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LM3406: Shut Off time after VINS low & Length of first on time pulse

Jeroen Fortanier
Prodigy 10 points
Part Number: LM3406
Other Parts Discussed in Thread: LM3409, TPS92515

Dear Team,

I am using the LM3406 as a LED current source from a 24V PWM voltage.

I am using a diode in series with VIN and a connection from the anode to VINS as in the application. What I see is that the LM3406 keeps on PWMing until approximately 1.5us after VINS is below 70%. Is this normal?

I am PWMing the 24V to the circuit at 500Hz. When I slowly ramp up the 24V PWM, The first on time out of the SW pin is at first stable at approx. 1.7us. When increasing 24V PWM further the on time drops a bit and then, with 24V PWM stable, increases over the next couple of seconds to approx 3us. Obviously this can be seen in the LED brightness. Why does this happen and is there something to be done about this?

Thanks, and with best regards,

  • 0
    TI__Mastermind 28655 points
    Hello Jeroen,

    This part has compensation so it's trying to stabilize when it is first powered up (every time you send power to it). If there is a capacitor on Vin/Vins it will try to run until it reaches shut-off threshold. This part has a PWM pin meant to do what you are trying to do. If you need to be able to do this from the power source I would recommend switching to either the LM3409 or TPS92515 since they are hysteretic parts.

    Best Regards,