Help me establish the EA of an LDO

[jefferson]

Hi,I'm doing something on LDO which needs an EA and a Pmos pass element to regulate the Vout.When the Gain is small,about 30 dB,it seems that the Vout would not very precise,but when Gain is high,up to 60dB,the circute loop tends to be no stable?
How much should the EA'Gain be?

Any reply would be appreciated

 

[IanP]

Re: EA of an LDO

"Stability analysis of low-dropout linear regulators with a PMOS pass element"
https://focus.ti.com/lit/an/slyt194/slyt194.pdf

Regards,
IanP

 

[jefferson]

EA of an LDO

Very thanks for Ianp's reply.

Anyone else who is also doing sth about LDO could post here or send message to me for disscusing.

 

[surianova]

Re: EA of an LDO

Hi,I'm doing something on LDO which needs an EA and a Pmos pass element to regulate the Vout.When the Gain is small,about 30 dB,it seems that the Vout would not very precise,but when Gain is high,up to 60dB,the circute loop tends to be no stable?
How much should the EA'Gain be?

Any reply would be appreciated

60 dB gain for the EA is good enuogh. and if u use Pmos as pass element, u need external cap for phase margin compensation.

 

[jefferson]

EA of an LDO

Hi,surianova.
I know that the PMOS pass element would make a pole of the whole circute loop. If I use an external cap,seems that it need high ESR for phase margin compensation.
How can I compensate without Cl and its ESR?
Thanks.

 

[smartdream]

Re: EA of an LDO

Hi, Jefferson.

Maybe you can use Miller compensation using a capacitor and a nulling resistor which connect between the Gate of PMOS and its output terminal. this approach is feasible especially for light loading conditions.
BTW, for the line & load and compensation consideration, the close loop gain is good enough for about 60~80dB
A usefull paper for your information.

 

[Chethan]

Re: EA of an LDO

Hi all,
I am supposed to design an LDO with 1.5V(fixed) output and a max current of 100mA. How can i find out the specifications for the Error amplifier , ie., what should be its gain, slewrate, BW etc. How can I estimate these parameters. Any material regarding the same would be very helpful.


thanx in advance
chethan

 

[nagula suresh]

Re: EA of an LDO

keeping the compensation capacitor makes u r system very slow.,
so u cant get the start up time specification.

to make the system stable,and anyway u should have output capcitor when u r driving other blocks or soc.,

so,by using the optimum Resr .,and adjusting the poles locations ,we can make the system stable.

 

[jefferson]

EA of an LDO

I think a Miller compensation using a capacitor and a nulling resistor will help some,but I found that the capacitor would be very huge.When I do simulation,I need more than 100p to keep enough phase margin.As nagula suresh said,the system is slower.

So I use ESR for compensation.It does works.But actually in use,few would use a Cap with high ESR.

The location of poles seems no easy to shift.
Use a buffer ?

Any other method for compensation?

Thanks.

 

[smartdream]

EA of an LDO

yeah, a buffer is also a circumvented method, you can perefer to Mora's Ph.D paper.

 

[jefferson]

EA of an LDO

Thank you all.I've found some papers on ldo compensation.Anyone else want it could refer to this: