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TPS548B28: Pre-biased startup feature of TPS548B28

Xiaoying Lu
Expert 4290 points
Part Number: TPS548B28

Hi,

I am an FAE NCG and in my rotation program now. When I read the datasheet of TPS548B28, I find it supports pre-biased startup.

Can you help explain how this device realizes pre-biased startup? Or do you have any materials for me to learn? Moreover, I notice that this device can support both skip mode and FCCM, so does prebiased startup feature keep similar for both two modes? Thanks.

Regards,

Xiaoying

  • 0
    TI__Mastermind 24265 points

    The device has an FB pin that connects to the output via feedback resistors. 

    If during power up/enable, the FB pin is greater than the internal reference, the device is considered pre-biased.   

    During pre-biased startup the low side fet should not sink current and pull the output node down, ie supports pre-biased startup.

    When operating in skip mode, the low side fet does not sink current from the output, the low side fet only sources the current.

    During the FCCM, the low side fet is gated until a high side fet is turn on multiple times (16 times) before allowing the the low side fet to turn on and sink current. 

  • 0 in reply to David_Daniels
    TI__Expert 4290 points

    Hi David,

    David_Daniels said:
    When operating in skip mode, the low side fet does not sink current from the output, the low side fet only sources the current.

    You say that operating in skip mode, the low fed does not sink current, but only sources the current. I am curious that why the low side current can source the current? Based on my understanding, for skip mode, during startup, the gate signal of the low side fet is always off, and only the high side fet is controlled to be on/off, is that right? Thanks.

    Regards,

    Xiaoying

  • 0 in reply to Xiaoying Lu
    TI__Mastermind 24265 points

    See the first 7 pages of the following seminar paper. 

    The paper does a very good of  discussing the switching of the fets.

       www.ti.com/.../slup317.pdf          

    The low side fet is used to reduce power dissipation and increase efficiency compared to using a low side diode.

    In fccm mode, during the high side off time, the low side fet should alway conducts. 

    In skip mode, during the high side off time, the low side fet should conduct when inductor current is flowing to output.  

    The low side fet is used to reduce power dissipation and increase efficiency compared to using only a low side diode. 

    When converters power up, the converter soft starts, ie the output ramps up.   The soft start is typically implemented

    by ramping the internal reference voltage from 0V to the reference value.   The converter regulates to the reference value.

    If the converter is set in FCCM, and a pre bias is present during power up, the output voltage would be pulled low to regulate to the soft start voltage, sinking current from the output capacitor.

    If the converter is set in Skip, and a pre bias is present during powerup, the output voltage would not be pulled low since the low side does switch until after a high side switches.