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LM3502 open circuit protection

DanR
Prodigy 240 points

Hi, I have a customer with the following issue...

We are using a TI (National) LM3502 LED driver on one of our boards.  The device works fine, however, if the LED backlight is NOT plugged in, the device will smoke and die. According to the data sheet it has open-circuit protection. 

Can you contact someone at TI and let me know what to do?

Thanks!

Dan

  • 0
    TI__Intellectual 2060 points

    Hey Dan,

    Do you know which LM3502 they are using?  The 12V, 25V, 35V or 44V version? What is Vin? What is the ouput capacitor? Is Vout2/En2 being used? What are the voltages on Cntrl, En1 and En2?

    Regards,

    Mike

     

  • 0 in reply to Michael Mellitt
    Prodigy 240 points

    Thanks Mike. They are using the 44V version. I'm attaching the schematic...0842.Backlight.pdf

  • 0
    TI__Intellectual 1040 points

    Dan,

    The LM3502 does have an over voltage protection which should clamp the output at one of the predetermined levels.  However, there are a few reasons why the part could be getting destroyed when the LED string is removed. 

    1. The Schottky diode is getting destroyed because it is undersized and thus going open.  The open Schottky would then cause the device to get destroyed since it is not there to clamp the inductor voltage at VOUT.

    2. The didoe is not a Schottky.  If this is the case than the diode being used might be too slow to catch the inductor current during the open load condition, thus causing the SW node to go higher than the absolute maximum voltage of the device.

    3. The output capacitor is undersized or not ceramic.  An undersized output capacitor can lead to excessive voltage ripple on out that can overvoltage VOUT1.  Undersized meaning that the nominal value is too low and/or the DC voltage rating is too low.   Also, the output capacitor must be a ceramic type.

    4.  The PCB layout has too much inductance in series with the diode, the output capacitor, and/or the GND bump.  Too much PCB inductance will cause excessive voltage spikes on VOUT1 and SW which can overvolatge the SW and/or VOUT1 bumps.

    Thanks

    Travis

  • 0 in reply to Travis Eichhorn
    Prodigy 240 points

    Thanks! It looks like the diode could be the issue...

    My diode is a Schottky and I assume that is my problem.  Currently 30V, 05.A.  I am changing to a 60V, 1A

    ...I'll let you know if that solves it.