• State TI Thinks Resolved
  • Locked Locked
  • Replies 2 replies
  • Answers 1 answer
  • Subscribers 64 subscribers
  • Views 219 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMR51430: Cff cap

Sith Domrongkitchaiporn
Prodigy 50 points
Part Number: LMR51430

Hi,

I have questions about the Cff cap for LMR51430YFDDCR (1.1MHz Forced PWM). I don't see any mention of it on the datasheet. This cap showed up when I used Webench to design the following configurations:

  • 3.3V @ 0.4A (1.24A doesn't seem to require Cff cap)
  • 1.5V @ 0.57A

How is the value calculated and at what current is the cap required? We are interested in using the same value cap for both the 3.3V and 1.5V rail. Is this possible? If so, would 47pF work for both or is there a better value?

Thank you

  • 0 in reply to Denislav Petkov
    TI__Mastermind 19891 points

    Hello Sith, 

    The webench tool takes into account the internal compensation and the external components and tries to optimize the loop response for best load transient performance. 

    The feedforward capacitor can help with this optimization. It is usually a good idea to have a place holder in the schematic (and layout footprint) so that you can optimize the performance if needed by your load. 

    You can disable the Cff optimization with the panel on the left side of the tool:

    If you want to have it as a placeholder, here is what you can do. 

    If you have two designs and you want to re-use the same value, one suggestion is to create two separate webench designs for the two output voltages.

    Then, you can adjust the feedforward capacitor value manually and look at the calculated loop cross over and phase margin values. If the results are satisfactory, you can re-use the same value perhaps. You can see the calculated performance values on the bottom of the design page, as you scroll down below the schematic.

    Regards, 
    Denislav