Why should the power supply speed up the discharge function! Doesn’t it work?

“Many loads have power-on sequence and voltage conversion rate (slew rate) requirements for the power supply. For example, when the load requires multiple power supplies, these power supplies must have a sequence of power-on and power-off, and must also meet certain power-up and power-off sequences. Power-off slope requirements.

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Many loads have power-on sequence and voltage conversion rate (slew rate) requirements for the power supply. For example, when the load requires multiple power supplies, these power supplies must have a sequence of power-on and power-off, and must also meet certain power-up and power-off sequences. Power-off slope requirements.

After turning off the power supply, the power supply voltage will drop slowly due to the large capacitance of the load Circuit. If the load capacitor is not completely discharged and the power is turned on immediately, the system may not start normally.

The screen may be blurred, the camera may be turned on abnormally, or the system may directly freeze.

Therefore, it is introduced in some product descriptions, such as routers: wait 10 seconds after power off, and then power on again.

For some old-fashioned TVs, the power indicator will go out after a few seconds after being turned off, which is caused by the lack of a fast discharge circuit.

In order to avoid power supply abnormalities caused by repeated switching of the load, it is generally necessary to add a fast discharge circuit. A very simple discharging circuit is to add a ground switch MOS. When discharging is needed, MOS is turned on, and when discharging is not needed, MOS is turned off.

The fast discharge function is integrated inside the LDO in the figure below.

The fast discharge function of the LDO is controlled by the EN pin. When the EN pin is high, the LDO outputs Vout. When the EN pin is low, the LDO turns off the output, the fast discharge path opens, and the voltage on the load capacitor is discharged through MOS was released.

The pink (positive) and green (negative) power supplies in the figure below are the power supply for the screen. The timing waveform after adding the fast discharge function shows that the power-off is very clean.

In the following figure, Vpos and Vneg are powered off very slowly, muddy and watery, and the phenomenon of flickering is very easy to appear.

The above is a brief introduction to the fast discharge function of the power supply