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LM5118: Buck Mosfet gate signal

Calin Bont
Prodigy 20 points
Part Number: LM5118


Hi TI,

I used WEBENCH to design a voltage regulator with Vin 5V to 30V and Vout 11.5V and 10mA current that is working as expected. The problem is that the signal of the buck Mosfet drive is not a clean pulse.  I am worried about long time performance. Where should I look for the cause of this.

Yelow - Vin; Blue - Mosfet signal

Thank You

,

  • 0
    TI__Mastermind 21720 points

    Calin,

    Is the blue waveform the measurement for the HO pin to GND pin? Just want to clarify. Also is is possible for you to post the schematic?

    For the first waveform it looks like there is a little bit of sub-harmonic oscillation occurring. I can tell this because there is one long pulse followed by a short pulse, indicating that there is not enough slope compensation. To increase the slope compensation the capacitor on the RAMP pin can be reduced slightly, increasing the amount of slope compensation. Use the quickstart calculator ensure that the current limit will not be triggered by decreasing the RAMP capacitor.

    One the second waveform is just appears that the load current is small enough that the converter is operating in discontinuous conduction mode (DCM). There is nothing wrong with this. It just indicates that the inductor current reaches 0A during the switching cycle. I can tell it is DCM because of the ringing that is present in the waveform before the next switching cycle. Here is information that will help explain this mode of operation.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 20 points

    I change Cramp from 470 pf to 220 pf and the signal is  fine in buck-boost mode.

    Thank You..