LMR51450: RFF formula

Hussam Sabbouh

Part Number: LMR51450

Tool/software:

Hi Team,

I know that CFF can introduce zero in the frequency response of the feedback loop, and that zero adds a phase boost to improve phase margin. RFF prevent CFF of conducting noise from the output of the circuit directly to the FB node, is that correct? what's the role of RFF?
How do we calculate the value of RFF? is there a specific formula I can use for that?

Thanks for the help

over 1 year ago

0 Hussam Sabbouh over 1 year ago

Prodigy 20 points

Additionally, does RFF add a pole in the feedback loop?

0 Andrew Kutzler over 1 year ago in reply to Hussam Sabbouh

TI__Mastermind 23755 points

Hi Hussam,

There's a large volume of posts on E2E right now. I'll get to this as soon as I can.

Thanks,

Andrew

0 Joshua Austria over 1 year ago in reply to Hussam Sabbouh

TI__Mastermind 24115 points

Hi Hussam,

Thank you for your question. There is a calculator tool for this device that can aid in your stability calculations. You can find more information about the feedforward capacitor here: Optimizing Transient Response of Internally Compensated dc-dc Converters with Feedforward Capacitor.

Additionally your assumption of RFF's role is correct as it is used to limit the noise from the output coupling into the FB node of the IC from the feed-forward capacitor (CFF). You can stick with RFF = 1 kOhm and the calculator provided above should be designed with this in mind.

Thank you,

Joshua Austria

Power management

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LMR51450: RFF formula