How calculate gain in a LDO? What's my problem?

[Fred556]

Hi,

I'm trying to calculate the gain in my LDO but I don't know what I'm doing wrong...

To calculate the gain, I'm doing dB20(VF("/vout")/VF("/vref)). But it's giving me an negative gain???

someone could help me?

Thanks in advance.

 

[erikl]

I'm trying to calculate the gain in my LDO but I don't know what I'm doing wrong...

In order to measure the ac gain you have to inject a frequency-dependent disturbance: insert a (unit) ac voltage source either in series with vref or between the inductor L0 and the Vplus input.

BTW: the upper node of L0 in any case needs grounding via a (large) cap.

PS: Usually not the gain of an LDO is interesting, but e.g. its Power Supply Rejection Ratio PSRR. In this case, the unit ac voltage source is inserted in series with the power supply (Vdd_High). PSRR = dB20(VF("/vout")/VF("/v unit ac voltage")).

As VF("/v unit ac voltage") ≡ 1 , PSRR = dB20(VF("/vout")) < 0 , e.g. -80 .. -20dB .

 

[Fred556]

In order to measure the ac gain you have to inject a frequency-dependent disturbance: insert a (unit) ac voltage source either in series with vref or between the inductor L0 and the Vplus input.

I've done that but it doesn't works...



I thinks it´s because my function but I don't know... dB20(VF("/vout")/VF("/vref"))

I have always the same negative gain...

 

[erikl]

I've done that but it doesn't works...



I thinks it´s because my function but I don't know... dB20(VF("/vout")/VF("/vref"))

Are you sure that vref is a series circuit of the correct dc ref. voltage and a unit (1V) ac voltage?

BTW: It looks like you have 2 ac sources in parallel: V3 & V9. May be they disturb each other if they aren't synchronous.

If V3||V9 really is a unit ac source (i.e. Vac=1V), you should plot just
dB20(VF("/vout"))
... as I told you before, because the ac input ref voltage (in the denominator) is 1V .
VF("/vref") may only present the dc part of the voltage.

 

[afujian]

you can use stb analysis to simulate the loop gain

 

[leo_o2]

Yes, stb is recommended.