Voltage monitoring devices perform multiple functions

Market trends have forced digital signal processor (DSP), microprocessor and field programmable gate array (FPGA) manufacturers to continuously increase the clock frequency to achieve higher performance, while also requiring lower power consumption. These two conflicting conditions have led to the development of multi-rail devices. Typically, a multi-rail device will have an I/O voltage, whose function is to power inputs and outputs, such as driving a system bus, communicating with existing logic devices or lighting an LED. The I/O voltage is usually 3.3V or 5.0V, which is one of the higher voltages on the circuit board. One or more lower core voltages are used to drive the high-frequency logic within the device. The low core voltage allows the logic to switch quickly while using less power because it reduces switching losses compared to higher voltages.

These high-performance multi-voltage devices require tight tolerances on the core voltage to enable the internal logic to operate correctly and to properly execute software code. In addition, in multi-voltage devices, there is generally a need for power-on voltage sequencing to avoid lockups and device damage during power-up and power-down. Voltage monitoring devices can be used in these high-performance devices to ensure smooth boot and code execution under different power conditions.

In its simplest form, a voltage monitor consists of a voltage detection circuit that triggers a timer reset output signal. Both the voltage monitor and the timer can use digital or analog electronics internally. Figure 1 shows a typical voltage monitor circuit. The voltage detection circuit is a comparator with hysteresis. The voltage to be monitored is typically divided by an external resistor divider and compared to a reference voltage. When the monitor is powered on, the /RESET output goes low. When the divided SENSE voltage exceeds the reference voltage, a retriggered one-shot timer is triggered. The timer pulls the /RESET signal low for a fixed period of time and sends the appropriate RESET signal to a DSP, microprocessor, FPGA, or other complex logic device.

Figure 1. Analog voltage monitoring setup.

In addition to simply monitoring the power rails, some monitoring devices also use a watchdog timer (WDT) to monitor the execution of the DSP or microprocessor program code. The watchdog timer is a special timing device that triggers a system reset if the DSP or microprocessor fails to reset the WDT regularly. Figure 2 shows a monitoring device with a voltage monitor and a watchdog timer. Generally, a DSP or microprocessor output pin can be used to drive the monitoring input of the monitoring device. The DSP must change the logic state of the output regularly to generate a pulse train. The watchdog timer monitors this pulse train. If the software enters an infinite loop or aborts the program, causing the DSP to stop generating the pulse train, the watchdog timer will time out and issue a reset signal to restart the DSP and recover from any software errors. The length of time from the stop pulse to the reset is determined by the oscillator and counter that make up the WDT circuit and depends on the device. The WDT time is generally 0.5 seconds to 2 seconds.

Figure 2. Monitoring device with voltage monitoring and watchdog timer.

The same monitor functionality can also be used digitally. Figure 3 shows the same voltage monitor implemented using a digital circuit. For a digital monitor, the voltage to be monitored is still divided down to provide a detection voltage (SENSE) within the input range of the analog-to-digital converter (ADC). The ADC then converts the SENSE voltage into a digital signal that the microprocessor can recognize. The microprocessor's I/O pin can be used to drive an open-drain MOSFET to provide a reset signal. Digital monitors can provide functionality that analog monitors do not.

Figure 3. Digital voltage monitoring device.

Digital monitors contain microprocessors, so they have more functionality and flexibility than analog monitors. Some digital monitors provide serial channels that can communicate with other power management devices or the main microprocessor. Key parameters and configuration information can be stored in the monitor and can be read and modified through the serial channel. This allows the reset threshold to be modified through software without changing the external voltage divider. In addition, the ADC allows the microprocessor to read a wide range of SENSE voltages, while the analog monitor voltage detection comparator only provides a signal to determine whether the voltage is above or below the reference voltage. The wide range of SENSE voltages allows digital monitors to be set up for variable undervoltage monitoring, variable overvoltage monitoring, and undervoltage detection. In addition, most microprocessors used in digital monitors have many I/O pins, so multiple voltage rails can be monitored with the same device. Digital voltage monitors can monitor 12 different rails, making them ideal for complex and large systems.

A voltage sequencer is a device derived from a voltage monitor. A voltage sequencer can be used to drive the enable pins of the power supplies in a system to provide power-up and power-down sequencing of multiple voltage rails. Figure 4 shows an example of a power-up/power-down configuration for a multi-voltage rail system. The voltage rises and falls at a given time with other system voltages. The voltage sequencer is used to control and monitor the power-up and power-down timing.

