TPS40422 VDD supply

Hi

Could you give me answer soon? I must respond to my customer as soon s possible early. I would appreciate an anser as soon as possible.

This is a holiday in the US.  You may not get a response until next week.

Thank you for your reply. I'm looking forward to your answer until next week.

Looking at the datasheet, I am thinking it may not work properly.  TPS40422 uses voltage feed forward to adjust the power stage gain characteristics.  Vin would have to be the same for VDD and FET drive for that to work.  If your 3.3 V rail never varies, it may be possible to adjust for this in the compensation network, but I have not tried it.  I'll see if i can get a better answer next week.

Thank you for your reply. Please tell me, if you can get a better answer next week.

Hi, 

As long as the compensation is tuned and the loop is stable, we don't think there is a problem for this application. 

Before turning on PWM, VIN and VDD need to be available first. 

Thanks

Qian

Thanks for clarifying...

Thank you for your reply.
I think my customer will need to adjust for the compensation network. So should TPS40422 is needed to input VDD and Vin from same power rail?

Best regards,

tateo

I think the answer is that it is not completely necessary if the two voltage do not vary much.  TPS40422 adjusts the voltage feed forward based on VDD and assumes VDD=VIN.  If both voltages are not the same but constant, then you can adjust the compensation to account for the difference between VIN and VDD.

Thank you for your reply.
I have one more question. When my customer adjust for the compensation network, what is the check point?
 ex. Resistor and capacitor of comp pin
       Phease margin

Best regards,

tateo