Theoretically, a capacitor cannot pull the OFF-channel output node down to ground in DC circumstances since the capacitor is regarded as an infinitely large impedance for DC signal, whatever the value of capacitor is. However, in the real test, a large capacitor does work to pull down the voltage at output.

Figure 3-5 Pull-down Capacitor

This is due to the non-designed for performance of capacitor products. The actual capacitor has a finite resistance value because a small amount of current flows between the insulated electrodes. This resistance value is called insulation resistance.

Figure 3-6 Real-world Capacitor Model

The insulation resistance can be calculated by looking up the data sheet of the capacitor. Take TDK’s capacitors as an example. According to the TDK Product Center, the insulation resistance is the smaller value of 10GΩ and 100MΩ·uF for capacitors with a rated voltage of 6.3 to approximately 16V. For capacitance of 1uF, the paralleled resistance of this capacitor is 100MΩ, which is small enough compared to the typical value of off-channel equivalent resistance calculated in Section 3.1 Therefore, this can pull down the output voltage to ground.

Plus, the capacitor can also reduce the AC interference coupled in the circuit, such as the 50Hz noise.

This design works well for DC input scenario. For AC input, the reactance of capacitance needs to also be taken into consideration, which is not discussed in this application note.