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6.25 ms

15.55 pF

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TPS3813K33-Q1, TPS3813I50-Q1

www.ti.com

SPRS288D – MAY 2008 – REVISED JUNE 2015

Typical Application (continued)

8.2.1 Design Requirements

Design requirements include any design parameters that are solely based on the watchdog timing desired by the

user. The Implemented Window-Watchdog Settings and Detailed Design Procedure sections describe these

timings. Select the TPS3813-Q1 device option based on desired threshold voltage of either 2.5 V, 3 V, 3.3 V, or

5 V.

8.2.2 Detailed Design Procedure

To configure the window watchdog function, two pins are provided by the TPS3813-Q1 family of devices. These

pins set the window timeout and ratio.

The window watchdog ratio is a fixed ratio, which determines the lower boundary of the window frame. This ratio

can be configured in two different frame sizes.

value based on a ratio calculation of the overall window timeout size. For the watchdog timeout pin (WDT)

for an external capacitor connected to WDT, the value is a ratio of 1:64.5.

If the window watchdog ratio pin (WDR) is set to GND (Position 2) the lower window frame is a value based on a

ratio calculation of the overall window timeout size. For the watchdog timeout pin (WDT) connected to GND, the

connected to WDT the value is a ratio of 1:25.8.

The watchdog timeout can be set in two fixed timings of 0.25 s and 2.5 s for the window or can by programmed

by connecting a external capacitor with a low leakage current at WDT.

boundary is 19.6 ms.

8.2.2.1 Programming Window-Watchdog Using an External Capacitor

The upper boundary of the watchdog timer can be set by an external capacitor connected between the WDT pin

and GND. Common consumer electronic capacitors can be used to implement this feature. The capacitors that

are used should have low ESR and low tolerances because the tolerances must be considered to perform the

calculations. The first formula is used to calculate the upper window frame. After calculating the upper window

frame, the lower boundary can be calculated. As in the last example, the most important values are the

The external capacitor should have a value between a minimum of 155 pF and a maximum of 63 nF.

(1)

Table 4. Watchdog Upper-Boundary Capacitor Programming

SELECTED OPERATION MODE

WINDOW FRAME

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