One of our terminals requires lighted buttons and on this particular board we are starting to see Leds failures.
I have included a page of our schematic which has the circuit driving the leds which is from your app notes and also the led spec.
The only change on this board from an earlier version is a reduction of number of leds from 4 to 2 in each circuit.
Annual usage of the LM3402MM is approximately 65k to 130k.
We have gone back to our pcb vendor to determine if there was a process issue and also to remove the leds and send them back for failure analysis.
Optek the led vendor came back with the following report:
“Electrical test showed these returned LEDs were short & leakage current. After chemical decapsulation, optical inspections found
burn mark existed on die surface where located at surrounding of metal trace and bond pad edges.
Burn mark encountered on die surface has led to conclusion that LED being experienced a high current spike or voltage spike.
LED failure was caused by EOS (electrical overstress), event that had occurred at post manufacturer process.”
The fact that the leds have shorted instead of opened, does your chip still protect for this type of failure?
If you need any more information please contact me.
Thank you
Joseph Gaccione
GTECH Corporation
55 Technology Way
West Greenwich RI 02817
401-392-2063