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LM3410: Suggestion to reduce undershoot

ikon
Genius 5355 points
Part Number: LM3410

Hi Support,

Using the LM3410 as the LCD backlight driver on the IVG, and attached is the switching voltage captured on the SW pin pin 6.
The undershoot is below 0V by more than 3V.

Could you help to advise whether his would cause any damage to the LM3410?
And any suggestion on how to reduce the undershoot?

Thanks.

  • 0
    TI__Mastermind 25500 points

    Hello,

    First of all are you sure the magnitude is real? There will always be some spikes at the switch edges but you have to be careful how you measure them since they are high frequency. If you have any sort of loop in your ground connection such as a ground clip lead noise will couple into it that the IC isn't actually seeing. You need to use as short a ground lead as possible and connect it directly next to the ground pin of the IC. A few turns of wire wrapped around the probe ground ring with the end sticking out to form a very short loop between ground and the probe tips works well. Differential probes work well also.

    However if it is real you could connect a schottky or an ultrafast diode between ground and the switch pin to prevent the voltage from going more than a diode drop below ground. You could also use an RC snubber from switch to ground which would reduce the spiking in both directions.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Genius 5355 points

    Dear Clint,

    Have tried using very short leads as ground for the probes, but the spikes are still there.
    While searching for more information on this, we came across (http://www.ti.com/lit/an/slva494a/slva494a.pdf) from TI explaining that such spikes are actually due to normal operation of the circuit and do not damage the IC.
    The minimum voltage rating of -0.3V or -0.5V actually refers to an externally applied DC voltage and not such spikes which is introduced by the inductor.
    Would the issue we see here be the same and the spikes observed should also not damage the LM3410?

    Thanks.

  • 0 in reply to ikon
    TI__Mastermind 25500 points

    That is correct. If you have a poor layout and they are extreme in magnitude and duration you might worry, but it doesn't look like you have that issue at all so you should not need to worry about damaging anything. You can reduce them as I mentioned if it concerns you, but I've never seen an IC get damaged that way with a clean layout.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Genius 5355 points

    Hi Clint,

    Thanks.

    In addition, could you advise whether there is any data on how much the LM3410 built-in MOSFET body diode forward voltage is?

    And how much undershoot is considered the norm?

    Thanks.

  • 0 in reply to ikon
    TI__Mastermind 25500 points

    Hello,

    I'm not exactly sure, but the body diodes are usually around 1V. Had I thought about it a little longer though I would have realized that this is a low side switch device. That means if there is undershoot the FET will be on and absorb the energy so you will not damage the body diode. In any case typical spiking is in the 1V to 3V range usually.

    Regards,

    Clint