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LM5122-Q1: Unstable operation

Matt Thompson
Prodigy 10 points
Part Number: LM5122-Q1

I'm trying to get this 48V 2A boost stabilized. It starts to behave erratically and singing when the load approaches 1A, showing small/large pulses on the SW pin which would indicate sub harmonic oscillation. I've tried reducing Rslope to 90k with no improvement.

SW @ 300mA load

HO @ 300mA load:

LO @ 300mA:

Unstable operation @ 900mA load:

SW:

HO:

LO:

I've also noticed that the switching frequency is about 10KHz higher than it should be at the SYNCOUT pin. The Rt resistor is 37.4k (verified correct part is installed) which should yield a Fsw of 240kHz, but it's measured at 253kHz as shown here.

 

Any advice would be greatly appreciated.

  • 0
    Prodigy 10 points

    I removed C12 on the CSP/CSN low pass filter, and stability has greatly improved. It's not perfect yet, still acting up under some scenarios. Possibly noise on CSP/CSN?

    Why would that filter be causing this issue? The rolloff of that filter should be up >1MHz.

  • 0 in reply to Matt Thompson
    TI__Genius 10375 points

    Hi Matt,

    Thank you for using E2E forums, regarding your query it seems that you have stability issues, I would suggest to recalculate your slope compensation and then also check your COMP circuit.

    I am not sure about the C12, that should not be the cause of the issue and if there was noise that capacitor should be helping remove that noise.

    Could you maybe share the layout is possible?

    BR,

    Haroon

  • 0 in reply to Haroon Wardak
    Prodigy 10 points

    Hi Haroon,

    The supply is stable now without C12, but I'm going to validate the phase margin with an injection transformer.

    This is the layout:

  • 0 in reply to Matt Thompson
    TI__Genius 10375 points

    Hi Matt,

    Yeah a bode plot would also be nice or just taking some waveforms with load transients on your system to see how the stability is. I still think the slope compensation needs to be recalculated again as shown in the datasheet.

    Let me know how it goes.

    Thank you

    BR,

    Haroon

  • 0 in reply to Haroon Wardak
    TI__Guru 58225 points

    Hello Matt,

    As we did not hear back from you, I expect the answer was sufficient or you have been able to solve your issue yourself. If my assumption is wrong, please post below or create a new thread if this one is already locked.

    Best regards,
    Brigitte