SBVS300A November 2016 – September 2021 TPS3851
As illustrated in Figure 8-1 there are three options for setting the watchdog timer. The design specifications in this application require the programmable timing option (external capacitor connected to CWD). When a capacitor is connected to the CWD pin, the watchdog timer is governed by Equation 1 for the standard timing version. Note that only the standard version is capable of meeting this timing requirement. Equation 1 is only valid for ideal capacitors, any temperature or voltage derating must be accounted for separately.
The nearest standard capacitor value to 2.9 nF is 2.7 nF. Selecting 2.7 nF for the CCWD capacitor gives the following minimum timing parameters:
Capacitor tolerance also influences tWD(MIN) and tWD(MAX). Select a ceramic COG dielectric capacitor for high accuracy. For 2.7 nF, COG capacitors are readily available with 5% tolerances. This selection results in a 5% decrease in tWD(MIN) and a 5% increase in tWD(MAX), giving 7.34 ms and 11 ms, respectively. To ensure proper functionality, a falling edge must be issued before tWD(min). Figure 8-6 illustrates that a WDI signal with a period of 5 ms keeps WDO from asserting.