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LM3433EVB / resister on drain of bottom FET

S.Satoshi
Guru 21685 points
Other Parts Discussed in Thread: LM5111

I'd like to know the purpose and effect of the resister and bead located on the drain of bottom FET, like R17-23, and L3-10 at  "LM3433 10A to 40A LED Driver Evaluation Board".

I believe that this can reduce switching noise, however I would like to have your comments in case.

Best Regards,

  • 0
    TI__Mastermind 25500 points

    The bead/resistor combination is to reduce switching noise, mainly the noise caused by the reverse recovery charge of the high side FETs when the low side FETs turn on. However, it isn't simply a matter of noise. This is a high current circuit with substantial FETs, the reverse recovery of the high side FETs can cause a substantial spike well below -VEE that can damage the HS pin of the LM3433. The beads reduce the spike to protect the IC and the resistors are there to dampen any potential resonance between the beads and the parasitic capacitances of the FETs.

  • 0 in reply to Clinton Jensen
    Guru 21685 points

    Hi Clinton Jensen-san,

    Thank you for your reply.

    I understood that the bead/resistor it to reduce switching noise and also protect the HS pin from substantial spike noise.

    Best Regards,

    Sonoki

  • 0 in reply to S.Satoshi
    Prodigy 140 points

    Hi

    I have put all the components as per the design but still find that in some of the PCBs, Q3 or Q4 and their respective LM5111 pops off.

    After that although I still get around 10A, I am unable to regulate the current through the Pot

    Could you please tell me the reason for this and how I can rectify it

    Regards

    Himanshu

  • 0 in reply to Himanshu Jain
    TI__Mastermind 25500 points
    There is very little information here. But the pot used on those boards have had issues over time and have resulted in board damage due to unregulated current that can go very high and destroy FETs and drivers very quickly. Also some people have removed (or never had) the two pin shunt that should connect the POT and ADJ pins of J2 and that will have the same result. Make sure the shunt is there and connecting the pins. I would recommend trying a new and/or different pot or removing the jumper and driving ADJ directly if it is already there.