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LM5176: BUCK mode SW2 wave is not correct

Song Youwei
Prodigy 50 points
Part Number: LM5176


d72545b442d109f5ecc21b216807c6d3.pngimage.png

boost mode is correct, but buck mode is incorrect. SW2 wave not always high level, I don't know why.

  • 0
    TI__Guru 60741 points

    Hi Song,

    I assume this i the recharge cycle to keep the BOOT2 cap charged. 

    In 100% duty cycle mode the BOOT cap of the corresponding phase needs to be recharged from time to time to keep the voltage level high enough to properly open the MOSFET ( in this case the BOOST  high side).

    Yellow Signal in the scope plot above is VOUT - right?

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 50 points

    Hello Stefan,

    Yellow Signal in the scope plot is VOUT，VOUT is 12V，this is correct. In LM5176 datasheet, SW2 wave should be high level during BUCK mode. like picture below. but I am curious why my SW2 wave have some low level in BUCK mode. I try change C23&C25 from 100nF to 220nF, but still have low level in blue wave. Should I change C23&C25 to more next? 

  • 0 in reply to Song Youwei
    Prodigy 50 points

    Or could it be that the datasheet only shows a partial view? In true buck mode, the SW2 waveform must stay low for a certain period to recharge the bootstrap capacitor, no matter how large that capacitor is chosen.

  • 0 in reply to Song Youwei
    TI__Guru 60741 points

    Hi Song,

    the recharge cycles are depending on the leakage of the MOSFET and in case used pull low resistors on the MOSFET gate signals.

    They can be reduced with using larger Boot Cap (but would not go larger then 220nF for proper charge up at startup).

    The scope plots in the datasheet do not show this short recharge pulses as on the EVM the leakage is very small.

    Best regards,

     Stefan

  • 0
    Prodigy 50 points

    hello Stefan,

    STP80NF70, this is MOSFET I choosed. R6/R11/R5/R10 gate driver I choosed 30Ω. Please check if the selected values of the gate resistors R6/R11/R5/R10 are appropriate. Additionally, for the STP80NF70 operating at a switching frequency of 300kHz, what capacitance values should I choose for the VCC pin-to-ground capacitor C20 and the BOOT capacitors C23/C25? thank you!

    C361041_场效应管(MOSFET)_STP80NF70_规格书_WJ274579.PDF

    BR，

    Youwei Song

  • 0 in reply to Song Youwei
    TI__Guru 60741 points

    Hi Song,

    30Ohm is much to high and can lead to issues with the dead time detection of the LM5176.

    We recommend to not use higher then 5 Ohm gate resistors with the LM5176.

    Boot Cap of 100nF and Vcc cap of 1uF should be OK.

    Best regards,

     Stefan