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Issue at start-up with multiple LM3414HV

Matteo Dal Lago
Prodigy 20 points
Other Parts Discussed in Thread: LM3414HV

Dear all,

I'm using the LM3414HV for 24V 500mA LED drivers and I'm having some troubles when I connect multiple pieces of these drivers to a properly dimensioned 24V power supply.

The initial current absorption of these ICs sometimes lead the power supply to protection and then everything start to blink. In some configurations the blinking stops after few seconds, in some never stops. The only way to solve the problem is to change the power supply with one much more powerful than necessary.

The used schematic  is almost identical to that indicated in the datasheet. 

I'm thinking to modify the schematic by inserting some circuitry that pumps current in Radj at the start-up; the idea is to keep the output off until the initial transient is finished.

Does anyone has ever faced this issue or has some suggestion on how to solve it?

Thanks a lot!

  • 0
    TI__Mastermind 25500 points

    Using more input capacitance may solve the issue as it will reduce the peak of the current spikes drawn from the supply. You generally want more input capacitance with multiple converters anyway to reduce the possibility of interactions. But injecting current into Radj may do the trick. You could also hold the DIM pins low until after the initial transient and you could even stagger pulling them high to reduce stress on the supply.

  • 0 in reply to Clinton Jensen
    Prodigy 20 points
    Thanks for the reply.
    I've tried to increase the input capacitance from 10uF to 100uF but it doesn't work. Unfortunately I cannot scope the current transients but I've noticed some sort of hysteresis in the input operation:
    - the driver at steady-state absorbs around 500mA@24V;
    - I've connected it with a bench power supply set to 24V;
    - if I limit the current of the power supply at 900mA (or lower) the voltage at the input reach 13V and do not rise to 24V until I move the current limit up to more than 1A.

    If I limit the current to 600mA and slowly increase the voltage I can reach 24V without any problems.
  • 0 in reply to Matteo Dal Lago
    TI__Mastermind 25500 points

    I see. It sounds like it's slew rate related and you have a fast slew rate supply. In that case the extra capacitance hurts you at startup. It's likely the slug of current required to charge that capacitor along with the regulators starting up forces the supply into current limit. Keeping them off using DIM until the input is fully up should help and may solve the issue. If not it is simply due to the input capacitance. You could always add an inrush limiter at the input that will slow the slew rate and then heat up and become low resistance.