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LM5008: The LM5008 Can't output

Big Bill
Expert 1980 points
Part Number: LM5008
Other Parts Discussed in Thread: TPS7A4001,

Dear  All:

      I  use the LM5008 to  design DC-DC  Vin = 60---84V  Vout= 12V ，when my system into the sleep mode， the system supply power come from the TPS7A4001，once the sleep mode shutdown, the system power supply come from  LM5008，the issue is  when I  connect the  the  BAT to LM5008 Vin Pin in active mode， the output voltage is 0 , the  LM5008 can't startup, but i   cut the FB resistor R65  R66，from  20K,5.1K to   10K , 2.5K  the output voltage is normal that is 12V. why cause the result? Can you  help  to resolve the issue? Thanks~

  • 0
    TI__Genius 11040 points

    Hello,

    I believe the problem is related to the ripple at the FB pin.  The injection into the FB pin should require a serial resistance with C32 of a minimum 0.84Ohm.  Consequently by shrinking your feedback resistances, you are increasing the current through the Feedback resistors and any noise will be less likely to interfere with the FB signal which is smaller than expected due to the missing resistances.

    I also checked this theory with the Quickstart Calculator from the product folder, and I confirmed that the calculator also recommends a serial resistance with Cout:

    Please let me know if anything remains unclear.

    Best Regards,

    Katelyn