The TPS3851 uses an open-drain configuration for the RESET circuit, as shown in Figure 8-4. When the FET is off, the resistor pulls the drain of the transistor to VDD and when the FET is turned on, the FET attempts to pull the drain to ground, thus creating an effective resistor divider. The resistors in this divider must be chosen to ensure that V_OL is below the 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 RESET pin current (I_RESET), and V_OL. The maximum V_OL is 0.4 V, meaning that the effective resistor divider created must be able to bring the voltage on the reset pin below 0.4 V with I_RESET kept below 10 mA. For this example, with a V_PU of 1.8 V, a resistor must be chosen to keep I_RESET below 50 μA because this value is the maximum consumption current allowed. To ensure this specification is met, a pullup resistor value of 100 kΩ was selected, which sinks a maximum of 18 μA when RESET or WDO is asserted. As illustrated in Figure 6-13, the RESET current is at
18 μA and the low-level output voltage is approximately zero.

Figure 8-4 RESET Open-Drain Configuration