Microcontroller directs supply sequencing and control

With the proliferation of dual-voltage architectures and multiprocessor boards, even simple applications can require several processor voltage rails. With each processor having its own power-up and -down requirements, power-rail sequencing and control can become a complex task. The challenge for power-supply designers is to consider each processor's timing and voltage requirements and assimilate these into a total system, ensuring that the final design meets the requirements of all processors.
Failure to properly power processors can lead to problems that range from the fairly benign, such as a reduction in MTBF, to the catastrophic, such as latch-up. Given the variety of available processors and the application challenges you expect when developing a power-sequencing and -control scheme, use of a microcontroller is desirable because of its programmability. The MSP430 is a good fit for such an application (Figure 1). The high-performance, low-cost 16-bit RISC processor has several high-quality analog peripherals and a JTAG interface.
Controlling power supplies that have enable pins, such as those on most "brick" dc/dc converters and low-dropout regulators, is simply a matter of using a GPIO (general-purpose I/O) line. If the power supply has no enable function, an inline switch, normally a MOSFET, can control the power supply, either with a GPIO or PWM signal. The circuit in Figure 1 uses the TPS725xx family of low-dropout regulators to provide 3.3, 2.5, and 1.8V from an input dc source. These regulators have an enable pin and a reset function. You can easily expand this circuit to any number of voltage rails. The MSP430 monitors a control variable to determine when to activate each rail. For power-sequencing applications, the two most commonly controlled variables are time and voltage. When time is the control variable, the controller enables the first rail. At a specific time thereafter, it enables the next rail. Some time after that, it enables the next rail, and so on until it has enabled all rails.

Figure 1: An ultra-low power microcontroller controls a system's power-supply sequencing

First Voltage Rail
The MSP430 provides the timing-sequence and the -control signals to turn on the power supplies. If voltage is the control variable, then the controller activates the first voltage rail and monitors its rise via an ADC. When the first voltage rail reaches a specific voltage level, the controller enables the next voltage rail and monitors its rise until it reaches a specified voltage level. At this point, the controller enables the next voltage rail and monitors it. This process continues until the controller has enabled all voltage rails. When using voltage as the control variable, the controller can use either a GPIO or a PWM signal as the enable signal, depending on whether the design requires rail tracking. You can also use a combination of voltage and timing control.
In Figure 1, each low-dropout regulator connects to two MSP430 lines-one for enabling and the other for monitoring. When time is the control variable, the monitoring takes place via Port 1 (GPIO); when voltage is the control variable the monitoring occurs via Port 6 (ADC). The MSP430 also provides a system reset and has an input for power-down. The code is fairly simple and does not require much programming experience. When time is the control variable, the first thing to do is initialize the MSP430 and setup the port and timer; this operation takes five lines of code (see Listing 1). The next operation is to load the capture-and-compare register zero (CCR0) with the first timing interval and start the timer. When CCR0's value is equal to the timer's value, the first voltage rail becomes enabled. CCR0 is then loaded with the next time interval, and the timer resets and restarts. When CCR0 is equal to the timer value, the second voltage rail becomes enabled. This operation repeats until all rails become enabled.
Once all rails are enabled, a delay loop enters the picture to ensure that the reset pins on the low-dropout regulators have time to come up. The TPS725xx family has an open-drain, 100-msec reset function. Once the delay is complete, the MSP430 checks each regulator's reset line to ensure that all rails are up. If all rails are up, the MSP430 issues a system reset. When voltage is the control variable, only five lines of code are necessary to initialize the MSP430 (Listing 2). The next operation is to load registers R9, R10, and R11 with values that represent 3.3, 2.5, and 1.8V, based on a 3V ADC reference. The first rail becomes enabled, and its output voltage undergoes monitoring until it is within specification, at which point the next rail is enabled and monitored. This operation repeats until all three rails become enabled. Once all the rails are enabled, the delay loop for regulator reset activates, and the system reset occurs.
Monitor Mode
Once the MSP430 turns on all the voltage rails and applies the system reset, it enters the monitor mode.
It continually checks the low-dropout regulator's output voltage, via the reset or output pins, depending on whether time or voltage is the control variable. If a fault occurs, the MSP430 enters an error routine. The most obvious fault would be the loss of a voltage rail, but other faults, such as overvoltage and undervoltage, are also amenable to monitoring. The actions that the error routine takes depend onthe application. The simplest actions would be to power down all rails, but programmability gives you complete control. One final function is the powering down of the voltage rails. An external signal, likely from the main processor, signals the MSP430 to power down the processor rails. In this example the power-down sequencing is just the opposite of the power-up sequence, but you can define any sequence. One addition to the power-down sequence could be to turn on dummy loads to discharge the output filter capacitors. This design uses the TPS725xx low-dropout regulators because they offer fast transient response and stability with any output capacitor. However, some applications may require large output capacitors to maintain stability and transient response. In these cases, the ability to discharge these filter capacitors improves MTBF.