ZHCSIJ7A July 2018 – October 2021 TPS3431
PRODUCTION DATA
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. This equation estimation is only valid for ideal capacitors and any temperature or voltage derating must be accounted for separately.
The nearest standard capacitor value is 2.7 nF. Selecting 2.7 nF for the CCWD capacitor gives the following minimum and maximum 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 213.16 ms and 318.75 ms, respectively. To ensure proper functionality, a falling edge must be issued before tWD(min). Figure 8-8 illustrates that a WDI signal with a period of 260 ms keeps WDO from asserting.