# Power Supply Sequence Specification ②: Sequence Operation at Power Shutoff

2022.12.07

Points of this article

・In sequence ②, three power supply systems are turned on in order, and are shut off in the same order.

・In order to aid understanding of sequence operation, the operation at the time of shutoff of each of the three power supply systems is shown separately.

In the previous article, we explained sequence operation at power turn-on for power supply sequence specification ②. This time, we explain sequence operation at power shutoff.

## Power Supply Sequence Specification②: Sequence Operation at Power Shutoff

As explained in the previous article, in the sequence for the specification ②, the outputs of the three power supply systems for 1.2 V, 1.5 V, and 3.3 V are turned on in this order, and are shut off in the same order. In this explanation, the operation in which 1.2 V is shut off is the first stage, the 1.5 V shutoff operation is the second stage, the 3.3 V shutoff operation is the third stage. In diagrams of each of the stages, the part relevant to the explanation is shown red.

●Operation for first-stage power supply shutoff

• 1) The Enable pin is set to “L” level to shut off the power supply.
• 2) The EN pin of DCDC 1 goes to “L”, so that DCDC 1 is shut off.
• 3) At the same time, the IN pin of Discharge 1 also goes to “L”, and so the OUT pin of Discharge 1 changes to low impedance. As a result, the output voltage of DCDC 1 rapidly shifts to 0 V.
• 4) When the DCDC 1 output voltage falls, the output voltage of Power Good 1 changes from “H” to “L”, and so the next-stage DCDC 2 EN pin and Discharge 2 IN pin go to “L”.

●Operation for second-stage power shutoff

• 1) The EN pin of DCDC 2 has gone to “L”, and so DCDC 2 shuts off.
• 2) At the same time, the IN pin of Discharge 2 also goes to “L”, and so the OUT pin of Discharge 2 changes to low impedance. As a result, the output voltage of DCDC 2 rapidly shifts to 0 V.
• 3) When the DCDC 2 output voltage falls, the output voltage of Power Good 2 changes from “H” to “L”, and so the next-stage DCDC 3 EN pin and Discharge 3 IN pin go to “L”.

●Operation for third-stage power shutoff

• 1) The EN pin of DCDC 3 has gone to “L”, and so DCDC 3 shuts off.
• 2) At the same time, the IN pin of Discharge 3 also goes to “L”, and so the OUT pin of Discharge 3 changes to low impedance. As a result, the output voltage of DCDC 3 rapidly shifts to 0 V.
• 3) With this, shutoff of all of the three power supply systems is completed.

The above is sequence operation at power shutoff for power supply sequence specification ②. In the next article, examples of an actual circuit and settings of component values etc. will be presented.

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### 【HAND BOOK】 Linear Regulator Basics

This is a hand book for understanding the basics of linear regulators, such as operating principles, classification, characteristics by circuit configuration, advantages and disadvantages. In addition, typical specifications of linear regulators, efficiency and thermal calculations are also explained.