LM3488: Output FET permanently on

Hi Ken,

Thanks for reaching out with your question. Sorry to hear that you are having issues.

Is there any condition that changes after about 3ms of operation? For example does the output load turn on? Are you starting up unloaded or loaded? Also does this happen for every input voltage or does it change with other operating conditions?

Thanks,

Garrett

Hi Garrett,

The conditions on start up are stable. Initial testing was without any load. Tested with a combination of charged battery supply (7.6V and was well in excess of 5A being drawn with no load.)

The other supply was 12.8V current limited to 2.5A, and the current limit was being reached. (has a decent output capacitor and can deliver the peak current needed to start the supply)

I also tried starting with a 120R resistor as a load (should max out at 100mA), and the result was the same.

The output voltage peaks at approx 2.7V but drops back to about 2.2 volts and is steady. (for the time connected which is limited because things start getting very hot)

The input voltage to the circuit drops to 5.6V on the battery supply (batteries themselves reaching supply limits), and drops to 4.1V on the current limited supply.

I assembled a second prototype board to use as a comparison and have identical results.

I've used the LM3488 on several other projects without any real issues (selected again due to having stock here and I like its flexibility), however this is the first coupled coil one I have done.

As per previous designs it is based on the web bench design but I added the enable option as I need to be able to turn it off and on, on demand for maximum efficiency. (intended duty cycle of load is 10-15Sec once or twice a day at peak load and 2 hours at 1A every 6-12 months.)

Thanks

Keith,

So you are saying the only thing different between the two designs is the MOSFET Q2 used to disable and enable the device?

Thanks,

Garrett

Hi Garrett,

Yes that is the only difference between to two designs.

Regards

Ken

Ken,
Please try shorting out the MOSFET to see if that improves the issue. It is a possibility that something might be damaged on the board already.
Thanks,
Garrett

Hi Garrett,

sorry about the delay, I have just returned from 2 weeks away.

I had tried shorting the FET (Q2) to pull R18 directly to ground. This made no difference to the operation.

It appears the LM3488 tries to fire up initially, and the output switches as expected for the 3mS (approx 500nS on pulses). After the 3mS its stops oscillating, however the output is high rather than the expected low if there is an error.

Regards

Ken

Hi Ken,
Can you please check, the COMP pin, FB pin, FA/SYNC pin and the ISENSE pin voltages. Assuming the MOSFET is on all time the current limit should trigger, turning the MOSFET off.
Garrett