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TPS22965-Q1: Slew Rate Capacitor Design value

PANKAJ GANGWAR
Prodigy 40 points
Part Number: TPS22965-Q1
Other Parts Discussed in Thread: TPS22965, TPS22990

Slew Rate Capacitor (Ct) to be chosen in TPS22965 (n-load Switch), The Load Capacitance is about 162uF for our Design. We can not maintain Cin/Cload ratio of 10:1 as per datasheet.

 

But when we calculated the rise time by using,  In rush current = Cload* dV/dt as per datasheet.

In rush current = 4A is considered as per limiting values of TPS22965. dV being 3.3V,

Cload=162uF, dt= 135us. But dt value what is calculated is not matching with the table given in the data-sheet because Ct=0 for Vin = 3.3V & rise time = 136us as per Table-1 of TPS22965 Datasheet. Please Comment on this issue as soon as possible.

  • 0
    Prodigy 40 points

    Also Comment about same issue on TPS22990 switch

  • 0
    TI__Mastermind 32650 points

    Hi User,

    The 10:1 Cin/Cload ratio is not a strict recommendation. This exists to protect weaker, power limited supplies for dropping voltage during the inrush phase. If the inrush is clamped and the supply strong, the 10:1 ratio can be ignored.

    Table 1 only shows the typical timing value when the test conditions are as mentioned in the table. The VBIAS also is 5V for the timing values in the table. Please note that you will see some variation depending on VIN, VBIAS, load and temperature as timing is a very sensitive parameter. Please note that this is not an issue as the CT capacitance can be increased to further clamp the inrush current. This is true for TPS22990 as well.

    I don't understand the issue you are seeing here as with my and your calculation, the dt for an inrush of 4A is 133.65us. If the inrush is too high, please use a larger CT capacitor. If the inrush is acceptable, you can leave the CT capacitor floating. Do note that the part is able to survive a pulse of up to 6A.