How to Calculate the PSRR of double Power supply LDO?

[ckseu]

Deal All:
Such as the Figure follow, we use a Vdd for Error Amp and other control block
And a Vin for N-type Power element
I know the PSRR of the Vout to Vdd,
But How about the PSRR of the Vout to Vin?
Thanks a lot and expect for any suggestion!

 

[hmsheng]

The gain from Vin to Vout is very small when the NMOS operating in saturation region. It should be
Gain = Rseries/Rin@Vin = Rseries/(gm*ro*Rseries+ro+Rseries)
where, gm is transconductance of NMOS, Rin@Vin is the input impedcance looking into the NMOS from Vin, ro=1/λID is the output resistance of NMOS, and Rseries is the sum resistance of the two resistors.
But if the NMOS operating in linear region, the gain from Vin to Vout will rise.
According to this you can analyze the PSRR with the circuit parameters.