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TPS40211 PMP4702

Hello,

Basically, I want to drive a LED strip with the design proposed in PMP4702.

But I did a slight modification by adding a PWM capability. Please accept my apology for the hand drawing. That was just the easiet and quickest way to do it.

So I was wondering whether this would work or not? Am I missing something?

Any feedback and advice would be appreciated.

Kind Regards

Martin Botsov

  • Martin:
    Although this PWM dimming circuit is in the TPS4011 data sheet, it may not work well for this SEPIC design. If the PWM dimming frequency is low, you can do PWM dimming using the EN pin. If you do that, you have to reduce the soft start time. If the PWM dimming frequency is high, you can do shut FET PWM dimming by put a MOSFET in parallel with the LED string.
    David Zhang
  • David,

    Thank you very much for your response.

    I thought of using the DIS/EN pin for PWM dimming at first. But unfortunately I already have a driving signal on that pin which shuts down the whole chip at particular circumstances. So, it's taken and I can't use it for anything alse. And that is how I came to this concept to use the proposed dimming circuit from the datasheet.

    "If the PWM dimming frequency is high, you can do shut FET PWM dimming by put a MOSFET in parallel with the LED string." By putting a MOSFET in parallel (as you are saying) with the LED string I would be able to interupt (by applying a PWM signal at the MOSFET Gate) the primary switching current path. Please correct me if I've misunderstood you.

    Forgive me my ignorance, but this raises an interesting question for me. If I have to switch the current in order to have the PWM dimming dunctionality, then why is the additional MOSFET in parallel with the LED string and not in series?

    I am really curious about this. I don't mean to disrespect you or offend you in any way. I have to manufacture this thing, so I am taking your advices and guidance seriously.

    Martin

  • Martin:

     

       LED strings have distributed inductance. If the MOSFET is in series with the LED string, there is a voltage spike after the MOSFET is switched off.That is why MOSFET in parallel is the preffered way of PWM dimming.

     

    David Zhang

  • David,

    If I have understood you correctly, your advice should practically look something like this.

    I've covered the initial circuit with a few white sheets of paper because I ran out of corretion tape. But basically I have removed the transistor, capacitor and the two resistors that I had put there at first.

    Now, this is a close up of the output with the transistor in parallel to the LED string.

    It's drawn with a pencil.

    And that is the whole circuit.

    Have I understood your advice correctly?

    Martin Botsov

  • Martin:
    The circuit is correct. 2N7002 is probably not the right MOSFET to do the dimming. It is a 300mA MOSFET.
    David Zhang
  • Dear David,

    I don't know how to say thank you for your responses, advices and guidance.
    You have no idea how much you have helped me. May be it's something usual
    for you, but it means a lot to me. Anyway, thank you once again.

    Oh, yes. I figured out that this transistor won't be a good fit. But I had
    already uploaded my post and so I left it anyway. I picked up another MOSFET
    that can withstand those currents. So, I am updating the design first thing
    tomorrow morning.

    Martin Botsov
  • Martin:

     

       You are welcome.

     

    David Zhang