Figure 4. Power-up and power-down sequencing example.

Sequencers provide enable signals to power supplies instead of providing a system RESET signal. Sequencers are typically digital circuit or microprocessor type devices that can be configured for various power-on and power-off sequencing possibilities and combinations that are unique to individual systems. Voltage sequencers enable power supplies based on time or the status of other power supplies. After enabling a power supply, the power sequencer monitors the power supply for undervoltage or overvoltage events and takes appropriate action when these events occur. Appropriate actions include disabling the monitored power supply and other power supplies, retrying a failed power supply, de-sequencing all power supplies, or taking other set actions. Voltage sequencers can control up to 8 power supplies with a single sequencing IC. Sequencer ICs can be connected in series to provide sequencing of more than 8 power supplies when necessary. Figure 5 shows the general configuration of an 8-channel voltage sequencer.

Figure 5. 8-Channel Voltage Sequencer The voltage level triggering single basic function of the monitor can be used in many applications beyond its intended use. For example, one common application of the voltage monitor is to provide a bounce-free mechanism for push button switches. When the push button switch is normally turned on, it is not pressed, so the contacts do not touch to complete the circuit. The mechanical parts will rebound quickly, so the connection and disconnection time is only a few milliseconds before the mechanical parts finally settle to complete the circuit. If the switch is used to trigger edge-triggered logic, there will be many cases of logic false triggering when the switch rebounds. The switch will also rebound when the button is released to open the circuit.

Most monitors have an input for a manual reset switch connected to a pin (usually labeled MR). A reset is initiated when the switch is pressed, as if the monitored voltage were to drop. Monitors can be used to connect a push button to this pin to prevent the push button switch from bouncing. The reset time must be longer than the time it takes to debounce or bounce the switch. Figure 6 shows an example of a voltage monitor as a switch bounce prevention circuit with a push button switch connected to the MR pin of the TPS3808. The switch specifications specify a maximum bounce time of 8 milliseconds and a maximum non-bounce time of 10 milliseconds. The external 2.2nF capacitor sets the reset time length to 13 milliseconds, which is longer than either the debounce or bounce time. Figure 7 shows the expected waveforms for the push button input and reset output.

Figure 6. Monitoring device for switch rebound prevention

Figure 7. Timing of switch bounce prevention circuit

Figure 8 shows another example of a monitor used as a simple timer in a camera flash circuit to drive a high current white LED (WLED). In this example, the high power WLED is driven with 1A of current, thus producing the high brightness flash required for digital photography. However, the WLED cannot sustain this amount of current indefinitely due to the thermal limitations of its package. When the flash button is released, the monitor provides a 570 millisecond LED flash.

Figure 8. WLED flash timer using voltage monitoring device.

The monitoring function of the monitoring device can also be used for other purposes outside of the intended application. The monitoring function can be used to detect when the pulse train stops. For example, the monitoring function can monitor the serial communication line to determine when the serial flow of data or frequency stops, or detect a stalled or locked motor rotor. Figure 9 shows the watchdog timer used to detect a locked motor rotor. In this example, a microprocessor is used to drive the motor. A fiber-optic slot tachometer is attached to the end of the motor shaft, and the shaft provides a pulse train as long as the motor is turning. The tachometer's phototransistor drives the watchdog timer input WDI with the tachometer pulse train. If the motor rotor stops turning, the pulse train stops, so the watchdog timer is not reset. After 1.4 seconds, if there are no pulses from the tachometer, the TPS3128 will drive the output of the reset pin low. This low signal drives the input pin of the microprocessor, so the microprocessor can adjust the drive output to compensate for the locked rotor condition. Figure 10 shows the timing of the locked rotor condition.

Figure 9. Locked motor rotor detection using a watchdog timer.

Figure 10. Locked rotor timing.

summary

Voltage monitoring devices and sequencers have been available for many years. Newer devices now operate digitally, allowing for increased flexibility, the number of channels monitored or sequenced, variable voltage thresholds, and varying timing parameters. However, even with these new high-performance digital monitoring devices, older analog monitoring devices are still suitable for a small number of monitored channels, or to provide other functions in systems such as switch bounce prevention, or as timing elements.