Many DSP chips, microprocessors, FPGAs, and ASICs require multiple power supplies that must deliver different voltages in a specific start-up sequence. Out-of-sequence voltages can cause excessive input current, logic errors, or even device failure. To sequence different supplies, a common approach is to regulate a lower voltage from a higher voltage using a linear regulator. Another approach is to use a series of Schottky diodes. Although simple in concept, these approaches can be expensive and difficult to implement in applications that require more than two power supplies. Figure 1 shows a simple, low-cost approach that requires only one pnp transistor to provide the necessary logic. Figure 2 shows a dual power supply that uses the described circuitry to sequence the outputs.

When V_OUT1 is low, V_OSENSE2, the voltage feedback for V_OUT2, goes high, and the second supply, V_OUT2, shuts off (Figure 3). When V_OUT1 approaches its nominal level, Q₁ turns off. Q₁ then relinquishes control of V_OSENSE2, and V_OUT2 resumes its normal start-up process. The process is similar for power-down sequencing. When V_OUT1 is high, V_OUT2 operates normally. When V_OUT1 goes from high to low, V_OSENSE2 goes high and shuts off V_OUT2. More specifically, R₃ and R₄ set the clamping voltage for the V_OSENSE2 pin when V_OUT1 is low, and R₁ and R₂ determine the V_OUT1 voltage level at which Q₁ turns off. In cases of multiple supplies, you need only add ORing diodes at the collector of Q₁ (Figure 3). The design uses an LTC1628 dual-output controller. You can see the sequenced-output waveforms in Figures 4a (at turn-on) and 4b (at turn-off).

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