9.1.2.2 Adjustable Capacitor Timing

Adjustable tWD timing is achievable by connecting a capacitor to the CWD pin. If this method is used, please

consult Equation 1 for calculating typical tWD values using ideal capacitors. Capacitor tolerances cause the

actual device timing to vary such that the minimum of tWD can decrease and the maximum of tWD can increase

by the capacitor tolerance. For the most accurate timing, use ceramic capacitors with COG dielectric material.

9.2 Typical Applications

9.2.1 Design 1: Monitoring a Standard Microcontroller for Timeouts

This example application uses the TPS3435CDDBBDDFR to monitor a microcontroller to ensure it is not stalled

during operation.

VDD

GND

TPS3435

3.3V

Microcontroller

SET1

VCC

WD-EN

GND

MR

SET0

WDI

GPIO

Copyright © 2022, Texas Instruments Incorporated

RESET

WDO

Figure 9-1. Microcontroller Watchdog Monitoring Circuit

9.2.1.1 Design Requirements

PARAMETER

DESIGN REQUIREMENT

DESIGN RESULT

Watchdog Timeout Period

Typical timeout period of 40 ms

Typical timeout period of 40 ms

Watchdog Output Assert Delay

Typical output assert of 2 ms

Typical output assert of 2 ms

Startup Delay

Minimum startup delay of 700 ms

Minimum startup delay of 900 ms

Output logic voltage

Open-drain

Open-drain

Maximum device current

consumption

20 µA

250 nA typical, 3.0 μA maximum(1)

(1)

Only includes the current consumption of the TPS3435.

9.2.1.2 Detailed Design Procedure

9.2.1.2.1 Setting the Watchdog Timeout Period

The watchdog timeout design requirement can be met either by using a fixed-timeout version of the TPS3435 or

by connecting a capacitor between the CWD pin and GND. The typical values can be met with preprogrammed

fixed time options, hence a pinout offering fixed time options is selected. Please see the Section 7.6 for a list of

fixed timeouts. If using the CWD feature, please refer to Section 8.3.1.1 for instructions on how to program the

timout period. In this application example, the 40 ms timeout watchdog period is achieved by using watchdog

time of 10ms (option D) and watchdog time scaling of 4 (option B). Connect SET[1:0] = 0b'10 to select watchdog

time scaling of 4.

9.2.1.2.2 Setting Output Assert Delay

Please see the Section 7.7 for a list of fixed timeouts. Timeout option B was chosen in order to meet the design

requirement for a 2 ms typical timeout.

9.2.1.2.3 Setting the Startup Delay

Startup delay option D is chosen, which offers a startup delay of 1 s. This accounts for the minimum specification

of 700 ms.

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TPS3435

SNVSCF7 – DECEMBER 2022

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