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How to choose the GWB of a LDO?

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horzonbluz

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How to choose the GWB of a LDO?
Usually a LDO have three poles and a zero, how to compensate them and choose
the GWB to provide a reazonable setting time?
I am newer, who can tell me?
Thanks in advance!
 

Re: The GBW of LDO?

Dear horzonbluz :

Be care for use the series resistance of the output capacitor (ESR) to
compensate the ldo system, and the stable range of ESR value.


mpig
 

Re: The GBW of LDO?

Oh, dear mpig09.
Because of the ESR changing with the output capacitor.
We usually cannot use the ESR to compensate the ldo system.
 

Re: The GBW of LDO?

horzonbluz said:
Oh, dear mpig09.
Because of the ESR changing with the output capacitor.
We usually cannot use the ESR to compensate the ldo system.
Click to expand...

I think if we put the first pole at the output of error amplifier, the esr of
capacitor is not very important. But the GBW may be lower.
 

The GBW of LDO?

Many LDO structures use load cap in the output to generate the dominant pole and ESR zero to compesate LDO to a surficient PM.
of coure the zero varies with the ESR,so, many LDOs use specified Capacitor to provide stability.
Some structures' use novel compensation scheme to provide stability not rely on ESR .
 

Re: The GBW of LDO?

Any idea on using no output capacitor for LDO?
 

Re: The GBW of LDO?

locate the output pole of the error amplifier at higher frequency.
 

Re: The GBW of LDO?

in order to get large bandwidth, you should pull all the high frequencies pole into higher frequencies.
the first thing is to locate the dominant pole.
 

Re: The GBW of LDO?

I like to think of an LDO as a two stage amplifier, where the opamp is the first stage and the pass transistor + resistor divider are the second stage. Thus, you want to have your compensation happening at the output of the opamp.

Choosing the right GBW depends on your PSR frequency response. because the pole of the LDO and the unity gain frequency determine that (plus other factors that I don't have on my mind now).

Hope that this helps.
 

Re: The GBW of LDO?

Hi, dear jonashat!
What's the PSR frequency response?
Or you mean the PSRR of LDO system?
 

The GBW of LDO?

I think PSR (power supply rejection) is used for LDO's. It is defined as the inverse supply-to-output gain. It is a positive figure and the higher the number the better.

PSRR (power supply rejection ratio) is an OPamp parameter and is defined as the ratio of the supply-to-output gain over input-to-output gain. Obviously, the lower the number the better.
 

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    horzonbluz

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Re: The GBW of LDO?

Dear jonashat.
Thank you for your advice! But it usually use PSRR in LDO system as same as OP AMP though they have different definition.
 

The GBW of LDO?

Not that I'm an expert, but I have looked at a couple of papers and I recall seeing PSR. However, when I did my assignment on LDO's I used the terms PSRR and PSR interchangeably :)
 

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