8.2.1.2.4 Calculating the WDO Pullup Resistor

The TPS3435 uses an open-drain configuration for the WDO output, as shown in Figure 8-2. When the FET

is off, the resistor pulls the drain of the transistor to VDD and when the FET is turned on, the FET pulls the

output to ground, thus creating an effective resistor divider. The resistors in this divider must be chosen to

make sure that VOL is below the maximum value. To choose the proper pullup resistor, there are three key

specifications to keep in mind: the pullup voltage (VPU), the recommended maximum WDO pin current (ISink),

and VOL. The maximum VOL is 0.3V, meaning that the effective resistor divider created must be able to bring the

voltage on the reset pin below 0.3V with ISink kept below 2mA for VDD ≥ 3V and 500μA for VDD = 1.5V. For this

example, with a VPU =VDD = 1.5V, a resistor must be chosen to keep ISink below 500μA because this value is the

maximum consumption current allowed. To make sure this specification is met, a pullup resistor value of 10kΩ

was selected, which sinks a maximum of 150μA when WDO is asserted.

WDO

VDD

WDO

CONTROL

Figure 8-2. Open-Drain WDO Configuration

8.2.1.3 Application Curves

Figure 8-3. Typical WDI Pulse

Figure 8-4. Watchdog Timeout Fault

8.3 Power Supply Recommendations

This device is designed to operate from an input supply with a voltage range between 1.04V and 6V. An input

supply capacitor is not required for this device; however, if the input supply is noisy, then good analog practice is

to place a 0.1µF capacitor between the VDD pin and the GND pin.

8.4 Layout

8.4.1 Layout Guidelines

Make sure that the connection to the VDD pin is low impedance. Good analog design practice recommends

placing a 0.1µF ceramic capacitor as near as possible to the VDD pin. If a capacitor is not connected to the

CRST pin, then minimize parasitic capacitance on this pin so the WDO delay time is not adversely affected.

• Make sure that the connection to the VDD pin is low impedance. Good analog design practice is to place a

0.1µF ceramic capacitor as near as possible to the VDD pin.

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TPS3435

SNVSCF7A – DECEMBER 2022 – REVISED AUGUST 2024

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Product Folder Links: TPS3435