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• Power Supply Sequence Specification ①: Sequence Operation at Power Shutoff

2021.10.27 DC/DC

# Power Supply Sequence Specification ①: Sequence Operation at Power Shutoff

Circuits to Implement Power Supply Sequences Using General-Purpose Power Supply ICs

In the previous article "Sequence Operation at Power Turn-on" in Power Supply Sequence Specification ① was explained. Here, sequence operation at power shutoff is explained.

Power Supply Sequence Specification①：Sequence Operation at Power Shutoff

As explained previously, in the sequence for Specification ① the outputs of the three power supply systems for 1.2 V, 3.3 V, and 1.5 V are turned on in order and shut off in the reverse order. In succession to the sequence at power turn-on of the previous article, the sequence operation at power shutoff is explained. Power is shut off in the order 1.5 V, 3.3 V, 1.2 V. In this explanation, the operation in which the 1.5 V power is shut off is called the first stage, the 3.3 V shutoff operation is the second stage, and the 1.2 V shutoff operation is the third stage. In diagrams of each of the stages, the part relevant to the explanation is shown in red.

●Operation for first-stage power shutoff

• 1) The Enable pin is set to "L" level to shut off the power supply.
• 2) The EN pin of DCDC 3 goes to "L" via the diode D6, so that DCDC 3 is shut off.
• 3) 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.
• 4) When the DCDC 3 output voltage falls, the output voltage of Power Good 4 changes from "H" to "L", and so the previous-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 3 changes from "H" to "L", and so the previous-stage DCDC 1 EN pin and Discharge 1 IN pin go to "L".

●Operation for third-stage power shutoff

• 1) The EN pin of DCDC 1 has gone to "L", and so DCDC 1 shuts off.
• 2) 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.
• 3) With this, shutoff of all of the three power supply systems is completed.

In the next article, examples of an actual circuit and settings of component values etc. will be presented.

#### Key Points:

・In sequence ①, power supplies with three supply voltages are turned on in order, and shut off in the reverse order.

・In order to facilitate understanding of sequence operations, the operations at power shutoff for each of the three power supply systems were illustrated in terms of three stages.