The TPS3435 uses an open-drain configuration for the WDO output, as shown in Figure 9-2. 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 ensure that V_OL is below its maximum value. To choose the proper pullup resistor, there are three key specifications to keep in mind: the pullup voltage (V_PU), the recommended maximum WDO pin current (I_Sink), and V_OL. The maximum V_OL is 0.3 V, meaning that the effective resistor divider created must be able to bring the voltage on the reset pin below 0.3 V with I_Sink kept below 2 mA for V_DD ≥ 3 V and 500 μA for V_DD = 1.5 V. For this example, with a V_PU =V_DD = 1.5 V, a resistor must be chosen to keep I_Sink below 500 μA because this value is the maximum consumption current allowed. To ensure this specification is met, a pullup resistor value of 10 kΩ was selected, which sinks a maximum of 150 μA when WDO is asserted.