TPS3890: Accurate Supervisor TPS3890 - How to calculate tPD(r)

Destin,

Thank you for your comment. The equation to calculate tPD(r) is derived from the capacitor equation:

I = C (dv/dt) -> delay [seconds] = (C_CT x V_CT ÷ I_CT) + (nominal delay with no cap at specific Vdd)

So depending on what supply voltage Vdd you are using and if you are calculating the rising or falling delay, the (nominal delay with no cap) will change value. In the table you posted, the value is 25us (nom) for rising delay. In table 7.6 for Timing Requirements, you can see what the nominal delay with no capacitor is for Vdd = 3.3V and Vdd = 5.5V shown below for falling delay and also see the nominal rising delay with no capacitor at Vdd = 3.3V:

So using this approximation (nominal means can vary slightly from device to device) you must then calculate the delay using the above equation in bold. Note that the delay capacitor voltage and current also have tolerance as shown below from Table 7.5:

So if you assume typical values and assume Vdd = 5.5V and assume C_CT = 26uF , you arrive at:

Delay_Falling [unit: seconds] = C_CT [unit: uF] x 1.23 [unit Volts] / 1.15 [unit: uA] + 8 [unit: micro seconds] = 27.8 Seconds

But if you calculate using the minimum values for V_CT and I_CT: 33.8 seconds.

There will be slight device variation and even tolerance in the capacitor can cause slight variation.

I hope this helps! Let me know if you have any more questions.

-Michael