TPS22810-Q1: Calculation of QOD fall time

Hi Hayashi-san,

I believe I know what the confusion is here :). The equation for calculating the discharge time is below:

VCAP = VIN x e^(-t / RQOD x CL)

All of your numbers seem correct, except for the RQOD value. At 5V, the typical value is 250Ω, and this will discharge the output capacitance much slower than the 79mΩ you are using. The 79mΩ value is for the resistance across the main pass FET, not the QOD pulldown on the QOD pin.

Using 250Ω, the equation becomes:

0.5V = 4.5V * e^(-t/(250Ω*100uF))

Solving for t gives a value of 55000us, about 5000us away from the value shown in the applications section of the datasheet. Looking into this further, I see that the RQOD section in the electrical characteristics page and the RQOD values in Figure 6 (graph) don't quite match up.

If I use the RQOD value in Figure 6 (~228Ω) then I get a discharge of 50000us, as shown in the applications section, so that is how the table was generated. I will look into this discrepancy, but for your calculations I would use the above equation and the RQOD values shown in Figure 6.

Thanks,

Alek Kaknevicius