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Current sharing with e.g. LM5642(x)

Niclas Arndt
Intellectual 425 points
Other Parts Discussed in Thread: LM2642, LM5642, LM5642X

Hi,


I would like to connect one or two DC PSUs to a buck switch in order to get both voltage level conversion and current sharing into 12V, 2*15A.

Am I right to suppose that a dual current-mode controller (e.g. LM5642, LM5642X, or LM2642) could be used? (Or two separate ones?)

This differs from the standard one-output design of LMx642 in the sense that the two V_in are separate.

The two DC sources are identical.

It would be a plus if it were possible to have only one DC source connected. (If possible - which controller channel would have to be used?)

Grateful for your input.

Thanks in advance.

Best regards

Niclas

  • 0
    TI__Expert 3535 points

    Niclas,

    You are correct, you can use one LM5642 dual channel controller, and parallel the outputs.  You can refer to the application note AN-1292 which will help you with the connects for parallel operation.

    Terry

  • 0 in reply to Terry Allinder
    Intellectual 425 points

    Thank you Terry,

    Looking at AN-1292 page 4: If I separate the two channels' inputs and only connect one of them to a PSU, would the output creeping up through the un-powered channel's top FET's parasitic diode cause the un-powered channel to act up even if I disable that channel by pulling its ON/SSx low?


    Cheers

    Niclas

  • 0 in reply to Niclas Arndt
    TI__Expert 3535 points

    Niclas,

    The parasitic diode will be reversed biased, so the only current will be the leakage current.  The leakage current should be very small.  You have output resistive divider for the feedback signal which should act as a load and prevent the leakage current from biasing up the output.

    Terry

  • 0 in reply to Terry Allinder
    Intellectual 425 points

    Terry,

    This is true for the bottom FET, but the parasitic diode of the top FET will be forward biased, leading current from Vout into RSNS, KS, and ILIM of the channel that is unpowered.

    The question is if this will damage the LM5642 or if it will cause erratic behaviour even if ON/SS is pulled low.

    Cheers

    Niclas

  • 0 in reply to Niclas Arndt
    TI__Expert 3535 points

    Niclas,

    Your correct, sorry I didn't pay enough attention to the fact that you have two Vin sources, and one would be off.  I will need to get with the designer, I hope to be able to get back to you in a day or so.

    Terry

  • 0 in reply to Terry Allinder
    Intellectual 425 points

    Thanks Terry,


    No worries. This isn't a critical point - I could instead use one of the input connector pins to signal "PSU present". This could be used to control back-to-back PFETs that bridge the two inputs in case only one source is connected (in addition to controlling both ON/SS). It's a matter of BOM count and possibly efficiency at low load (by using only one channel).

    Cheers

    Niclas