Simple Circuit for Power Supply Sequencing

　　ASICs, FPGAs, and DSPs may require multiple supply voltages with various constraints on the order in which they can be started up. The highest voltage I/O voltage must often be started up first, followed by the other voltages in descending order, with the core voltage being the last. This may also require that the voltage on one supply line not exceed the voltage on another by more than one diode drop; otherwise, excessive current can flow back from the I/O voltage through the IC to the lower voltage, potentially damaging the expensive IC. The usual way you control this sequencing is to connect external diodes between the adjacent voltage lines being sequenced to clamp a higher voltage to within one diode drop of a lower voltage, thereby preventing possible latch-up in the IC. Diodes conduct only when a lower voltage rises above a higher voltage after power is applied, and turn off when the higher voltage rises above all the lower voltages because the diodes are reverse biased. A better approach is to use a power supply controller to precisely control the startup voltage sequencing of the supply lines. Figure 1 shows a simple op amp circuit with a dual switching power supply integrated to provide parallel output voltage sequencing.

　　In this power sequencing circuit, the three output voltages start up in sequence, with each output voltage tracking the next highest voltage during startup until it reaches a fixed, regulated voltage. Assuming that a 3.3V "master" I/O voltage (not shown) is powered up properly, the controller for that voltage uses its soft-start function to provide a
smooth, linear ramp of its voltage. The TPS5120 dual switching regulator generates the other two voltages, 2.5V and 1.8V. In most standard switching regulator circuits, the lower ends of R4 and R10 are connected to ground, thereby fixing the output voltage set point. In this circuit, the amplifier output controls the voltage at the lower ends of these resistors. A zero output voltage at the amplifier sets the output voltage to a predetermined fixed voltage, but any voltage greater than zero forces the output voltage below its set point.
　　These amplifiers use an inverter circuit that uses the next highest output voltage as its input or "sense" voltage. Therefore, at power-up, if the 3.3V output is at 0V, the output voltage of amplifier IC1 is high, forcing the TPS5120 controller to regulate its output voltage to 0V. The output voltage of amplifier IC3 is also high because the 2.5V output (also at 0V) controls the input voltage. As the 3.3V output voltage rises linearly, the output voltage of the amplifier falls linearly to 0V. Therefore, the 2.5V output voltage rises from 0V to its maximum set point of 2.5V. The 1.8V output voltage tracks the 2.5V output in the same way. The amplifier component values ​​are set so that the output voltage of the amplifier reaches 0V just when the sense voltage (for example, 3.3V) reaches the level of the tracking voltage (here, 2.5V). Thus, increasing the sense voltage beyond 2.5V does not further increase the tracking output voltage because the output voltage of the amplifier is already saturated at ground.
　　Parallel tracking requires several important design criteria. The amplifier feedback ratio R5-R6 must be equal to the feedback resistor divider ratio set by R1 and R4, and you must use the TPS5120 controller's reference voltage (0.85V in this case) as the amplifier's non-inverting input. Any reference voltage different from this value will force the tracking voltage output to a voltage that is not equal to the sense voltage. The amplifier you choose should have a low input offset voltage and be able to produce an output voltage at least as large as the controller reference voltage.

　　Rail-to-rail amplifiers are well suited for this purpose. Separate amplifiers allow for localized component placement and avoid routing near any noise sources. This design uses an additional decoupling capacitor near the amplifier’s noninverting input, which is used as a reference voltage. It uses a small soft-start capacitor value for the TPS5120 controller, which allows the controller to inherently start up faster than the 3.3V sense voltage. Larger soft-start capacitor values ​​do not track the output quickly enough. Too small a capacitor value can cause the output voltage to overshoot when the power supply is initialized. Figure 2 shows the startup voltages for the three synchronous buck converters. 3.3V is used as the main voltage, and 2.5V and 1.8V track their high voltages, respectively. You can set the sense voltage for the 1.8V output to track the 3.3V output instead of the 2.5V output, and you’ll still get just as good linear tracking at power supply startup. You can also add this sequencing circuit to any power supply controller that can take advantage of its reference voltage, soft-start capacitor, and output voltage resistor divider network.