Vin = 18 - 36 Vdc
Vout = 40Vdc I out range 0 - 4 A
Mode pin terminated with 93Kohm for CCM & Hiccup mode.
150KHz
L=12uH
If Vin is lowered below about 24Vdc and the output load is more than 2A, the HDRV2 (boost high side gate drive) stops working for a large number of units.
But some boards built the same way work OK to minimum Vin and max load.
This is non-destructive, and recovers and works fine if load is reduced or Vin is raised a bit.
There is 3A Schottky diode across S-D of the upper boost mosfet, and current flow thru this is continuous even when the upper HDRV2 stops, but power efficiency is unacceptable when the mosfet in parallel does not turn on.
It seems as if the unit is switching from CCM to DCM mode to prevent reverse flow of current by shutting off the high side boost gate drive. But Mode pin correctly measures 1.8Vdc. We suspected noise on the Mode pin was bumping it to a different mode, so added a 100pF cap across the 93K to reduce 500mVpk-pk noise down to less than 100mVpk-pk but no benefit from that.
Also suspected that maybe UVLO on the high side driver was preventing gate drive because of too low a bootstrap voltage. But the bootstrap voltage stays 6.5V higher than the SW2 node, as it should.
Bootstrap cap is 0.1uF Vcc bypass is 1.0uF Ripple is seen on Vcc when the bootstrap cap pulls low to have its charge replenished. Tried doubling up on the 1.0uF Vcc bypass, and even tried 10uF, but no benefit there either.
Vcc powers many function blocks inside the IC. Could noise on Vcc be upsetting those functions? Any other ideas for root cause that we can look for? Thanks.
