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LM317: 3pcs devices work in parallel?

Paul Yeh
Genius 14725 points
Part Number: LM317

Tool/software:

Hi team,

My customer designed LM317 in their system but seems thermal problem, so they consider to use 3pcs devices to share thermal.

Can they just do parallel design? Or any circuit we need to modify?

  • 0
    TI__Expert 5270 points

    Hello Paul,

    You can ask the customer to follow the circuit from Fig 22 on the datasheet for paralleling this device. Please note that while the theory should work for your application, we have not actually attempted it. We highly recommend that you prototype your application before committing to it.

    In order to adjust the voltage, you will need to modify the resistor(s) on the non-inverting input for the amp. This is your R2 for the resistor divider. R1 is the 100 Ohm resistor. You will also have any voltage inaccuracies in your transistor and op-amp.

    Other than that, you need to make sure that your transistor and op-amp are sized properly for your application voltages.

    When you layout your board, it is important to minimize any resistance between the ADJUST pins of your LM317 regulators. Any resistance in these pins will reduce the current sharing accuracy.

    Please refer to fig 9 from the datasheet and request the customer to consider adding D1 and D2 since they are using a Cbyp and large output capacitor.

    Please also refer your customer to this TI tool for paralelling LDOs: PARALLEL-LDO-CALC Calculation tool | TI.com

    Best Regards

    Ishaan

  • 0 in reply to Ishaan Shetye
    TI__Genius 14725 points

    Ishaan,

    According to your suggestion, I should ask customer to change another suitable LDO.

    Thank for comment.

  • 0 in reply to Paul Yeh
    TI__Genius 14725 points

    Ishaan,

    BTW, my customer use below circuit for parallel application. And the testing result is ok.

    So my question is this circuit ok to use?

    Thanks

  • +1 in reply to Paul Yeh
    TI__Expert 5270 points

    Hi Paul,

    Yes, this circuit is okay to use.

    Best regards

    Ishaan