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TL4242: Possible to use as current sink without LEDs?

David Dean
Expert 2305 points
Part Number: TL4242

I am wondering if it is possible to use TL4242 as a constant current sink but without actually using LEDs.  I believe I would do this by simply connecting Q directly to REF.  It seems like this should work, and the TL4242 would drop all of the voltage from VIN to REF.  One concern is that the datasheet mentions protection from short circuit, although gives no further details.  What I would be doing is essentially shorting out the LEDs.  What is the short circuit protection?  Does it simply make sure the part will not overheat?  Or does it reduce or disable the output? 

  • 0
    TI__Genius 13001 points

    Hi David,

    TL4242 is a constant current source. Device controls the output current  though the voltage feedback based on current sensing resister, so there won't be any current flowing through PNP transistor if tie Q and REF together.

    Could you provide more application information or your design goals? Then we can recommend proper solution.

    BR,

    Robin

  • 0
    TI__Mastermind 22745 points

    Hi David,

        First, yes, you can use the TL4242 as the constant current source without LEDs.

        For the short protection, it will be limited by the internal current limit and the output will not be disabled until thermal shutdown.

    BR

    Sean

  • 0
    TI__Mastermind 22745 points

    Hi David,

        I'm sorry that you can't connect "Q" and "REF" together, this device can only support the current source structure and the load should be placed between "Q" and "REF".  You can share what's your detail requirement and we can help to provide some suggestions.

    BR

    Sean

  • 0 in reply to Sean Zhou
    Expert 2305 points

    First, I am a little confused between these two replies.  Can I use the TL4242 without LEDs, are not?  

    Why would there be no current through the PNP if Q and REF are tied together?  Is there some minimum LED forward voltage required?  If so, why?  Perhaps I could use a dummy diode there to maintain whatever minimum voltage is required while keeping most of the power dissipation in the TL4242?

    I am basically trying to make a zener diode with some sort of current limit and thermal protection.  My current design has a simple zener in a relatively large SMB package, and I would like to replace that with a smaller, thermally-enhanced package, and with thermal protection in case the temperature gets higher than expected. 

    My plan to do this with the TL4242 is to set the current to about 250mA and use a small zener (along with a resistor, FET, etc) to enable the TL4242 when VIN is above my zener voltage, and disabled when below the zener voltage.

    I am open to recommendations if there is some other type of component that I should be looking at.

  • 0 in reply to David Dean
    TI__Genius 13001 points

    Hi David,

    Do you want to apply TL4242 as a current flowing path when VIN is above Zener voltage? That also means Zener will parallel TL4242, is my understanding right?

    If so, will the Zener be cut off once TL4242 begins work?

    BR,

    Robin

  • 0 in reply to Yuting Shao
    Expert 2305 points

    Yes, a low-power Zener would be in parallel with the TL4242.  Here is the general idea:

    This is basically the same as how you might use a BJT to make a high-power Zener:

    High power equvalient for a zener - Electrical Engineering Stack Exchange

    But instead of the simple transistor, using the TL4242 as a "smart" device - or, at least a device that has overtemperature protection.

  • 0 in reply to David Dean
    TI__Genius 13001 points

    Hi David,

    Understand.

    I have a simulation based on your design. It seems it's OK. But I also suggest you request a TL4242 EVM to have a further bench test.

    One issue should be careful: power dissipation of TL4242 may lead to high temperature.

    BR,

    Robin