WDI

ttWDOt

Ignore

ttWDt

ttWDt

ttWDt

ttSDt

tP-WD

WDO

tWD

Figure 8-6. Timeout Watchdog Timer Operation

8.3.1.1 tWD Timer

The tWD timer for TPS3435 can be set using an external capacitor connected between CWD pin and GND pin.

This feature is available with pinout options A or B or K. Applications which are space constrained or need timer

values which meet offered timer options, can benefit when using pinout options C or J. The TPS3435 offers

multiple fixed timer options ranging from 1 msec up-to 100 sec.

The TPS3435, when using capacitance based timer, senses the capacitance value during the power up. The

capacitor is charged and discharged with known internal current source for one cycle to sense the capacitance

value. The sensed value is used to arrive at tWD timer for the watchdog operation. This unique implementation

helps reduce the continuous charge and discharge current for the capacitor, thus reducing overall current

consumption. Continuous charge and discharge of capacitance creates wider dead time (no watchdog monitor

functionality) when capacitor is discharging. The dead time is higher for high value of capacitance. The unique

implementation of TPS3435 helps avoid the dead time as the capacitance is not continuously charging or

discharging under normal operation. Ensure CCWD is < 200 x CCRST for accurate calibration of capacitance.

Equation 1 highlights the relationship between tWD in second and CWD capacitance in farad. The tWD timer is

20% accurate for an ideal capacitor. Accuracy of the capacitance will have additional impact on the tWD time.

Ensure the capacitance meets the recommended operating range. Capacitance outside the recommended range

can lead to incorrect operation of the device.

tWD (sec) = 4.95 x 106 x CCWD (F)

(1)

The TPS3435 also offers wide selection of high accuracy fixed timer options starting from 1 msec to 100 sec

including various industry standard values. The TPS3435 fixed time options are ±10% accurate for tWD ≥ 10

msec. For tWD < 10 msec, the accuracy is ±20%. tWD value relevant to application can be identified from the

orderable part number. Refer Section 5 section to identify mapping of orderable part number to tWD value.

The TPS3435 offers flexibility to change the tWD value on the fly by controlling the logic levels on the SETx pins.

Section 8.3.1.4 section explains the advantages offered by this feature and the device behavior with various

SETx pin combinations.

8.3.1.2 Watchdog Enable Disable Operation

The TPS3435 supports watchdog enable or disable functionality. This functionality is critical for different use

cases as listed below.

• Disable watchdog during firmware update to avoid host RESET.

• Disable watchdog during software step-by-step debug operation.

• Disable watchdog when performing critical task to avoid watchdog error interrupt.

• Keep watchdog disabled until host boots up.

The TPS3435 supports watchdog enable or disable functionality through either WD-EN pin or SET[1:0] = 0b'01

logic combination or by keeping WDI pin in the floating state. For a given pinout only one of these three methods

is available for the user to disable watchdog operation.

For a pinout which offers a WD-EN pin, the watchdog enable disable functionality is controlled by the logic state

of WD-EN pin. Drive WD-EN = 1 to enable the watchdog operation or drive WD-EN = 0 to disable the watchdog

TPS3435

SNVSCF7 – DECEMBER 2022

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