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LM3424: Where should TREF, TSENSE, TGAIN terminal be connected to when thermal foldback is not needed?

Kazuya Nakai59
Guru 21515 points
Part Number: LM3424

 Hello guys,

 One of my customers is evaluating LM3424 on their own PCB board. At this moment, they don't need any thermal foldback. So they'd like to know how to ignore the foldbak function.

 Could you please tell me where TREF, TSENSE, TGAIN terminal should be connected to for ignoring of thermal foldback?  

 Your reply would be much appreciated.

 Best regards,

 Kazuya Nakai.

  • 0
    TI__Mastermind 25500 points

    Hello Kazuya,

    If not using the foldback function leave Tgain open, tie Tsense to ground, and tie Tref to VS. VS should also have some minimal loading to ensure stability so connect a resistor of 100k from VS to ground.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Guru 21515 points
    Hello Clint,

    Thank you for your reply.

    Could I ask you an additional question?
    The customer usage conditions for LM3424 are the following.
    VIN=24V, IOUT=3A, VOUT=about 40V(Depends on LED Vf)
    They are dimming LED with PWM signal which is input to nDIM terminal. The PWM signal frequency is 200Hz.
    When the PWM signal on duty is 100%, LED current is 3.0A. But the LED current is increased to 3.24A when the on duty is 20%. Also the LED current is increased to 3.40A when the on duty is 8%. They are not using the foldback function and their terminal conditions are TGAIN=OPEN, TSENSE and TREF are connected to GND.

    Have you ever seen the LED current increasing phenomenon?
    Could you please tell me any comment why the LED current increasing phenomenon is happen if you have?

    Thank you again and best regards,
    Kazuya Nakai.
  • 0 in reply to Kazuya Nakai59
    TI__Mastermind 25500 points

    Hello Kazuya,

    There is a minimum pulse width to keep the current in full regulation. This is described in the "PWM Dimming" section of the datasheet. But without your component values and exact operating conditions I can't be sure if you are close to that min tpulse. I would check that first, but sometimes increasing bandwidth can help a little as well (by reducing Ccomp and/or Cout usually).

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Guru 21515 points
    Hello Clint,

    Thank you for your reply and I'm sorry for my late response.

    Best regards,
    Kazuya Nakai.