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Analog signal on LM3401 DIM-pin

MikkoS
Prodigy 50 points
Other Parts Discussed in Thread: LM3401

Hello,

I'm troubleshooting a device that has LM3401 LED driver with an LDR-based ambient light switch. Some devices fail on the field and I found out a rather horrible design error (not me..).

The LDR is connected through a darlington + pullup, so the DIM-pin voltage will rise from 0 to VCC smoothly under suitable circumstances. How will the LM3401 react on this ?

Our options are that either the LED driver board or the LED itself are failing. To make things worse, the few failed devices are far away and it'll be a few days until we get them back.

  • 0
    Prodigy 50 points

    Found it - looks like optical feedback oscillator afterall. In suitable conditions when the ambient light goes down smoothly, the LDR sees some very small reflection from the IR light. The darlington completes the problem by destroying the hysteresis in the DIM pin.