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LM5143-Q1: 4 phase single output LM5143-Q1 stuck with a 5V output. Why??

Craig Beiferman
Prodigy 250 points
Part Number: LM5143-Q1
Other Parts Discussed in Thread: LM5143, , CSD18563Q5A

LM5143-Q1.pdf

Can someone check if my schematic is correct?  I have 48V input, and wanted either 24 or 36 Volt output. (selectable via Q6)
But for some reason, the output is always 5V.

  • 0
    TI__Mastermind 49317 points

    Hi Craig,

    The likelihood is that FB is measuring low at startup. Please ensure the Thevenin resistance at FB is > 5k.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 250 points

    LM5143-Q1_rev2.pdf

    Tim,

      Thanks, I updated the schematic so the Thevinin resistance was > 5K (as shown in the upper left of the attached schematic)
    However, now I am getting no voltage out, and the PG1 of the master controller is constantly reporting a phase error.
    The other 3 phases do not report an error? Any Ideas what this could be?

    Thanks,

        Craig

  • 0 in reply to Craig Beiferman
    TI__Mastermind 49317 points

    Hi Craig,

    Check your design using the LM5143-Q1 quickstart calculator. For example, a 33mOhm shunt gives a 75mV/33mOhm = 2.3A current limit per phase...not 7.5A as expected.

    Also, check the soft-start time as the current required to charge the 4 x 33uF to 24V could be significant and may engage current limit. Also, the RT resistor on the second IC should match the first, i.e. 27k for 800kHz.

    Additional comments:

    1. Some ceramic output capacitance e.g. 10uF/50V is also useful to filter the inductor ripple current at this switching frequency as the output electrolytics have parasitics and may not be totally effective at that frequency.
    2. Note that two phases may be enough here for a 24V output with 30A current limit, especially if a lower switching frequency (e.g. 400kHz) and 5 x 6mm MOSFETs (CSD18563Q5A) are feasible.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 250 points

    revX2.pdf

    Thanks Tim,

    I attached my circuit to a electronic D.C. load set at 15 Watts, so I would not be affected by a current limit.

    I made both of the Rt resistors = 27K

    I changed my softstart capacitor from 0.1uF to 2.2uF to allow more time to turn on.
    The soft start voltage ramps to about 125mV, and then turns off??? I thought it should climb to 0.6 volts? (see attached sawtooth image)

    The other image is my Vout? It jumps to about 2.5 Volts, and then starts decaying??

    Any ideas?

  • 0 in reply to Craig Beiferman
    TI__Mastermind 49317 points

    Looks like current limit is engaging at startup. Is 33mOhm the correct shunt value? This seems very low.

    Maybe try disabling some phases for debug purposes. Please send a completed quickstart calculator file for this design. I can also review the layout if needed.

  • 0 in reply to Timothy Hegarty
    Prodigy 250 points

    LM5143-Q1 Quickstart Tool - revA9.xlsm

    Tim,  I changed my current shunts to 0.004 Ohms (I had these on hand), but its still overcurrenting.

    But the actual requirements are that I need to switch the output to either 24V or 36V (using my microcontroller)

    at 24V @ up to 480 Watts output

    or 36V @ up to 900 Watts output.

    Attached is my quickstart calculator, now that I looked up some of the ESR of the capacitors, it looks like I am out of spec.I am guessing that electrolytic capacitors are not going to work for this application?

    Also, I am confused about the input and output capacitor calculator, is the calculator for a single capacitor at each output phase (4 output capacitors), or a single capacitor for all of the combined phases. (1 output capacitor?)  Maybe I have way too many capacitors.

  • 0 in reply to Craig Beiferman
    TI__Mastermind 49317 points

    Yes, you need some output ceramics to take the inductor ripple current. The quickstart takes the total Cout, not the per-phase value. Try testing with 1 or 2 phases for debug purposes.