ldo psrr

[Shilpa poddar]

How miller cap affect the ldo psrr? Can anybody explain me this.

 

[dick_freebird]

The most common Miller comp scheme puts the
cap across the pass FET D, G terminals against a
weak transconductance amplifier.

Consider that there is an external capacitor that
is significantly larger in value. That makes VOUT
for purposes of discussion, DC-constant.

Now if you perturb VIN upward, you will increase
the voltage across the compensation capacitor
(attached to D = VOUT) as its gate is in-the-moment
positioned relative to VIN (=S) by the gate overdrive
required to make IOUT. The increased comp cap
voltage must pull on pass FET G, downward. That
increases pass FET current at the same time VIN
is rising, so obviously degrades PSRR; the pass FET
has significant gain when not in "dropout" (linear
region) and a bit remaining even in dropout.

 

[Shilpa poddar]

The most common Miller comp scheme puts the
cap across the pass FET D, G terminals against a
weak transconductance amplifier.

Consider that there is an external capacitor that
is significantly larger in value. That makes VOUT
for purposes of discussion, DC-constant.

Now if you perturb VIN upward, you will increase
the voltage across the compensation capacitor
(attached to D = VOUT) as its gate is in-the-moment
positioned relative to VIN (=S) by the gate overdrive
required to make IOUT. The increased comp cap
voltage must pull on pass FET G, downward. That
increases pass FET current at the same time VIN
is rising, so obviously degrades PSRR; the pass FET
has significant gain when not in "dropout" (linear
region) and a bit remaining even in dropout.

Thanks for good explaination.I have one more doubt. which frequency range this effect will be visible? According to my knowledge at low frequency loop gain of ldo decide pssr as we go to high frequency loop gain deteriorate and output capacitor decide the pssr.

 

[dick_freebird]

At DC the Miller cap has no authority and PSRR is
about the same as the AVOL. As frequency (or dV/dt)
rises the Miller cap begins to fight against the loop
gain and error amp output impedance. At very
high frequencies the loop cannot keep up and
the ability of the error amp to tamp down supply
noise goes away, and all that's left is the external
filter.