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LM5045: SS&SSSR @MAX Capacitance

T Kishi
Genius 4425 points
Part Number: LM5045

Hi

My Customer has question.

Increasing the capacity value of SS and SSSR at the customer occurred, SSSR should start to rise at SS = 2 V,
but a phenomenon occurred in which the SSSR starts to rise at SS = 5 V.
Please look at the attached.

The difference is only the capacity value of SS and SSSR, but is there an upper limit of capacity value?

LM5045.pdf

Best Regard

T Kishi

  • 0
    TI__Mastermind 34465 points
    Hello T Kishi,
    I think whats happening is this.
    SSSR becomes active when the feedback signal into the COMP pin is less than 800uA (and also SS=2V )
    You have set the soft start time to 100mS with a 1uF cap and your control loop may have some non-linearity that requires a larger error signal to generate 200uA.
    You can test for this if you can disconnect your feedback path so that ICOMP = 0mA.
    You should then get a synchronous rectifier drive signal when SS>2V.
    Regards
    John
  • 0 in reply to John Griffin1
    Genius 4425 points
    Hi John

    Thank you for reply.

    I will check with customers.
    If the COMP has been affected, what is the maximum capacity that can be connected to the SS pin?

    Regard
    T Kishi
  • 0 in reply to T Kishi
    TI__Mastermind 34465 points
    Hi T KIshi,
    That number is not specified but I think the 100mS you are expecting is an unreasonable number for any power supply.
    Remember you are looking at the synchronous rectifier start time.
    Synchronous rectification has no benefit during startup because the output currents is so low.
    The power supply will still start up through the body diodes of the FETs.
    If you want the soft start time you need to look at the gate drive signals.
    John
  • 0 in reply to John Griffin1
    Genius 4425 points
    Hi John

    As you say, 100 ms is too big. I anticipate that at most 10 ms is the limit, but customers are looking for the largest figure.
    Could you please cooperate?

    Regard
    T kishi
  • 0 in reply to T Kishi
    TI__Mastermind 34465 points
    Hi T Kishi,
    You are welcome.
    Regards
    John