max668 problems

Is the CPU "chattering" the ON/OFF pin?

Have you tried 100K between CPU and ON/OFF pin?

...Jim Thompson

What compensation capacitor are you using on the FB pin?. If this is not right, the circuit could oscillate beautifully...

Best Wishes

The CPU shouldn't be "chattering" the ON/OFF as I'm running with an emulator and switching it by hand at the moment. Plus, the shutdown pin is also sync so shouldn't mind chattering.

To answer roger

Currently 1nF+18k is the feedback but it was working before I added the CPU.

I've added a 33k in series when I replaced the blown part. My theory is that perhaps the GND at the MAX668 is lifting due to high currents and that means CPU 0V is below MAX668 0V so shutdown is taken negative and that blows the chip.

I've just replaced the blown part and check for shorts/continuity with a DVM so make sure it is all soldered right. I've left the mosfet out and use a bench supply via a 10K to feedback node to check the part is working before any big currents fly about. As expected enable it and output is 80% odd pulse train. Raise FB to Vref and it is a 0% pulse train. BUT, FB pin is taking 1-2mA and I haven't run this part in anger yet. No mosfet, no large currents. I work on a static mat area and the DVM I used to check shorts/continuity measures 0.4V, 0.6uA on another meter so why is this part broken?

Very odd.

Its easy to check the compensation: get a unit running, and use a FET to apply a step load (on and off) and look at the output voltage. If its somewhere between a critically-damped 2nd-order and a first-order response, the compensation is fine.

if you think your layout might be pulling FB below ground and breaking it (a distinct possibility), slap a schottky directly across the chip.

Cheers Terry

I don't see how. The layout is good and it is a 4 layer board with planes.

I have it working again. I built it very careful measuring with a DVM as I went. I especially check FB leakage before any voltage was applied. On this unit I did find 1k5 leakage on the CS pin. I checked a new part and that was ok so either this part had it before I soldered it in or soldering it damaged it. On the CS pin the leakage does not affect the performance as impedances are low. They are tiny parts and so tricky to solder but it is an odd failure mode and the are loads of other parts I have soldered without any problem. I even tried putting the iron on a new part in the same way to see if I could reproduce the failure, but there was no effect.

Very odd.

you may have fried it with static or heat then.

Cheers Terry