Pull-up and pull-down resistors, essentially, are both resistors at work. The reason they're called pull-up and pull-down is simply because they're used in different scenarios.

For example, in practical circuits, we often encounter situations where the device's output voltage amplitude is insufficient. If the preceding stage can only output voltage levels from 0V to 3V, but the subsequent system requires a high or low level of 0V to 5V, you can use a pull-up resistor to forcibly increase the output voltage of the preceding stage. However, this method can only raise the voltage to between 3V and 5V.

Similarly, when the output low level is not low enough, a pull-down resistor can be used to pull the low level down. The voltage it can pull down to depends on the resistance value.

In the MOS tube switch power supply, a pull-down resistor is generally added to the NMOS tube and a pull-up resistor to the gate of the PMOS tube, typically around 10K. Usually, they serve three purposes:

1. To set the gate voltage of the MOS tube.

2. To prevent electrostatic breakdown.

3. To provide a discharge path.

When the MOS tube is powered on, there is a period when it is in a high-impedance state. In the conducting state, this state is uncontrolled. The NMOS may be affected by high-frequency interference, and the PMOS may be affected by low-frequency interference, which can easily lead to the destruction of the switch tube.

Adding resistance at this time allows the gate to be at a fixed voltage, no longer in a high-impedance state. In other words, the MOS tube will remain closed when powered on until the microcontroller controls its switch.

Additionally, it's easy to accumulate some static charges when in a high-impedance state, which can, to a certain extent, form a high voltage between GS, making the MOS tube prone to breakdown and damage. Resistance is needed to provide a fixed level.

Providing a discharge path is something we've discussed before. This is because there is parasitic capacitance between the GS of the MOS tube, and the resistor provides a discharge path for this parasitic capacitance when power is turned off.