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TPS748A: How to calculate Soft-start-time

user6236094
Intellectual 310 points
Part Number: TPS748A

Hi,Team

Please tell me how to calculate soft-start-time.
There is a soft start time calculation formula in 8.2.2 Detailed Design Procedure in the data sheet below.
I believe the formula for calculating tss is as follows.

tss = (Vss×Css)／Iss

We also confirmed the contents of 6.5 Electrical Characteristics below.
Iss was 7.5[uA] when Vss=0.4[V], and Tss (Soft-start-time) was 1.2[ms] when Css=10[nF].
And I calculated it using the above formula.

tss = (Vss×Css)／Iss = (0.4[V]×10[nF])／7.5[uA] = 0.5333[ms？]

However, the calculation result was not tss = 1.2[ms].
Please tell me how to calculate Soft-start-time.

Best Regard

  • +1
    TI__Intellectual 2010 points

    Hi,

    The datasheet lists the voltage where Iss is measured, but Iss will be almost constant across Vss voltage.

    You can use the same equation you pointed out to calculate soft start time.

    But Vss is not mentioned with the equation. You can calculate it using tss = 1.2 ms, Css = 10 nF, Iss = 7.5 uA giving Vss = 0.9 V

    Regards,

    Bobby