Question

How do you design a simple power supply sequencing circuit for a multi-rail power system?

Answer 1

Designing a simple power supply sequencing circuit for a multi-rail power system involves ensuring that the different voltage rails power up and power down in a specified order to avoid potential issues or damage to the connected circuitry. This can be achieved using a few basic electronic components and a control logic circuit. Below, I'll outline a general approach to design such a sequencing circuit:

Step 1: Identify Power Rails and Sequencing Order
First, identify the different power rails in your system and determine the desired power-up and power-down sequencing order. For example, you may have three voltage rails: VDD1, VDD2, and VDD3, and the required sequencing order could be VDD1 → VDD2 → VDD3 during power-up and VDD3 → VDD2 → VDD1 during power-down.

Step 2: Gather Necessary Components
You'll need the following components for the sequencing circuit:

Voltage Regulators: One for each power rail to regulate the voltages.
NPN Transistors: To control the enable/disable of each regulator.
Resistors and Capacitors: For biasing and timing purposes.
Diodes (optional): For ORing power-good signals from the regulators (if available).

Step 3: Design the Control Logic
The control logic will determine the sequencing order. One way to implement it is by using simple resistor-capacitor (RC) timing circuits. Each RC circuit will control the base voltage of an NPN transistor, which in turn controls the enable/disable signal of each voltage regulator.

Here's a basic example circuit for power-up sequencing:

csharp
Copy code
           +12V
            |
           [R1]
            |
            +-----> VDD1 Regulator Enable
            |
           [C1]
            |
            |
            +-----> VDD2 Regulator Enable
            |
           [R2]
            |
            +-----> VDD3 Regulator Enable
            |
           [C2]
            |
            |
            +-----> Ground


In this circuit, R1 and C1 form an RC timing circuit that determines the delay for VDD1 to power up. Similarly, R2 and C2 form an RC timing circuit that determines the delay for VDD2 to power up after VDD1, and so on.

Step 4: Implementing Power-Down Sequencing (Optional)
For power-down sequencing, you can use the same concept but with different timing circuits or simply reverse the order of the RC circuits. Additionally, you may use diodes to OR the power-good signals from the regulators if they provide a power-good output.

Step 5: Adjusting Timing
The timing components (R1, R2, C1, C2, etc.) can be adjusted to achieve the desired delay between each power rail's activation or deactivation. Experiment with different resistor and capacitor values to get the timing right.

Step 6: Test and Verification
Before integrating the sequencing circuit into your system, thoroughly test it on a breadboard or a prototyping board. Verify that the power rails activate and deactivate in the correct order and within the desired timeframes.

Keep in mind that this is a basic example, and the complexity of your sequencing circuit may vary depending on the specific requirements of your multi-rail power system. If you're dealing with high currents or complex power management, you may need to use more advanced sequencing solutions or consider using dedicated power management ICs.