Hi,
Anyone know anything about this part. I have a design with 2 of these. The first is a standard 5V boost, 0.3A. That seems to work fine. The second is 10W flyback giving +/-70V on the output. This uses a transformer. I'm blowing up max668s like they are going out of fashion :-(. One board now has damaged tracks so I've moved on to board 2. On this board I made sure the 70V psu was working before fitting the CPU etc. It was working fine. I loaded the supply and took measurements. But first time I tried the supply after fitting the CPU it has blown up.
The odd thing about the way it fails is that the feedback pin takes too much current. The current "blown" part takes about 2mA into the feedback pin. I've taken out the external mosfet and put a 10K to the feedback node to a bench supply. The max668 output is running at 80% as expected. When I increase the bench supply the feedback pin voltage increases until it hits about 1.25V (Vref) then the output drops to 0%. All as expected, except that I'm pushing ~2mA in and the FB resistors are 18K and 1M and the feedback pin should only need
20nA.It is the high current that causes the problems. In effect the feedback fails, the transformer is driven flat out and the mosfet blows up.
I don't understand what is causing this failure and it is odd that the max668 works, except for the feedback pin current. The only thing different from when I had it working is the addition of the CPU. The on/off pin is now driven hard from a CPU port rather than a 100k pull down and 10K link to 5V. Could a port blow a max668?
TIA
Malcolm