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LM5118 Ilimit varies

NORM NOVOTNEY
Prodigy 30 points
Other Parts Discussed in Thread: LM5118

Problem: LM5118...when VCCx is changed from ext 5V to ext 12V, the current limit/knee changes from about 2.5 amps to about 1.9 amps The Vin is about 11.8V and vout is 27V . Using a zener drop to feed it back to Vccx  ...should Vccx affect the current limit? using a 0.028 ohm current sense resistor.  I do note at 1.9 amps there is about 150mv p-p ripple

  • 0
    TI__Guru 58941 points

    Hi

    LM5118 current limit changes by changing duty cycle. Duty cycle changes by Vout / Vin ratio. Please refer 'Typical Performance Chanracteristics' on the page 8 of LM5118 datasheet.

    Regards,

  • 0 in reply to EricLee
    Prodigy 30 points

    Eric:

    The Vin is not changing, it is around 11.8V, Vout=28V...What is changing is VCCx, the internal/external regualtor input.  When it is connected from 5V to 12V, the current limit goes down.  Any reason?   The 5V is clean, but perhaps the Vccx=12V has some ripple. 

    Also, does Vccx affect the gate drive?  That is, not connected vs 5V in vs 12V in on VCCX?

     

     

     

  • 0 in reply to NORM NOVOTNEY
    TI__Guru 58941 points

    Hi

    Current limit can be changed if Rramp is connected to VCC / VCCX.

    Driver strength has proportional relationship with VCC/VCCX voltgae.

    Regards,