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TPS40057 SwitchePro schematic C33

K. Sasaki14
Guru 10200 points
Other Parts Discussed in Thread: SWITCHERPRO, TPS40057, TPS40055

TPS40057 schematic in SwitcherPro has gate capacitor C33. This is not described in datasheet.

How is this value calculated? 

According to TPS40055 EVM user's guide,  this capacitor value is decided from the capacitance ratio between the gate-source and drain-gate, reducing the voltage which appears on the lower MOSFET gate.

(http://www.ti.com/lit/ug/sluu190/sluu190.pdf)

However, the capacitor value changes depending on external components or input and output condition though same low side FET is used in SwitcherPro. Thus our customer requires the calculation method.

5751.TPS40057 DesignReport.pdf

Best Regards,

Kohei Sasaki

  • 0
    TI__Mastermind 21755 points

    Hi Kohei,

    After digging into some details for this device: 

     When we implemented this logic, we did not have access to the two voltages we need, the Vgs and the Vds voltages that were used to measure the gate drain charge and the gate source charge.   As a very crude approximation, we assume Vds is about half the max FET voltage ( Vds = 0.5* Vbreakdown) and Vgs is 1/3 of that ( Vgs= Vds/3).                                                                             

       QgdApplication = FETGateDrainCharge * (VinMax + Vgs) / (Vds + Vgs);

    If the ratio of Qgd(application)/Qgs(datasheet) > 1, then we need to add the dV/dT cap to the gate of the lower FET.

       QgsDataSheet = FETGateSourceCharge;                      

     Cgs = (1.25 * QgdApplication - QgsDataSheet) / FETThresholdVoltage

    You might end up with slightly different values depending on your assumptions.

    Hope this is of some help.

     

    Best Regards,

    Amod Vaze

  • 0 in reply to Amod Vaze
    Guru 10200 points

    Amod-san,

     

    Thank you for your reply.

     

    Is the initial C33 value in SwitchePro changed when FET is changed?

    According to my observation, C33 is not changed although FET is changed. I think C33 value should be changed when using FET is changed because the FET parameter is difference.

     

    Best Regards,

    Kohei Sasaki

  • 0 in reply to K. Sasaki14
    TI__Mastermind 21755 points

    Kohei-san,

    No, C33 is not redesigned when the FET is changed. You might benefit by using the above formulae and the specific FET parameters in your design.

    Best Regards,

    Amod Vaze

  • 0 in reply to Amod Vaze
    Guru 10200 points

    Amod-san,

    Which BOM value is redesigned when the FET is changed? Please let me know if you have this information.

    Best Regards,

    Kohei Sasaki