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LM2576 Feedforward capacitor

John Jones
Intellectual 380 points
Other Parts Discussed in Thread: LM2576

Hi,

In the datasheet for the LM2576 why isn't a feedforward capacitor used on the adjustable version across the upper resistor like other simple switchers?

Would adding this capacitor add any benefits to the circuit and why do other simple switchers mention feedforward capacitors in their datasheets?

Thanks.

  • 0
    TI__Genius 11085 points

    Hi John,

    I think that's because at the time this part was developed, it was not common practice to use very low ESR Cout. Now, things change. I can see in the datasheet, it is mentioned that "However, when operating in the continuous mode, reducing the ESR below 0.03 Ω can cause instability in the regulator." in page 18, OUTPUT CAPACITOR section. That would be the case where CFF can help improve stability. I believe carefully selected Cff is beneficial when paired with low ESR Cout, same as other SIMPLE SWITCHER parts.

    regards,

    Yang

     

  • 0 in reply to YangZhang
    Intellectual 380 points

    Thankyou for your reply Yang,

    I did have a small suspicion that it was something to do with the chips age or perhaps its rather low 52khz switching frequency, but wasn't sure.

    I am using a Panasonic EEUFC1E102 which has an ESR of 38 milliohms @100khz, would something like a 2.2nF feedforward capacitor give a better output response and if so what areas of the circuit should I be probing and comparing?

    Thanks again.

  • 0 in reply to John Jones
    Intellectual 380 points

    Shameless self bump.

  • 0 in reply to John Jones
    TI__Genius 11085 points

    For the transient performance, you can monitor the output voltage across Cout. When the load have a transient, the response time and undershoot/overshoot on the Vout show the loop performance. Pick the BOM with lower overshoot/undershoot magnitude and faster recovery time.

    Yang

  • 0 in reply to YangZhang
    Intellectual 380 points

    Cheers! a 1nF cap seems to do the trick.