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TPS3808: Equation1 for CT(Reset period programming pin)

Part Number: TPS3808

Hi all

Would you mind if we ask TPS3808?

There is the description as follows;
CT(nF)=[tD(s) - 0.5×10^-3(s)]×175
Delay(s)=CT(nF)/175+(0.5×10^-3(s))

We calculated using  Equation1, the result were follows;
-CT = 100 pF -> 1.07ms
-CT = 180 nF -> 1.079s

There is the difference compared with 7.6 Switching Characteristics showing.
So, could you let us know why there is the difference?

Kind regards,

Hirotaka Matsumoto

  • Matsumoto-san,

    The values calculated for the RESET delay time in the TPS3808 as you mentioned are considered ideal values in the datasheet under typical. For this calculation, there are a few real-world factors that go into the value including the tolerance of the internal current source in the device as well as the having an ideal capacitor. These variations can change the value as you saw in your calculations.

    The equations provided in the datasheet should be mostly used as an estimate of the delay that specific device should generate rather than what all devices will provide. This being said, the values you provided are well within the MIN-MAX range (0.75ms-1.75ms and 0.7s-1.7s) of the datasheet values and also close to the typical value. So to summarize, the difference mostly just comes from internal device characteristics such as the current source tolerance.

    Thank you,
    Abhinav