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LM5176: does not start-up when Vout set to 51V - breaks right after ramp-up

Part Number: LM5176

I am designing a power supply with an adjustable output voltage from 5.4V to 50.4V. The LM5176-based buck-boost converter should start in boost mode regulating to 50.4V. But right after turning on the input voltage it just ramps-up to the desired output voltage and then goes off and never works again. When setting the output voltage to the 43V (removing resistor between feedback and output of the DAC), the controller works fine. The device is run at the input voltage of 32V and with no load at the output. The schematic of the device:

The layout of the device:

After using TI webench tool, I tuned the COMP and SLOPE components only for boost mode. The results where the same. I did sampled Vout (channel C1), COMP pin (channel C2) and FB pin (channel C3).

The first picture shows when Vout is set to 43V, it ramps up and regulates as desired:

And the following pictures show the failing of the LM5176 to regulate to 50.6V after ramp-up:

  • Hi Justinas,

    Thanks for reaching out with your question.

    When you say the LM5176 never works again do you mean it is damaged? Or does it work after the power is reset? If it appear to be damaged does there appear to be any pin damaged? Are all the start-up conditions when VIN = 32V?

    Can you please also provide waveforms of the SW1 and SW2 nodes during start-up? This will help to confirm that the LM5176 is switching properly.

    Can you please try removing the DAC from the circuit and change RC103 to set the output voltage to 50.6V? Maybe the DAC is causing some issue.

    Thanks,

    Garrett
  • Hi Garrett,
    by "never works again" I mean it's damaged:)
    I found the problem. It was related to the 150MHz ringing on the gates of the MOSFETs. I cut the trace and put 33R in serries - it now works as expected. BTW, the ringing on gate resulted in spikes of 30Vpp on SW2 - probably that's why controllers kept burning.