The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> Miller compensation && Low drop out regulators
https://designers-guide.org/forum/YaBB.pl?num=1380314989

Message started by hchanda on Sep 27^th, 2013, 1:49pm

Title: Miller compensation && Low drop out regulators
Post by hchanda on Sep 27^th, 2013, 1:49pm

Hi all,

I am working on a LDO which as PMOS as it's pass transistor. This regulator has output load cap that can vary from (1uF to 8uF) which is big. I have a single stage amplifier output driving the pass transistor.
I am trying to compensate the LDO using miller cap, which does not seem to help.

In the above structure I described above, before compensation the dominant pole is at output (near output load cap) and the next pole is at the output of the amplifier.

When we apply miller cap does it push the pole at the output of error amplifier to lower frequencies and the pole at the output (near cload) does not move much because of huge capacitance (1uF). Doesn't this worsen the LDO compensation because now miller is actually pushing poles together?

I also need some good understanding on the miller compensation
I read a paper "re examination of pole splitting of generic single stage amplifier". This paper states that miller cap actually pushes the dominant pole to lower frequency and non dominant pole to higher frequency, whether your dominant pole is at input or output.
Is this true? I always thought that miller cap always pushes the pole at the input to lower frequencies and the pole at the output to higher frequency. Can anyone clarify on this and explain me intuitively?

So again back to LDO compensation, if the LDO has to drive large capacitive loads, does miller compensation work very well? Any other better and simple techniques to achieve good stability.

I found couple of papers on compensating the LDO (still going over them), none of them discusses the cons of using miller compesation in detail.

Appreciate your time and response.

Thanks

Title: Re: Miller compensation && Low drop out regulators
Post by raja.cedt on Sep 28^th, 2013, 3:23am

Hello,
Miller compensations makes dominate more dominate and non dominate makes even more non dominate. So i would say go please post your schematics and results, i guess it will help you may be your capacitor is not sufficient. Many people use miller but in ldo it is not often preferred because of PSRR. few people use rc series circuit which will add a zero in the circuit.

Thanks,
Raj.

Title: Re: Miller compensation && Low drop out regulators
Post by Aman on Nov 7^th, 2013, 3:25pm

hchanda,

You have a two-pole system that needs compensation. Try to first figure out where you
would like to place the dominant pole, this will determine PSRR response of your LDO.
Compare PSRR response with ideal VCVS for OTA, with a OTA model using VCCS(gm)
and shunt Rout(close enough) and a parasitic capacitor(or use your amplifier), this should
give you some sort of bounds for best-case/worst-case PSRR response.
Now, the centerpiece of Miller compensation is pole-spitting, and remember how a miller
capacitor works by multiplying the input side capacitance with gain(as long as you have
phase inversion). So by adding a miller across your power device will work only if
1) If there is any gain across power PMOS.
2) Location of output pole(this is a known once you fix your PMOS size).

Good Luck!

-Aman

Title: Re: Miller compensation && Low drop out regulators
Post by analog_rf on Nov 26^th, 2013, 5:25am

Since you already have 1uF cap sitting at the output this will be the dominant pole. In LDO's you will have implicit miller compensation :meaning you dont have to place a cap between first stage and output node, its already present because of the size of your pass transistor. This parasitic Cgd will provide miller compensation ,but in your case since you have an output pole which needs to be dominant, you will need to take care that you do not push this miller pole too close to the output pole as this will make your ldo unstable.Feel free to ask any questions you may have. Also i suggest you to go through rincon mora's thesis carefully as you will see all the aspects of ldo design explained in a detailed manner.

Thanks
analog_rf

Title: Re: Miller compensation && Low drop out regulators
Post by Kevin Aylward on Dec 15^th, 2013, 4:39am

THE STANDARD LDO problem driving large cap is the large cap. "Standard" LDOs usually require esr in the load cap and are SPECIFIED as such in their data sheets. There are "special" "any cap" LDOs which use additional tricks. One is to use a series, low value output resister mimicking esr, with duel feedback paths. A comp feedback cap from the drain to somewhere backwards, and the DC setting resistive feedback from the final output. Often the cap feedback is to a cascode low Z point referenced to ground to increase PSR.