LDO's another problem

hello

i met this problem, when i changed input voltage from 1.3 to 3.3V. The input voltage is also supply of the opamp. I designed the opamp for the lowest value of power supply 1.3V. Dropout voltage is 0.5V When i change Vin to 2V and set feedback resistors to give 0.9V (Vref is

0.8) everything is ok yet.

But when i change it to 3.3V and let feedback be the same, transient response is very improper.

For what supply values i should assign W/L ratios? The opamp is simple p-input differential amplifier with active load.

Pass device is set to be in the saturation.

thanks in advantage

Reply to
jutek
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did i ask OT question?

Reply to
jutek

No, merely ambiguous and without context. You gave no indication of what you're trying to accomplish, what you have now, what it's doing that it's not supposed to do, what it's not doing that it _is_ supposed to do, that sort of thing.

What _do_ you have now?

Thanks, Rich

Reply to
Rich Grise

:> did i ask OT question?

: No, merely ambiguous and without context. You gave no indication of what : you're trying to accomplish, what you have now, what it's doing that it's : not supposed to do, what it's not doing that it _is_ supposed to do, that : sort of thing.

: What _do_ you have now?

LDO = Low Dropout Regulator. The original poster is having a problem where the transient response of his LDO is varying with supply voltage, resulting in an "improper" transient response.

To answer the original poster's question (with a question): How are you compensating your amplifier?

"Improper" transient response seems like a problem in your compensation. Generally, you would compensate your amplifier by putting a gigantic cap. on the output of your regulator, making your output pole the dominant pole. You need to make sure that the output pole remains the dominant pole over all supply voltages (i.e. the gm of your output/pass device will increase with supply, moving your output pole to a higher frequency. This could cause your first-stage pole to become to dominant pole, and result in the "improper" transient response that you are seeing, depending upon the resulting pole locations.)

If you are not putting a gigantic cap. on your output, you need to spend lots of time figuring out how you will effectively compensate your regulator.

Good Luck,

Joe

Reply to
<jwelser

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