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LM5106: LM5106 HS Undershoot to around -6.6V

Kok Khuan P'ng
Intellectual 450 points
Part Number: LM5106

Hi TI,

I have a buck converter during start up showing -6.6V at HS pin to COM pin. All traces is direct and short to reduce the parasitic inductance. However, there is still visible undershoot of around -4V across the bottom MOSFET D-S and HS pin to COM pin. In LM5106 datasheet, its allowable HS-COM undershoot is VDD - 15V = 12V - 15V = -3V only. Is it OK to proceed with undershoot more than 3V since we do not see any part damaged under such operating condition?

Regards,

KK

  • 0
    TI__Mastermind 21555 points

    Hello KK,

    The datasheet does specify the HS voltage as you mention. In order to ensure the IC is not damaged we recommend you follow the datasheet ratings. The probing of the HS node should be with a short probe connection measured as close to the IC pins as possible. With high dV/dt you can see higher voltage spikes than actual if the probe connections are not short. Use the spring coil ground and probe tip connection for best measurement.

    The switching speed can be reduced by increasing the gate resistance which can reduce the HS voltage dV/dt at peak voltages. Also another option, if possible could be to reduce VDD to 10V which would increase the allowed negative voltage to -5V. This along with some small spike reduction will work.

    Confirm if this addresses your questions, or you can post additional questions on this thread.

    Regards,