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LM3478 Boost Start up Current

Charles Ladd
Prodigy 50 points
Other Parts Discussed in Thread: LM3478

I have a Boost LM3478 that on occasion (4 boards out of 100+) has turned on and drawn large amounts of current burning the weakest trace on the board (2oz 25mil, about 2 inches long). I have replaced the trace and done testing and have never been able to duplicate the problem.

There is no evidence of other damage - the switching transistor seems fine and still works properly, the down stream circuitry also works.

The only signal(s) that look a little suspicious are the switch voltage and switch current - both have a few tenths of a microsecond of ringing - the overshoot / undershoot of the switch voltage is 5-10 volts and the overshoot / undershoot of the sense voltage is a 0.5V

Any suggestion on what to look at or how to duplicate the problem?

  • 0
    TI__Intellectual 2225 points

    Hi Charles,

    It seems like there are definitely very high currents flowing for a short period of time.  In general, the overcurrent limit int he LM3478 should prevent that from happening, however, if the inductor reaches saturation, then the current can ramp up more quickly than the LM3478 can react.

    One thing to double-check would be the rating on your inductor.  Check to make sure that the saturation current of your inductor with the worst-case tolerance is higher than the worst-case current limit that is set by the sense resistor and the LM3478 Vsense threshold.  The highest Vsense is 190mV.

    Perry

  • 0 in reply to Perry Tsao96839
    Prodigy 50 points

    The sense resistor is 16 mOhm, therefore the worst-case current limit is 11.9A. My inductor is a DO5040 which has an Isat of 7.8A.

    So you suggest finding an inductor with a higher Isat.

    Is there any other way the FET could latch on at start-up?

  • 0 in reply to Charles Ladd
    TI__Intellectual 2225 points
    Yes, finding an inductor with higher Isat would work, or you could use a larger sense resistor to limit the current to a lower value.
    There is a tendency to hit current limit at startup especially if there is a large output capacitance. I can't really think of any other reasons why this would happen.