MP3428A synchro switch not driven properly

DC-DC Power Converters
1

Operating conditions:

  • circuit based on fig. 7 from the datasheet
  • Vin=12V (high power output battery)
  • Vout=19V@2A
  • t(on)=800ns
  • t(off)=1.47us
    The issue is, that the synchronous rectifier switch is not properly driven: the gate is driven only for about 120ns at the beginning of the off-time of the boost switch. Measurements have shown that the bootstrap capacitor gets charged to Vout+5V and stays that way all the time, so the problem is not about the bootstrap circuit. The IC itself would not drive the gate for the whole boost switch off time, for some reason, and I ran out of ideas what could be the cause. The PCB layout around the converter is nearly identical to the example from the datasheet, but with one difference, namely guideline 5 is not met, both AGND and PGND are connected to a single big ground plane, and the VDD capacitor is also connected to that ground plane. Could this be the source of the problem?
2

What was the load at the time? I have no experience with this part, but it is possible at light loads that it drives the output rectifier in “diode mode” to avoid discharging the output. I would try loading it up and seeing what happens

3

The test load is about 2 amps. I also measured the inductor current with a non-invasive magnetic probe, and it is sure that the converter works in CCM - no way the diode mode would be used while switching in CCM, since the off-time inductor current never falls to 0. The next step was to capture the exact operating point of the converter (Vin, Vout, Iout, ton, toff, L, Rload) and simulate it, which I did, and it confirmed CCM. It seems that the SR driver is kind of “getting tired” after 120ns of driving. It is suspicious that there is always these 120ns of drive signal regardless of line or load,…
Have not much ideas left what went wrong and where to look for error… As last resort I will replace the chip, but I don’t expect that it will help…

4

Maybe your bootstrap cap is wrong value or damaged. I would change that first as it is easier