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LMR51625: What is the smallest output LC combination to make sure PWM frequency stable, at low power condition (4ohm, 1W)

Shadow He
Mastermind 49260 points
Part Number: LMR51625

Tool/software:

Hi Expert

For some special testing requirement, I'm trying to decrease the LC filter value on the EVM LM51630VAL. The silicon on the EVM is my new requested LMR51625YFDDCR samples. My target as:

- Output LC filter better to close to value 3.3uH+1uF.

- Light load condition, 1W on 4ohm load is enough. A little higher power is also acceptable, if it helps.

- Need PWM to be fixed 1.1MHz. 

- Not care about other performance, like the voltage/current ripple, load transience, etc.

What I get from EVM, after change LC to 3.3uH+1uF, PWM as picture shows. The output voltage is also not stable.

The datasheet listed calculation method, will get a very large L and C value. And I don't really need to get a very good general performance, just make sure the PWM stable is fine.

Could you please let me know anything could be done to reach my target? If 3.3uH+1uF is indeed not possible, what would be the minimum value could have a try? Thanks a lot.

  • 0
    TI__Genius 14236 points

     Hello Shadow


    I would suggest using WEBENCH for this optimization. I have created a preliminary design you can edit on top of that - webench.ti.com/.../SDP.cgi. My suggestion would be start with 10uF output cap and 10pF feedforward cap (Cff). Increase Cout further if phase margin is lower than 45 deg.

    By the way what is the application you are looking for, what is the Vin (nom, max), Vout, Io (min, max)?

    Thank you

    Regards

    Onkar Bhakare