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TPS22975: Rise time calculation gap against Table 1

Takashi Onawa
Genius 13200 points
Part Number: TPS22975

Hi Team,

According to the datasheet, we can estimate the rise time by equation (1). But the calculation results do not match the information in Table 1. Which result is correct?


[Example]

SR[µs/V] = 0.43 x Ct[pf] + 26      Equation(1)

When Ct = 220pf case, the SR will be;
0.43 x 220 + 26 = 120.6 [us/V]

therefore Vin = 5V case, Rise time will be;
120.6 [us/V] x 5V = 603 [us]

On the other hand, the table show it should be around 520us. Over 10% error is existed between the calculated estimation and table contents.

Regards,

Takashi Onawa

  • 0
    TI__Mastermind 32650 points

    Hi Onawa-san,

    The equation is a rough approximation for the slew rate but it is not exact. A lot of factors such as voltage, temperature, PCB come into play when finding the true timing of the device. The table was built from typical data gained from testing in house. 

    I would suggest referencing the equation to see what CT cap is a good fit for your ideal slew rate and then refer to the table for the actual rise times seen when using that capacitor under the test conditions mentioned on the datasheet. You can then adjust your CT value as you need.