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LM3409HV: LM3409HV IC heated and LED flickering issue.

Nicky Lim
Genius 3195 points
Part Number: LM3409HV
Other Parts Discussed in Thread: LM3409

Hello,

Make LED lighting circuit as attached.

Using 4pcs of  LM3409HV to drive 4color(RGBW) LED (Osram LE RTDUW S2W) and 4 LEDs are connected in series.

During ON test LM3409HV and FET(IRF5210(Q1)) are heated, and then LED flickering is occurred after 1 hour ON.

Could you check attached circuit and let me know why this LED Flickering is occurred?

I also attached LED spec sheet.

Regards,

Nicky

LE RTDUW S2W.pdf

  • 0
    TI__Mastermind 25500 points

    Hello Nicky,

    It sounds like a thermal issue to me, it usually is if it runs fine for a while then starts blinking. It's likely going into thermal shutdown. Are the part numbers in the schematic correct? If so I have two comments:

    1. The FET you are using has a very high gate charge. I know you are using a relatively low switching frequency, but with that gate charge you are still running near the VCC current limit of the LM3409. That could lead to some working and some not for beginners, but more importantly here with 48V input running near the current limit means you are dissipating about 1W in the IC just driving the FET. That is the most likely cause of the excessive heat in both the IC and the FET. I would recommend with this device to use a FET with a Qg of no more than maybe 60nC maximum, but the lower the better. It that input voltage the Qg is often more important than the Rds(on) for efficiency and thermals.

    2. The diode is also really oversized. I'm not sure if this contributes or not, but you generally do not want to really oversize the diode because the parasitics increase if you do. That can lead to noise issues and even more heating due to the noise generated. I would recommend using something like a 3A schottky for this design.

    I hope that helps.

    Regards,

    Clint