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Need help in LM3407 LED driver for 3 Watt LED(Three 1 Watt LED in series, 350mA)

Rohit V
Prodigy 60 points
Other Parts Discussed in Thread: LM3407, LM3414, TPS40211

I am trying to implement a LED driver using LM3407 IC to drive three 1 Watt LEDs connected in series(350mA). The circuit is not performing the buck operation as the input power is reflected at the output across the LEDs. I have attached the schematic and PCB layout herewith. Please suggest me modifications if any.

  • 0
    TI__Mastermind 25500 points
    I'm not exactly sure what you mean by the input power is reflected at the output across the LEDs. Do you mean it is out of regulation? Could you let me know what the LED stack voltage and input voltage is exactly? I do notice that you are switching at a very high switching frequency. That is fine for certain applications, but it limits your minimum duty cycle so if the input voltage is too high it will not regulate properly.
  • 0 in reply to Clinton Jensen
    Prodigy 60 points
    It is not performing the buck operation, that is because the input current and voltage is same as the output current and voltage. I am connecting three 1 watt LEDs in series, so the total output voltage should be around 9 V and the output current has to be constant around 350mA. So will the high switching frequency be a problem in this case?
  • 0 in reply to Rohit V
    TI__Mastermind 25500 points

    What is the input voltage you are using?

  • 0 in reply to Clinton Jensen
    Prodigy 60 points
    I am using 9V - 12 V supply.
  • 0 in reply to Rohit V
    TI__Mastermind 25500 points

    Does it switch and regulate at 12V? The actual output voltage will be 3*Vf + 200mV. If they are truly 3V LEDs (in my experience white LEDs are generally more like 3.2V to 3.5V) then your output will be 9.2V. Due to losses in the switch, diode, and inductor you will probably go into 100% duty cycle linear mode below about 10.2V and will be out of regulation. Obviously this voltage will go up if the LEDs are more than 3V. If you need to maintain regulation over the full 9V to 12V input range you will need a buck-boost converter.

  • 0 in reply to Clinton Jensen
    Prodigy 60 points

    Thanks for your explanation Clinton. In that case, I think the output should atleast be regulated at 12 V input, correct? But, in this case it is not able to.

    Also, it would be of great help if you can suggest a schematic for:

    1) driving three 1 watt LEDs(Vf of which can be above 3V) in series

    2) driving two 1 watt LEDs in series(as it seems that a input voltage range of 9V-12V will not be able to cater three 1 watt LEDs in series)

    Thanks.

  • 0 in reply to Rohit V
    TI__Mastermind 25500 points

    For case 2 the LM3407 would work great. If you don't need precise current matching and/or you don't mind putting a ballast resistor in series with each LED (improves current matching but lowers efficiency) you could use the LM3414 and drive the 3 in parallel.

    If you need to drive 3 in series with that input voltage range you will need a buck-boost such as a SEPIC or Cuk. The TPS40211 is probably the simplest that can cover that voltage range. You can look at PMP5541 and PMP8943 both of which are pretty close to what you need but probably have some extra circuitry you don't and could cut out.