Monitoring and sequencing supply voltages - Part 1

For most electronic systems, monitoring system voltages with a power-on reset (POR) ensures proper initialisation at power-up.

Detecting brownout conditions with a POR minimises possible code-execution problems that can corrupt memory or cause a system to execute improperly.

To improve reliability in high-end systems, it is often necessary to sequence a system's power supplies in the correct order to prevent its microcontrollers, microprocessors, DSPs or ASICs from latching up - a problem that can cause damage or long-term reliability issues.

In most cases, one or more microprocessor supervisor ICs can perform these sequencing and monitoring functions.

A simple way to monitor a system supply voltage is with a voltage detector, an IC that combines a comparator and an internal reference.

When the supply voltage falls below the voltage detector's threshold, its output asserts to notify the system's microcontroller of an impending power failure.

This warning gives the microcontroller an opportunity to back up its memory, turn on or off power supplies and shut down the system in a controlled manner.

When a voltage detector changes states during power up or down, its output asserts after a short propagation delay. This is fine for a power-fail warning.

In most cases, though, a microcontroller's reset input requires a longer delay; during power-up, the system clocks and power supplies must stabilise and the processor's registers must initialise before releasing the microcontroller from its reset state.

A power-on reset (POR) is a form of microprocessor supervisor IC that provides this longer delay (called a reset timeout) to permit the system to fully initialise before allowing the microcontroller to operate.

Also, when the supply voltage falls temporarily below the POR's threshold after power-up has occurred, this same delay applies after the supply voltage returns above the POR threshold.

Power-on resets are available with a number of different fixed timeout periods and threshold voltages and some provide capacitor-adjustable timeout periods.

Most systems monitor the 3.3 V I/O logic supply. For systems requiring higher reliability, it may be necessary to monitor additional supplies, such as those that power cores and memory.

Numerous multi-voltage microprocessor supervisors are capable of performing this task, but the specific requirements of a given system can quickly reduce the number of choices.

Although most supervisors monitor standard voltages such as 5, 3.3, 2.5, and 1.8, it is often necessary to monitor additional voltages because various components (eg, memories, PLDs, ASICs) have unique power-supply requirements.

As a result, you must decide whether to use a fixed-threshold device (which requires no external resistors) or a more flexible adjustable-threshold device that accommodates changes as needed (but that requires external resistors).

A device with a combination of fixed and adjustable thresholds might provide the best solution. When selecting a device, it's important to choose one with a reference whose voltage is low enough to monitor the system's lowest voltage.

When working with 0.8, 0.9, and 1 V supplies, for example, a device with a standard 1.2 V reference won't work.

The number of supply voltages present in high-reliability systems has increased in recent years; 10 or more voltages is common. When monitoring a large number of voltages, you can end up using several supervisor devices.

Figure 1: Two multi-voltage supervisors with open-drain outputs monitor eight voltages and provide a single reset output.

Multi-voltage supervisors with open-drain outputs are often advantageous in these situations because their outputs can be ORd together to provide a single output.

Figure 1 shows an example of two MAX6710s connected to provide one reset signal while monitoring eight voltages.

It may be necessary to monitor some power supplies not just for undervoltage, but for overvoltage conditions as well. Overvoltage monitoring has become necessary in many systems to prevent damage to expensive processors and ASICs.

A window detector that monitors both overvoltage and undervoltage conditions can be constructed with two voltage detectors and a reference. Alternatively, you can use a dedicated window detector IC such as the MAX6754. Another type of voltage protection circuit includes an external p-channel MOSFET that shuts off a supply if the supply voltage exceeds a specified level. (See Figure 2.)

You can conveniently sequence power by using the enable or shutdown pins of the power regulators. Under this 'daisy-chaining' scheme, when a power supply first comes up, it asserts its power OK (POK) signal (if it has one) to notify other circuitry that its voltage is within tolerance.

Figure 2: When this supervisory circuit detects an overvoltage condition, the p-channel MOSFET disconnects the supply.

The POK output connects to the shutdown or enable pin of the second regulator and turns on that regulator when it goes active.

Figure 3 shows a diagram of this approach. For those situations where a longer delay is needed, some regulators include a POR; this arrangement allows a longer time delay before turning on the next power supply in the sequence.

When a POK signal isn't available, you can monitor a power supply's output with a voltage detector or a POR by connecting the detector or POR output to the second supply's shutdown or enable input.

Figure 3: A power supply with a POK output provides a convenient way to sequence other supplies.

The second supply turns on when the monitored voltage exceeds a specific threshold. When used with noisy power supplies, a voltage detector might unnecessarily turn a regulator on and off several times, especially if the monitored voltage level is near its trip threshold.

In these situations, a power-on reset circuit can minimise this effect - a benefit of the POR's timeout period. When the monitored voltage falls below the supervisor's threshold, the POR's output asserts and remains asserted for at least the minimum reset timeout period after the monitored voltage returns above the threshold. The voltage must be above the reset threshold continuously during the timeout period for the supervisor to de-assert, thus preventing the power supply from cycling repeatedly.

In addition to these benefits, using a POR to generate a signal for the shutdown or enable pin allows you to control the turn-on time; PORs have reset timeouts that range from a few microseconds to more than a second.

Capacitor-adjustable devices allow you to change the timeout period of a given device.

A power-on-reset circuit also provides the ability to control other power-up sequencing situations. For instance, in a system with three power supplies, you might want the first two supplies to be valid before the third supply is activated.

If a single regulator without a POK output generates the first two supplies, you can use a dual-voltage POR to monitor its two voltages.

This POR's output then controls the sequencing of the third supply by feeding its enable or shutdown pin. To sequence larger numbers of supplies, you can use multi-voltage devices.

When using a 'silver box' or 'brick' power supply, turning on and off each voltage in a controlled order isn't always possible without additional circuitry.

These power supplies provide standard voltages, such as 5, 3.3, 2.5, and 1.8, that are often distributed throughout a system.

A 'brick' can provide, for example, a 3.3 V logic supply and 1.8 V core supply to two different ICs. In some situations, these ICs will require different power sequencing; one device needs the core supply to rise first while the second device requires that the I/O supply rises first.

One way to sequence supplies in this situation is to switch power through an external pass element.