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LM5575: There is no output

Jingtai
Genius 4575 points
Part Number: LM5575

Design : 4856155/81 LM5575MHX/NOPB
LM5575MHX/NOPB 24.0V-24.0V to -15.00V @ 0.5A

The PCB has finished but there is no output across Cout. I checked Vcc that is 7V. I didn't find out other check values from the datasheet. How is it checked to figure out causes?

  • 0
    TI__Expert 4100 points
    Hello,
    Please try to go through following steps:
    1. make sure Vin is present and stable
    2. check SD pin and verify that device is enabled
    3. place oscilloscope probe on SW pin and verify that device is swithcing
    4. check voltage on FB pin

    For all the signals above try to capture waveforms.
    Also if you could provide schematic of you design that would be helpful.
    Regards
    Brani
  • 0 in reply to Brani Dubocanin
    Genius 4575 points

    Thanks for your support.

    Vin=24V

    SD is not connected.

    FB=0.63V

    SW

    The schematic from WEBENCH

  • 0 in reply to Jingtai
    TI__Expert 4100 points
    From your screenshot it looks like the part is not operating. There is no activity on switch node other than some 50Hz noise you are showing.
    Suggest trying following:
    1. Enable SD pin by either pulling it high to ~2V (using an external power supply) or connecting it to Vin through appropriate resistive divider and observe part behavior. Make sure not to apply voltage higher than 14V to this pin. It will damage device.
    2. measure voltage on VCC pin
    3. with SD pin pulled high measure voltage on output and FB pin

    Regards
    Brani
  • 0 in reply to Brani Dubocanin
    Genius 4575 points

    Thanks for your support.

    I'll try to connect the voltage to SD but it  is not easier doing it on the ready PCB. The WEBENCH design guides me to wrong way.