LT3763: bottom gate driver doesn't pulse

Dear Power by Linear,

    I'm having trouble with a buck regulator based on the LT3763 control IC. I'm using it as a voltage regulator, with an input of 54.0VDC, an output of 46.0VDC, and a maximum output current of 10A. When I apply input power, the output voltage reaches 46.0VDC like it should, but then it immediately begins to drift downwards, and eventually settles at around 12-15VDC. All of my testing is being done with no load at the output. The switching frequency is 160 kHz. I've checked many different waveforms and compared the circuit against an LTspice simulation that's as accurate to real life as I can make it. I found that the BG pin doesn't pulse - not even during soft-start. In LTspice, even with no-load, the BG pin pulses periodically to refresh the bootstrap capacitor for the high-side FET drive, but since the BG never pulses, eventually the boot cap in my real circuit discharges, and so then the output voltage drifts downwards.

Ch.1 = Vout, Ch.3 = BG pin, Ch.4 = inductor current

Ch.1 = Vout, Ch.3 = TG pin (WRT GND), Ch.4 = inductor current

These captures of the startup look nearly identical to the LTspice, except that in LTspice the BG pin pulses during soft-start and then gives minimum pulses for bootstrap refresh each switching cycle.

The LT3763 datasheet states that the BG drive is disabled when inductor current is less than 1/10th of its nominal value, so naturally I expect this circuit to be in DCM at no-load, but I can't understand why BG doesn't pulse during soft-start, or to refresh the boot cap.

I'm going to try increasing the frequency to the stated minimum of 200 kHz. But what else could disable the BG drive?

Many thanks,

chris



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[edited by: JValeriani at 1:55 PM (GMT 0) on 14 Oct 2019]