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LM3409HV: Too high power dissipation at zero dimming using shunt FET

Franco Bagnalasta
Prodigy 20 points
Part Number: LM3409HV
Other Parts Discussed in Thread: TPS92641, LM3409

Good morning,

we are designing a new RGBW LED driver for TV studio; we need very high dimming range, so we plan to use parallel dimming.

We have been using LM3409HV for years in EN dimming with very good results, so we decided to use the same for this project.

We need 5A output with 48Vin and 36Vout on LED string.

The board can be set for EN dimming or parallel dimming; in EN dimming power at no light is almost zero, but with parallel dimming it raises to about 3.5Watt.

Power on parallel FET is about 0.5Watt, the remaining is on Schottky, Pmos and inductor.

There is some suggestion to lower dissipation at no light output? Some app note?

Thank You for the attention

Franco

  • 0
    TI__Mastermind 25500 points

    Hello Franco,

    The point of shunt FET dimming is to get very fast rise and fall times in the LED current. The way to do that is to keep the inductor current regulated even while the LED is off so current is already flowing when the FET turns off. The downside to that is that all of the components still burn power since you are still switching and regulating current even when the LED is off. So the only real ways to lower the power dissipation is to use better components (lower Rds(on) for the shunt FET and PMOS and Vf for the diode).

    Be sure you optimize the Roff2 value as well. The ideal operation is to have the inductor current ripple the same when the shunt FET is on as it is when the shunt FET is off.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 20 points
    Hello Clinton,

    Thank You for your last reply (very effective!)
    I've lowered Vf and Rds for components, and the power at zero dimming is now 30% less, and I'm working to further reduce it.
    Do You think that using a synchronous converter like TPS92641 it should be possible to have better results?

    Just another question: I planned to use this driver in a project where multiple units could be paralleled to drive a higher power LED
    (for example three modules each 100W to drive a COB at 300W).
    For what I see LM3409 can be used in parallel configuration, but I'm not sure that the same is for TPS92641.
    Do You have some app notes for paralleling LM3409HV and (maybe) TPS92641?
    Is there some other TI driver that You consider a better candidate for this type of application?

    Thanks again and best regards
    Franco
  • 0 in reply to Franco Bagnalasta
    TI__Mastermind 25500 points

    Hello Franco,

    The TPS92641 would have better results since it is synchronous. The low side FET (or diode in the case of the LM3409) is conducting most of the time during shunt FET dimming so the losses could decrease significantly depending on the FET Rds(on). I am not aware of any app notes regarding paralleling either, but both of them can be paralleled just by tying the outputs together. As with any switcher it may require some additional filtering on the input with multiple devices running.

    Regards,

    Clint