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LM3429-Q1: Output current limit, can't reach target output current.

Nick Winston
Prodigy 60 points
Part Number: LM3429-Q1

Hello,

I'm configuring my already built board using the LM3429-Q1 in the buck-boost topology with parallel LED strings. Baring any failed strings or thermal issues (I've solved those issues) here's my specs (My FET & Diode have well been oversized for this application so no worries there and my FET has a low enough Qg thus switching in time for the Fsw isn't a problem):

Vin: 40 - 65V (48V nominal)

Vout: 32.5 - 65V (Variable number of LEDs ranging from 9 LEDs to 18 LEDs depending upon application).

Iout: 4A (1A/string up to 4 strings in parallel)

L: 66uH (2x 33uH in series)

Cout: 4uF (4x 1uF in parallel)

Fsw: ~700kHz

Right now in my testing I'm unable to get more than ~750mA per string with 3 strings connected @ 9 LEDs per string (@ 48V Vin), and this is no matter how far I turn down my CSH resistance its as if I've hit some current limit. None of my components seem damaged.

Is there anyway to increase the current out of this?

Thanks,

Nick

  • 0
    TI__Mastermind 25500 points

    Hello Nick,

    I would need a schematic with values to be sure, but it does sound like it could be current limiting. The current limit is set by the IS pin and the resistor from IS to ground (I hope you are not Rds(on) sensing or you will blow the device when Vin+Vout is >75V). So you will need a lower RLIM value as described in the datasheet for such a high power level.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 60 points

    Can I send you my schematic privately through email so we can discuss it? Right now I'm only able to achieve ~0.6A/string at 16LEDs/string (Total VF of ~64V), however I followed your advice and was able to achieve the 1.1A/string at 9LEDs/string. Of course I'd like to be able to attain 1.1A/string.

    Thanks,

    Nick

  • 0 in reply to Nick Winston
    TI__Mastermind 25500 points

    Hello Nick,

    You are trying to get a lot of power. It should be achievable, but it may require an external gate driver. But sure, I would be happy to check out your schematic. It would really help to have a BOM and layout as well because there could be issues with one or both that could limit your ability to get the power you are trying to get. Send me a friend request and then I will be able to send you my direct e-mail address through a private message. Thanks.

    Regards,

    Clint