ZHCSFY8B January 2017 – September 2021 TPS3850-Q1
PRODUCTION DATA
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 CCWD until VCWD = 1.21 V. The TPS3850-Q1 determines the window watchdog upper boundary with the formula given in Equation 6, where CCWD is in microfarads and tWDU is in seconds.
The TPS3850-Q1 is designed and tested using CCWD capacitors between 100 pF and 1 µF. Note that Equation 6 is for ideal capacitors. Capacitor tolerances cause the actual device timing to vary such that the minimum of tWDU can decrease and the maximum of tWDU can increase by the capacitor tolerance. To allow for a valid watchdog window, choose a capacitor with tolerance such that tWDU(min) and tWDL(max) do not overlap. For the most accurate timing, use ceramic capacitors with COG dielectric material. As shown in Table 8-4, when using the minimum capacitor of 100 pF, the watchdog upper boundary is 62.74 ms; whereas with a 1-µF capacitor, the watchdog upper boundary is 77.455 seconds. If a CCWD capacitor is used, Equation 6 can be used to set tWDU the window watchdog upper boundary. The window watchdog lower boundary is dependent on the SET0 and SET1 pins because these pins set the window watchdog ratio of the lower boundary to upper boundary; Table 8-5 shows how tWDU can be used to calculate tWDL based on the SET0 and SET1 pins.