Adjustable capacitor timing is achievable by connecting a capacitor to the CWD pin. If a capacitor is connected to CWD, then a 375-nA, constant-current source charges C_CWD until V_CWD = 1.21 V. Table 8-2 shows how to calculate t_WD using Equation 1, Equation 2, and the SET1 pin. The TPS3851-Q1 determines the watchdog timeout with the formulas given in Equation 1 and Equation 2, where C_CWD is in nanofarads and t_WD is in milliseconds.

The TPS3851-Q1 is designed and tested using C_CWD capacitors between 100 pF and 1 µF. Equation 1 and Equation 2 are for ideal capacitors; capacitor tolerances vary the actual device timing. For the most accurate timing, use ceramic capacitors with COG dielectric material. If a C_CWD capacitor is used, Equation 1 can be used to set t_WD for standard timing. Use Equation 2 to calculate t_WD for extended timing. Table 8-3 shows the minimum and maximum calculated t_WD values using an ideal capacitor for both standard and extended timing.