I need help about this regulator circuit

[will_fung]

Hi Everyone,
I need help on this circuit, this circuit was designed by other engineers in my company, but, now, we have a problem.
It is use to provide power to the cell phone, while I switch S7 change the voltage from 3.9V to 3.2V, there is a big undershoot that make the phone crash down. could you tell me how to fix this problem?
I tried to add 10 pcs of 470uF Tan capacitors on the trace of VBATT_CAL and the overshoot can be reduced to 86mV, the cell phone can work well. but the quantity of the capacitor is too much, I also worry about the inrush current.
Thanks a lot.

 

[Velkarn]

maybe try to speed up the regulator. try to reduce capacitance C22 to 1n-300p and R26,32 set 20-100 ohm

 

[will_fung]

maybe try to speed up the regulator. try to reduce capacitance C22 to 1n-300p and R26,32 set 20-100 ohm

Thanks a lot.
I changed the C22 to 1nF, and R26 R32 changed to 270ohm, the undershoot got a big reduction, only about 60mV, it is good enough to make the phone work well.
But I still have a question, what is the C22 used for here?

 

[mtwieg]

Thanks a lot.
I changed the C22 to 1nF, and R26 R32 changed to 270ohm, the undershoot got a big reduction, only about 60mV, it is good enough to make the phone work well.
But I still have a question, what is the C22 used for here?

It gives the error amplifier the response of an integrator, and C22 controls its gain. If C22 is too small, you risk instability, but if it's too large the response will be slow. A more optimal response could probably be achieved by using a type II compensation scheme for the error amplifier.