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LM74700-Q1: Behavior in light load condition

Takashi Onawa
Genius 13200 points
Part Number: LM74700-Q1

Hi Team,

Could you tell me how LM74700 behaves in light load condition?

According to the datasheet, LM74700 controls FET's gate voltage to keep Vds to 20mV fundamentally. But I think it is difficult to keep the 20mV under light load like 100mA with normal power FET.

What kind of issues will come out if we choose 5mohm FET in such the use cass? 

Regards,

Takashi Onawa

  • 0
    TI__Genius 16305 points
    Hello Takashi,

    At Light load condition, MOSFET Vgs will be close to Vth of the MOSFET and can lead to turn ON/OFF
    Oscillation (Gate on/off) can occur at very low current, but this can be avoided by choosing a capacitor at the cathode with sufficient ESR.

    For a 5mohm MOSFET, oscillation can occur at less than 1mA and 100mA is not an issue.

    Regards,
    Kari.
  • 0 in reply to Karikalan Selvaraj
    TI__Genius 13200 points
    Hi Karikalan-san,

    Thanks for your prompt response on this.
    I understood there is a oscillation risk when smaller Ron FET is selected in light load condition.

    >For a 5mohm MOSFET, oscillation can occur at less than 1mA and 100mA is not an issue.
    Do you have any documents to estimate the oscillation threshold against Ron of FET? Or could you explain that?

    Regards,
    Takashi Onawa
  • 0 in reply to Takashi Onawa
    TI__Genius 16305 points
    Hi Takashi,

    Please note that oscialltion at very low light load currents, will not harm the output voltage if sufficient output capacitance is present.

    Oscillation is dependent on MOSFET Ron characteristics, Gate capacitance and Vth threshold.
    Simulation using Pspice with the MOSFET can show where the oscillation occurs.

    For example, with LM74700 driving MOSFET DMT6007LFG-13, it occurs at very low currents <1mA.
    At this low currents, if the output voltage will not see major droop due to output capacitance.

    For example, lets assume 1mA is where we see oscillation with DMT6007LFG MOSFET.
    If output has 1uF capacitance, we will see a droop of 1mV/1us.
    Oscillation frequency will be very slow of few kHz. So this will not affect the output.

    Regards,
    Kari.
  • 0 in reply to Karikalan Selvaraj
    TI__Genius 13200 points
    Hi Karikalan-san,

    OK, I understood.
    I will recommend Pspice simulation to my customer prior to their board design just in case.

    Regards,
    Takashi Onawa