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LM3489: USB-C Application Feedback Network Question

Daniels Kurgs
Prodigy 180 points
Part Number: LM3489
Other Parts Discussed in Thread: TPS65988, , TPS65987

Hello TI Team!

We are going to use the TPS65988 to supply USB-C Ports with PD capabilities. One of which must be capable of supplying 5V/9V/15V/20V @3A.

When examining the suggested solutions, the LM3489 was used with transistors switching the feedback resistors, though there are discrepancies with the feedback network between the LM3489 datasheet suggestion and the TPS65988 and TPS65987 evaluation kits.

1. In the datasheet, it states that Cff should reduce the (R1+R2)/R2 ratio to around 1 at the switching frequency.

2.In the TPS65988EVM schematic, R1 is parallel to 100nF and R2 is parallel to 1nF, effectively shorting the resistor ratio to 1 already at 1kHz.

3.The TPS65987 has more equal capacitors of 100pF on top and 1000pF at the bottom.

I understand that changing the feedback with transistors introduces stability issues, though what should be my thought process when designing for PD requirements? The input voltage is 24V, the inductor is 10uH.

Thanks for your support!

  • 0
    TI__Mastermind 22301 points

    Hi Daniels,

    Usually we do not suggest a capacitor across the lower FB resistor, however for multiple VOUT cases it is needed for a stable output voltage transition.

    The LM3489 converter is hysteretic and should be always stable with enough output capacitor ESR.

    The CFF will affect output ripple and transient response (lower cap = faster response).

    100nF is relatively large, the switching frequency would be low for TPS65988EVM.

    I suggest starting with the 1nF CFF and decrease towards 100pF as necessary.

    It should be always stable, just specifically look at the output ripple and load transient.

    Hope this helps let me know if you have further questions,

    -Orlando

  • 0 in reply to Orlando Murray
    Prodigy 180 points

    Hi Orlando,

    just wanted to thank you for the quick response!

    Kind regards,

    Daniels