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LM3642: No output from LED pin when I program LM3642 to torch mode

jilai wang
Prodigy 20 points
Part Number: LM3642

Hi,

I'm using LM3642 as the LED driver for the camera, and programed below two registers

0x2 --> Enable Register (0x0A)  

0x70 --> Current Control Register (0x09)

but when I measure LED pin using the scope, the output is 0V.

I have measured the voltage on pin "IN", it's 2.8 V. And all I2C registers have been programed properly.

Do i still missed something?

Thanks,

Jilai

  • 0
    TI__Intellectual 1940 points

    Hello,

    I have a few questions for you.

    • Do you have the current limit on the supply high enough to support the current?  If the level is set too low you can trip UVLO or even the onchip power on reset when you enable the part.
    • After you enable the device, do any of the flags in register 0x0B get set?  
    • If you raise VIN to something higher than 2.8V do things behave?
    • Are you seeing this issue on an EVM or is in on a customer board?

    Providing answers to these questions will help in figuring out what could be going on.  Thank you.

    Greg

  • 0 in reply to Greg Lubarsky
    Prodigy 20 points

    Hi Greg,

    for your questions:

    1. no, I don't have current limit.

    2. I checked register 0xB, it's 0.

    3. No, it's controlled by the voltage regulator, so should be constantly 2.8v.

    4. this is a customer board.

    My major concern is that if my register setting is OK. If yes, then I will suspect that the board itself has problem.

    Thanks,

    Jerry

  • 0 in reply to jilai wang
    TI__Intellectual 1940 points

    Hi Jerry,

    If you probe the VIN when you enable the device, does it dip at all?  Also, after you try to enable the torch mode and nothing happens, if you read register 0x0A, does it still read the correct value?  Have you looked at the switch and output voltage nodes when starting up?  VOUT should start at 0 and then rise to a value.  Can you share the scope photos so I can see what happens at the turn on instant?  Thank you.

    Greg