What are typical failure modes for the femtobuck circuit?
I’m running 5 identical femtobuck circuits from the same power supply. The power supply is a cheap boost converter from AliExpress, set to 30V output, powered by a 3S LiPo battery (i.e. around 12V input).
One of the femtobucks went kaboom when turning on (using a switch that disconnects the battery from the boost converter), with visible sparks. The other four are still working properly. I’m measuring a short (about 200mOhm) between the V_in and SW pins of the AL8805, as well as the SET and SW pins, which happens to also be the pins on either side of the 40V Schottky diode (MbRA140 according to the schematic).
I’m in South Africa, and I can’t seem to find a local supplier for new femtobucks, so I need to debug this.
Before I start desoldering the femtobuck: Which is more typical – the schottky diode failing as a short, or the AL8805 failing with a short between V_in and SW and SET?
Any idea what could have caused this? Maybe a turn-on spike in the boost converter’s output voltage?
I’m measuring a short (about 200mOhm) between the V_in and SW pins of the AL8805, as well as the SET and SW pins, which happens to also be the pins on either side of the 40V Schottky diode (MbRA140 according to the schematic).
It should read something like:
I’m measuring a short between the V_in and SW pins (which happens to also be the pins on either side of the 40V Schottky diode), as well as between the SET and SW pins.
So I desoldered the AL8805 and the Schottky diode, and tested both out of circuit. Both were blown. These components are not readily available in South Africa, so I substituted them with the following:
Diodes Inc ZXLD1350ET5TA 0A35 LED Driver
Vishay BYS10-45-E3 45V 1A5 Schottky Diode
It’s working very well now. There are some slight differences in the LED drivers, but they are pin compatible. Most notably, the replacement LED driver has: