Single transistor sequences multiple supplies
Edited by Bill Travis
David Chen, Linear Technology Corp, Milpitas, CA -- EDN, August 8, 2002
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 VOUT1 is low, VOSENSE2, the voltage feedback for VOUT2, goes high, and the second supply, VOUT2, shuts off (Figure 3). When VOUT1 approaches its nominal level, Q1 turns off. Q1 then relinquishes control of VOSENSE2, and VOUT2 resumes its normal start-up process. The process is similar for power-down sequencing. When VOUT1 is high, VOUT2 operates normally. When VOUT1 goes from high to low, VOSENSE2 goes high and shuts off VOUT2. More specifically, R3 and R4 set the clamping voltage for the VOSENSE2 pin when VOUT1 is low, and R1 and R2 determine the VOUT1 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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