LME49724: Output not functioning

Hi David,

Have you tried different supplies with the same results (bench supply, battery, etc)? Is this bias variation also observed at Vocm pin?
Perhaps you could place an additional bigger capacitor on Vocm pin to improve PSRR.

Best regards,
-Ivan Salazar
Applications Engineer - Low Power Audio & Actuators

Hi Ivan,

I have not tried other 24V power supplies, but I will.

I have actually replaced the 0.1uF Vocm cap with a 1uF capacitor with no change of operation. I also added 100pF capacitors across R33 and R43 just in case there was a stability issue. Nether of the modifications fixed the problem. The 15mA of power supply current in conjunction with the 24V power supply will result in about 360mW of power dissipation if the part is enabled. If the thermal characteristics match the datasheet, the junction temperature would be at about 18C above ambient. However, I did not see the suggested PCB footprint in the datasheet, and my copper for the exposed pad is about 1/2 the size of the suggested footprint. But with a 150C maximum junction temperature, I would expect that even if it had twice the rated Rthja, the junction temperature would be 36C above ambient (71C at the maximum ambient of 35C), far below the maximum.

I did not see in the datasheet under what conditions the Rthja was specified, but I am assuming that it was for the suggested footprint. There is some voiding in the solder under the part that was observed via x-ray, but it was less than 25%.

The problem is that once the part heats up the very first time, it no longer functions and I am not equipped to replace the part (i.e. it has to be sent back to the CM to have the part replaced).

I have a thermal camera looking at the PCB when I first power it up and see that part heats up over a second or so (not so hot from the outside, but maybe 10C above ambient) then the part immediately cools down and does not function again.

David,

Did you got a chance to test with different supply as above?
In addition, could you share your layout so we could take a look. From your description it seems like device is somehow being damaged, is the device heating up even when there is no signal at the inputs?

Best regards,
-Ivan Salazar
Applications Engineer - Low Power Audio & Actuators

Ivan,

The problem turned out to be rather trivial  It was the result of soldering with resin based solder and flux without completely cleaning the PCB before testing it.  Once cleaning the boards with an Ultrasonic cleaner, the amplifier seems to be working on at least some of the boards that it previously did not work.

I also noticed that I had not followed the datasheet's recommendation on the PCB footprint.  In the next revision of the board, the PCB footprint will be as shown in the datasheet.

Thank you for taking the time to look into this.

Regards,

David