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LM3409: LM3409N Driver Design problem

Vidya Jadhav
Prodigy 140 points
Part Number: LM3409


I am facing an issue with the LM3409 driver. This buck regulator is not working. It just passes the input voltage (24V) to output.

Design Specification(I have Attached a Schematic)

Vin:24V, 

Vout : 12V

Current: 1.2A

I used a dimming through adjusting the voltage at Iadj pin.  Initially, i have connected the Enable Pin to 24V, later i have kept open. It does not have any effect on working of driver

As I have connected one Led strip(4 LEDs in series, Vf -3V/Led), so led strip should be ON as per design.but It is not working as output is 24V instead of 12V, so i have connected 2 Led strips in series and its working

also dimming of Leds are working very well.

My problem is that, as per design specification output should be 12V.

 I might be missing something obvious.

Any ideas on how to troubleshoot this would be very helpful.

Thank You.leddriverlm3409.pdf

  • 0
    TI__Mastermind 25500 points

    Hello Vidya,

    It looks to me like you have the drain and source reversed according to the schematic. If that is the case all you really have is a FET body diode from VIN to the output. If that isn't the case let me know and we can investigate further.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 140 points
    Thanks Clinton.

    I have reversed the Drain & Source connection, still the problem is exist. Output is 24V not 12V.
    I have checked without Load.
  • 0 in reply to Vidya Jadhav
    TI__Mastermind 25500 points

    Hello Vidya,

    Without a load connected that is normal operation. The LM3409 will turn the switch on and keep it on until the peak set current in the current sense resistor is reached, then normal switching starts. If the output is open the current never increases to the peak so the switch just stays turned on indefinitely (or until it is disabled). You need to try it with a load for correct operation since this is a current regulator and not a voltage regulator.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 140 points
    Yes Clinton you are right.

    But Even after connecting a Load, Output Voltage is 24V.
    The Peak current (1.2A in my application) flows through FET when Vadj is 1.24V and at same time Led brights at its Peak because of high current.
    For Vadj: 0-1.24V, current should vary from 0-1.2A and Output voltage should be near to 12V.
  • 0 in reply to Vidya Jadhav
    TI__Mastermind 25500 points

    Hello Vidya,

    Have you verified the forward voltage of the LED string you are using? If the string voltage is close to the input voltage it will run in 100% duty cycle mode and the switch will just stay on. If the Vf is close to Vin you could even have around the right set current.

    I ask because if you connected a 12V string with a 24V input and the FET stays on continuously your current would run away and the LEDs would blow up. The only way you could prevent destroying the LED load is by current limiting the input supply, and even then a 12V string would still be nearer to 12V.

    Regards,

    Clint