When a design based on multiple point-of-load dc/dc converters requires a specific power-supply-start-up sequence, wiring each converter's power-good output to the next converter's enable input produces the desired voltage cascade. Although this approach works well for simple designs, it fails to satisfy a requirement of many modern microprocessors and DSPs: that, during shutdown, the power-supply rails switch off in reverse order. Although various vendors provide programmable-sequencing ICs, these components are usually too expensive for cost-sensitive applications.

Offering an alternative to programmable-sequencing ICs, the circuit of Figure 1 can sequence and cheaply and effectively monitor four power-supply rails. Four dc/dc power supplies provide the application circuit with 3.3, 2.5, 1.8, and 1.2V. A quad supervisor circuit, IC₁, monitors each rail, generating the master POK (power-OK) signal and ensuring that, during power-up, the next supply in the sequence does not turn on until the preceding supply voltage is valid. Using an RC circuit comprising R₁, R₂, R₃, and C₁, a second quad supervisor, IC₂, creates the power-up and power-down sequences. Each supervisor's internally preset voltage threshold eliminates the need for external resistive-voltage dividers.

Connecting the power-on/off signal to 5V initiates a power-up sequence, which charges C₁ through R₂. As the capacitor's voltage gradually exceeds 1.2, 1.8, 2.5, and 3.3V, each of IC₂'s corresponding open-drain outputs floats, thereby allowing the power supplies to turn on in the prescribed sequence. After a time delay, which C₂ sets, and after all four supplies turn on, the POK signal asserts—that is, goes high.

To monitor the supply rails, allow the power-on/off-control input to float. Resistors R₁ and R₃ sustain the voltage across C₁ and maintain the POK signal high to keep the power supplies on. When an output-voltage fault occurs, POK rapidly deasserts, discharging C₁ through R₁ and shutting off all of the power supplies. To remove power in an orderly sequence, connect the power-on/off signal to ground. Capacitor C₁ discharges through R₂ and also through R₁ when POK deasserts, turning off each power supply in reverse order (Figure 2).