's impossible when dropout 35mV is considered.
You can make it feasible at dropout >>100mV, and more drop-out voltage means pass element with less area.
Also I'm quite confused that you consider 1V supply while folded cascode uses 2.5V.
It's not clear in your thread what's your min. supply? And do you use charge-pump for output NMOS driving or not?

There are a lot of references on this topic, if it's possible for you, you can start with this one (and use its references):
IEEE Circuits and Systems, Nov. 2010. High Power-Supply-Rejection (PSR) Current-Mode Low-Dropout (LDO) Regulator.
(Don't ask me to share).

 

[dick_freebird]

If you depend on the loop to provide PSRR then your bandwidth
needs to be high. But minimizing internal PSRR detractors such
as bias-rack AC deflection is a place to look as well. Decoupling
reference voltages to their referred rail, cascoding domain-to-
domain reference currents, shielding voltage-to-current resistors
and so on. What do you know about the elements of /PSRR as
it sits?