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Original question:

LM5143-Q1EVM-2100: Modify EVM config to single output, two phase

LM5143-Q1EVM-2100: The SW waveform seems strange between 2 phase after I modified EVM to single output

Remy Zhang
Intellectual 900 points
Part Number: LM5143-Q1EVM-2100

Hi team,

I modifed the EVM board to single output mode, and connected DITH to VDDA in order to disable the dither feature.

I powered up the EVM board with Vin 30V and heavy load(10A), but the SW waveform from 2 phases seems strange, you can check the capture below.

After this problem occured, we tried to use the calculation tool to correct the loop/compensation of phase 1, I replaced R29 from 20k ohm to 6.8k ohm, and C53 from 1nF to 4.3nF, but the SW waveform didn't change from before.

Could you help to analyse the SW waveform and share your comments/suggestions? Thank you.

Remy Zhang

  • 0
    TI__Guru 67712 points

    Hi Remy,

    Please ensure the COMP2 network is removed and the SS and COMP pins are connected via the switches on the bottom side of the PCB. Also, note the EVM is setup for Fsw = 2.1MHz. This is very high at Vin = 30V in terms of switching losses.

    Depending on Vout, the min on-time may be reached, in which case pulse skipping can occur. This appears to be what's happening in the plot above.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    TI__Intellectual 900 points

    Hi Tim,

    Thanks for your comments, this issue is indeed related to the min on-time, after I changed Vout to 5V, the SW waveform changed back to normal, but there is another issue I found during the test:

    The rising edge and falling edge of slew rate of switching is slow at no load condition(ch2=Vgs ch3=Vsw):

    If we put heavy load to the output(starting from ≈2.4A)，the slew rate go back to normal as below(ch2=Vgs ch3=Vsw):

    Could you kindly share your comments about why the slew rate become slow both in rising edge and falling edge? Thanks a lot.

    Remy

  • 0 in reply to Remy Zhang
    TI__Guru 67712 points

    Hi Remy,

    This is expected at light loads as the inductor ripple current slews the switch voltage (and switch-node capacitance) during the deadtime.

    Regards,

    Tim