How to improve LDO high frequency PSRR?

[colinwang]

ldo psrr

hi all,
How to improve the high frequency PSRR of LDO with a fixed output capacitor?
two conditions:
large output capacitor: uF
small output capacitor: nF or pF

Thank you all!

 

[saro_k_82]

high psrr ldo

A description of acrchitecture that you are using, or a schematic and the results that you currently get would be useful

 

[colinwang]

psrr ldo

The schematic and the psrr is shown as follows. The load capacitor is 10-100pF and the load current is up to 20mA.

 

[colinwang]

how to improve psrr

This is the psrr plot. How can i improve the psrr at high frequency? If i want to get more than 40dB psrr at 1MHz, are there any good methods?

Thank you!

 

[knights]

high psr ldo

If your loading is 100pF, is that still stable?

 

[colinwang]

high frequency psrr

yes, the stb analysis shows that it has enough phase margin and gain margin.

 

[saro_k_82]

psrr of ldo

What is the psrr nature of the bias current? If that is fine, the best charaterisitic possible is to delay the rolloff until the second pole...

 

[colinwang]

ldo psrr

Could you explain more clearly? Are there any special structure can be added to improve the high frequency psrr?

thank you!

 

[svraj]

improve psrr

Hi,

The PSRR curve can be broken into 3 regions:
Region 1 - 0 to 200Hz
Region 2 - 200Hz to 100KHz
Region 3 - above 100KHz

For high PSRR in Region 1, improve your Bandgap PSRR to be above 60dB at DC.
In this Region, the Open-loop gain of the Error Amplifier must be large of at least 70dB till a frequency of 10KHz.

For region 2, only the open-loop gain of the Amplifier comes into play
Region 3 is mainly dominated by the large output caps, bigger caps gives higher PSRR.

I have attached a good article on this here. Also the link to this article is at
focus.ti.com/lit/an/slyt202/slyt202.pdf

Regards,
svraj

 

[colinwang]

psrr ldo improve

Thank you very much!
I also want to know how to achieve high psrr in high frequency for cap-less LDO. Without the large output capacitor, the high frequency psrr is really a difficult to achieve. Are there any suggestions?