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Measuring Power Supply Rejection Ratio (PSRR) of LDO in Simulation

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J.Yuan

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I trying to measure Power Supply Rejection Ratio (PSRR) of LDO,
but the output ac voltage is bigger than the input ac voltage.

the output ac voltage should be smaller than the input ac voltage.

base on the formula, PSRR = 20log(Vout,ripple/Vin,ripple ) in dB.

is there any wrong with my circuit construction for PSRR simulation?
 

Hi,

I didn´t review the schematic...

But if the output AC voltage really is higher than the input AC voltage then this might be because the circuit has a stabilty problem close to this frequency.

Try to analyze stability
Try to analyze output waveform caused by an input step.

Klaus
 
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    J.Yuan

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That appears to be a high gain op amp type circuit with no compensation.
If so, then there's about a 100% chance that it will oscillate in a feedback loop.
Compensation is typically done with properly sized capacitor from the drain to gain of M10.
 
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    J.Yuan

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Actually, instead of doing a transient analysis with some frequency component on the suppy, I suggest you do small signal AC analysis and see the PSRR vs. frequency. May be that at the frequency that you are exciting the circuit, there is peaking in the PSRR i.e PSRR>0 dB. You want the PSRR to be less than 0 dB for all frequencies. Miller compensation always worsens the PSRR, so you should do some other kind of compensation if the loop does not have enough delay room.
Another problem with transient is that the amplitude you are applying could be unrealistic and could make the circuit nonlinear and increase the Q of the loop.
 
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    J.Yuan

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Small signal analysis shoulf be done but this is not where
analysis should end. LDO error amp and output stage may
well "wind up" under large signal conditions, at min or max
load, min or max line, and then small signal analysis may
entirely miss the operating point of interest.

PSRR of a PMOS LDO is often quite poor especially at high
frequencies. You have to choose compensation wisely,
some styles that are area efficient also act as "power
supply noise injection" paths into places where gain
will follow, and you're then going to see just what you
see.
 
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