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LM3421: LM3421T PWM dimming problem

switch
Expert 2655 points
Part Number: LM3421
Other Parts Discussed in Thread: LM3409, TPS92641

Hi ti team,

My client uses LM3421 for BUCK circuit.Have the following problem.

1,Stroboscopic problems occur with PWM Dimming,Analog Dimming is normal,Please help to analyze the reason.

2,When the PWM is dimmed, the output capacitance cannot be increased. If the output capacitance is increased, the PWM dimming will be abnormal and the continuous diode temperature will rise instantaneously.

The schematic is referenced below

  • 0
    TI__Genius 9630 points

    hi switch

    Do you have any idea why customer does not use LM3409 or TPS92641? they are also buck controller that can provide current up to 5A. The reason is that LM3409 and TPS92641 are more popular and commonly used. So I am wondering if there is any specific feature that only LM3421 has.

    back to the issue, first of all, would you mind sharing detailed schematic first that I can review schematic parameters first. 

    For Q1: what are the PWM dimming frequency,  Vin, VLED? pls let customer provide waveforms of VLED, ILED, nDIM, DS voltage of Q1.

    For Q2: pls clarify the capacitor values before and after changing. pls clarify abnormal PWM dimming also by waveforms shown above. 

  • 0 in reply to Mason Wang
    TI__Genius 9630 points

    hi switch

    I would like to resolve this thread if this has been resolved at your side. thanks