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CSD95373AQ5M: body diode during DEM

MINKYUNG KIM
Expert 1250 points
Part Number: CSD95373AQ5M

Hi there,

I have a quick/basic question on DEM of CSD95373AQ5M.

Can you explain the difference between the operation of dead time and the operation of DEM? I'm confusing them. Simply put, both are just turning off HS FET and LS FET from my understanding. Vout on the SW node (from the figure below) is due to latched-off LS FET, thereby short-circuited. Am I correct here? How come body diode of LS FET has an effect on SW node during the dead time, but doesn't have an effect during DEM?  

Could you kindly help my understanding? Thank you for your time!

Simply put,   

  • 0
    TI__Expert 3525 points

    Hi Minkyung,

    The deadtime is when we have both HS/LS FETs turned off to prevent shoot through failure (short circuit if we have both HS/LS FET ON). The deadtime is a time period during switching transient.

    While the diode emulation mode is an feature of power stage that we operate the power stage in discontinuous conduction mode (DCM).  When PWM signal is low, the LS FET is turn on when inductor current is still positive. The inductor current is dropping and flowing through LS FET until IL reaches zero. The power stage detects zero-crossing and turn off LS FET smoothly (LS FET also emulates the diode characteristics). So in DEM mode, the inductor current will not go to negative to reduce the conduction loss. You can see that from the waveform you attached. The DEM is helpful to improve efficiency especially under light load condition. 

    Best,

    Qingquan