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LM3409: Unexpected short PMOS gate pulse after UVLO recovery

David Naviaux
Intellectual 590 points
Part Number: LM3409

Hi, I have a design using the LM3409 that is intended to allow PWM output current with a 10us risetime.

My design forces an UVLO condition to shut the output off and at the same time discharges the output capacitors with a MOSFET through a small resistor.  The other unusual thing is that I have set the off-time to be fixed by suppling a fixed voltage to the COFT capacitor through the COFT resistor.  

What is happening is that when the forced OVLO condition is released, the initial current pulse lasts only 2us even though the output current is nowhere near the programmed level.

In the scope capture below, the blue trace is the gate signal to the p-channel MOSFET and the green trace is the LED current.  The scope and LM3409 are getting pulsed 1ms on with a 100ms period.

Here is a clip from the schematic:

Here are the scope capture:

Any help would be greatly appreciated.  

Thank you,

David

  • 0
    TI__Mastermind 28655 points

    Hello David,

    It appears to be working correct.  I calculated how much current is generated with a 2 us pulse at 24V input (output at zero volts) and get 4.8A.  What is happening is the LM3409 is ramping the current to the peak current threshold.  That current isn't going to the LEDs at first, it has to recharge the output capacitors.  This is also why you see the gate pulses start out narrow (after the first wide pulse) and get wider.

    With 5A going to 50 uF it gets to 3V after 30 us which is about when LED current starts flowing.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Intellectual 590 points

    Thank you Irwin! 

    I see the only resolution to speed things up would be to reduce the output capacitance.  I'll have to examine the effect on the LED ripple current (also a critical number in the existing design).

    Thanks again and regards, David