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LM3409 gate signal issue

Jerry Hsueh
Expert 2730 points
Other Parts Discussed in Thread: LM3409

LM3409 circuit parameters are Vin=53V, Vo=40V, L1=220uH, Coff=2.2nF, Roff=82kohm. 

while frequency below 30kHz, the work is normal, but frequency more than this. Q1 driver signals will be abnormal as below, will be from Low to High(red circle), normal should be Low. Could you help to explain this phenomenon?

I want to increase the frequency, so Coff or Roff value need to become smaller, then if I reduce the value of L1 inductance, this problem will not happen, the work is normal. But this problem will occur without reducing the L1 inductance value. I want to increase the frequency, and keep the value (220uH) of inductance. How to adjust? Do you have good suggestions? Thank you.

  • 0
    TI__Mastermind 25500 points

    Hello Jerry,

    I'm out of the office currently but I can look into this more or forward it on. But I wanted to check on some things. Have you checked the datasheet to make sure you are not exceeding the minimum voltage ripple? It specifies that you need at least 24mV ripple across the current sense resistor (CSP-CSN) which sounds like a possibility since it is working ok at lower frequencies or inductance values. If the ripple gets too low the comparator will misbehave, so if the LED current ripple needs to be lower than the inductor current ripple you may need an output capacitor to accomplish that. Do you have analog and/or PWM dimming requirements for your application? Let us know and we can try and help further. 

    Thanks,

    Clint