LM73605: About the external components

tsuyoshi tokumoto

Part Number: LM73605

Hi team,

I calculated the external components values
by using the equations on the datasheet under the following conditions.

   Vin min:10V
Vin typ:12V
   Vin max:16V
   Vout:5V
   Iout max:5A
   fsw:2.2MHz

The external components I selected are as follows
regarding L,Cout,RFBT,RFBB and Cff.
     L=1.2uH(r=0.26 at Vin max)
    Cout=44uF(ceramic capacitors with low ESR)
     RFBT=100kΩ
　　 RFBB=25kΩ
     Cff: not use

In the above case,I have two questions about increasing the Cout
value(e.g. 100uF with very low ESR) in order to reduce the Vout undershoot
caused by a fast large load transient.

Q1) I think that the phase margin will decrease.
       Is my understanding correct?

       If yes,how much capacitor value is acceptable?
       Is there something like a guide, or is it necessary to
       evaluate the phase margin by using FRA?

Q2) I think that although the phase margin decreases by increasing
the Cout value,the phase margin will be improved by adding the Cff
capacitor calculated based on the equations 18 and 21.

Is my understanding is correct?

Best regards,
Tsuyoshi Tokumoto

over 7 years ago

0 YangZhang over 7 years ago

TI__Genius 11085 points

Hi, I've run a Webench simulation based on your design parameters and component values. The simulation shows a 60 degree phase margin with 100uF Cout and no Cff. Yes, Cff could help if more phase margin is needed. You can also check by transient performance of the circuit.

-Yang

0 tsuyoshi tokumoto over 7 years ago in reply to YangZhang

Expert 2630 points

Hi Yang-san,

Thank you very much for your prompt reply and
I apologize for the delay in my reply.

I had a bit of trouble to re-simulate by changing the parameter of the parts,
but I was also able to check the phase margin by Webench simulation.

Thank you and best regards,
Tsuyoshi Tokumoto

Power management

Power management forum

LM73605: About the external components