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Design >> Analog Design >> relationship between regulator supply noise sensitivity and RC filter
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Message started by analog geek on Jan 28^th, 2014, 6:29pm

Title: relationship between regulator supply noise sensitivity and RC filter
Post by analog geek on Jan 28^th, 2014, 6:29pm

Hello Guys,
I am currently working on Low Drop Out Regulators (LDO) for PLL circuits with Replica Compensation technique .
I had some doubts regarding RC filtering of the supply noise.
1. How is supply noise is filter out by output RC filter formed at the output of the regulator?...As this is Low Pass Filter (i think so) so it will pass the supply noise frequency till its cut off frequency and attenuate other higher frequency noise components?....Does this mean that it should be required to have minimum noise frequency which should be higher than bandwidth of the RC filter so that all the noise gets suppessed?...

2.basically tell me the relationship of supply sensitivity and RC filter at the output of the LDO?..

3.Having higher LPF bandwidth is good or bad?....

4. There is one statement in the paper as follows;"Past the closed loop bandwidth of the regulator (Wbw), the regulator can no longer attenuate supply noise and it is set by entirely RC filter. At closed loop bandwidth of the regulator (Wbw) the attenuation of the RC filter approaches open loop gain of the regulator and past the regulator bandwidth supply sensitivity falls with single pole roll of of RC filter, "..
my question is how supply sensitivity falls after single pole roll off of the RC filter.? isnt it good to fall supply sensitivity or not?....

I HAVE ATTACHED THE IEEE PAPER I AM REFERRING

Title: Re: relationship between regulator supply noise sensitivity and RC filter
Post by BackerShu on Mar 1^st, 2014, 3:46pm

An intuitive way to understand this is after the dominant pole of the regulator, the output branch act as a resistive divider for the supply noise to appear at the output. As the impedance roll off at 20dB/dec, the supply noise sensitivity falls at the same speed.

A major advantage to have replica biased regulator is to have one degree of freedom to control the regulator loop, 3 poles in replica-biased compared to 2 poles in conventional case. In conventional case, it requires very large caps to make the output pole dominant, whereas in replica-based case, the capacitor to make the RC filter node to be dominant is much less.

The following reference gives you much more details:
A. Arakali, S. Member, S. Gondi, A. Abstract, and P. K. Hanumolu, “Low-Power Supply-Regulation Techniques for Ring Oscillators in Phase-Locked Loops Using a Split-Tuned Architecture,” Solid-State Circuits, IEEE J., vol. 44, no. 8, pp. 2169–2181, 2009.