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LM3421: How to increase Vout voltage and Iout current?

daniel
Genius 5095 points
Part Number: LM3421

Hi Team,

How to increase Vout voltage and Iout current?

Below is our schematic, Now the Vout is 32V.

Could you give us some suggestion to increase Vout to 34V?

And, what is the maximum Iout current can LM3421 support?

Thanks

  • 0
    TI__Mastermind 25500 points

    Hello Daniel,

    The output voltage is determined by the LED stack voltage since this is a current regulator. But if you need higher current you can disregard CSH and COMP, you just need to adjust R808 for the right output current.

    Then you need to calculate the peak inductor current at the minimum input voltage and use an inductor with a saturation current rating higher than that (at least 20% higher to be safe) and also size R809 for a current limit less than the inductor saturation rating.

    The RCT values only need to be adjusted if you need a lower switching frequency for thermal reasons due to AC losses.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Genius 5095 points
    Dear Clinton,

    Many thanks for your support.
    Now we use below component value, and measure Vout=32V, Iout=400mA.
    R808=0.1 ohm
    R809=0.1 ohm
    R803=12.4K (SCH)
    C802=100nF (COMP)
    L801=3A (2.5A)

    Please give us some modify suggestions if we want Iout increase to 600mA or 800mA.

    Thanks
  • 0 in reply to daniel
    TI__Mastermind 25500 points

    Hello,

    If you can send a BOM I could be sure, but design wise it looks like you should have 1A output current given component values. Have you checked the FET drain waveform and LED current waveform? Perhaps the 400mA is coming from an oscillation due to current limit or something else tripping because of noise? It could be a layout issue if so, because the design values make it look like 1A should be going through the LEDs.

    I would be happy to look at the layout for issues if you can provide it.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Genius 5095 points
    Dear Clint,

    Many thanks for your help.
    I am ask for the BOM list from customer.
    For this application, could you suggest TI part for these two MOSFET (Q802, Q803)?
    Vout: 34V~36V
    Iout: 0.8A~1A

    Thanks