Voltage monitors improve the reliability of microcontroller-based systems by monitoring the power supply and putting the microcontroller in reset mode in the event of a power failure, preventing system errors and failures. However, power supply defects such as noise, voltage glitches, and transients can all cause false reset issues that can affect system behavior. This article describes how voltage supervisors can address factors that can trigger false resets to improve system performance and reliability.
For applications that require field-programmable gate arrays (FPGAs), microprocessors, digital signal processors, and microcontrollers for data computing and processing, it is important to ensure that each device can operate reliably and reliably. Since these devices can only operate within a certain power supply tolerance, the power supply requirements are high. Voltage monitors can be used to keep systems running reliably. In the event of an unexpected power failure, such as undervoltage or overvoltage, the voltage monitor can immediately trigger action to put the system in reset mode. However, it also faces some disturbances when monitoring the voltage in the power rail, which can trigger an unwanted false reset output. These disturbances include power supply noise, voltage transients, and glitches that can come from the power supply circuit itself.
This article will discuss the different parameters in voltage supervisors that can help address these supply noise, voltage transients, and glitches. In addition, it will be discussed how these parameters can improve the reliability of the voltage monitor when monitoring the power supply to improve the reliability of the system in the application.
Power supply noise, voltage transients, and glitches in the system
There is a defect in the power supply itself. There are always coupled noise artifacts in DC circuits, which can come from the power supply circuit components themselves, noise from other power supplies, and other noise generated by the system. If the DC power supply is a switching power supply (SMPS), these problems can be exacerbated. SMPS generates switching ripple that is dependent on the switching frequency. High-frequency switching transients also occur during switching transitions. These switching transitions are caused by fast turn-off of the power MOSFETs. Figure 1 shows an application circuit where the MAX705 supervisor is used to monitor the output of the switching regulator (i.e., the voltage source of the microcontroller) for any problems.
Figure 1The MAX705 monitor is used for monitoring.
Controls the output of the switching regulator, which is also the input voltage source for the microcontroller.
In addition to steady-state operating noise artifacts, there are also cases where voltage transients are noticeable in the power supply. During start-up, it is common to observe a voltage output overshoot associated with the power supply feedback loop response, followed by a period of voltage ringing until the voltage stabilizes. If the feedback loop compensation value is not optimized, this ringing can be more severe. Voltage overshoot and undershoot can also be observed during transient or dynamic loads. In specific applications, sometimes the load requires more current to perform complex processes, resulting in voltage undershoot. On the other hand, reducing the load immediately or at a fast ramp rate will result in a voltage overshoot. Short-time voltage glitches can also occur in the power supply due to external factors. Figure 2 shows that the supply voltage can have different voltage transients and glitches in different scenarios.
Figure 2Observable voltage transients and glitches for supply voltages in different scenarios.
Voltage transients that are independent of the supply voltage can also occur in the system, such as in user interfaces such as mechanical switches or conductive cards in some applications. Turning switches on and off creates voltage transients and noise on the input pins (usually manual reset pins). All of these factors (supply noise, voltage transients, and glitches) can inadvertently reach the supervisor's undervoltage or overvoltage thresholds, triggering false resets if this possibility is not fully accounted for in the design. This can lead to oscillations and instability, which is not conducive to the stability and reliability of the system.
How does a voltage monitor solve noise and transients and prevent false system resets?With a few parameters, it can help to shield out transients associated with the power supply or monitor voltage. These parameters include the reset timeout period, reset threshold hysteresis, and the change in reset threshold overdrive versus duration. At the same time, for transients associated with mechanical contacts in the circuit, such as pushbutton switches in the manual reset pin, the transients can also be shielded by using the manual reset setting cycle and debounce time. These parameters make the voltage monitor more robust and immune to transients and glitches, preventing undesirable system responses.
Reset Timeout Period (TRP).
During start-up or when the supply voltage rises and exceeds a threshold due to an undervoltage event, the reset signal has an additional period of time before it becomes invalid, known as the reset timeout period (TRP). For example, Figure 3 shows that after the monitored voltage (in this case, the supply voltage labeled VCC) reaches a threshold from an undervoltage or start-up state, there is an additional delay before the active-low reset high is invalid. This extra time allows the monitored voltage to stabilize and shield the system from overshoot and ringing before enabling it or taking it out of reset mode. The reset timeout period helps improve system reliability by suppressing false system resets and preventing oscillations and potential faults.
Figure 3The Reset Timeout Period (TRP) helps to keep the system in reset mode when the supply voltage is stable.
Threshold hysteresis (VTH+).
There are two main benefits to threshold hysteresis. First, it ensures that the monitoring voltage has enough headroom to exceed the threshold level before the reset is lifted. Second, it allows the power supply to stabilize before resetting it. When dealing with signals with superimposed noise, there is a chance that the reset output will be converted multiple times as the power supply fluctuates and re-crosses the threshold region. As shown in Figure 4, noise signals and voltage fluctuations can occur at any time in applications such as industrial environments. If there is no threshold hysteresis, the reset output signal will be continuously switched between set and unset until the power supply is stable. This causes the system to oscillate. Threshold hysteresis eliminates oscillations by keeping the system in reset, preventing the system from exhibiting the undesirable behavior shown in the blue shaded area in Figure 4. This helps the monitor protect the system from triggering a false reset.
Figure 4Reset output response for unset threshold hysteresis and set threshold hysteresis (the reset timeout period is not shown to focus on the effects of hysteresis).
Reset threshold overdrive vs. duration
Voltage glitches caused by external factors can occur in either the short or long term in any system. There may also be voltage dips of varying magnitudes. The change in reset threshold overdrive and transient duration is related to the amplitude and duration of the voltage glitch or overdrive. A short-term glitch with a large amplitude does not trigger the reset signal set, while an overdrive with a smaller amplitude and longer duration will trigger a reset, as shown in Figure 5.
Figure 5A glitch with a small amplitude but longer duration will trigger a reset signal, while a short-term glitch with a larger amplitude will not trigger a reset signal.
Depending on the glitch duration, some voltage transients in the monitored power supply will be ignored. Ignoring these transients will protect the system from interfering resets, such as those caused by short-term glitches. These glitches can mistakenly trigger a system reset, which can lead to undesirable behavior in the system. In product data sheets, the reset threshold overdrive vs. duration is typically presented as a typical performance characteristic graph, as shown in Figure 6. Any value above the curve will trigger a reset output, while values within the curve will be ignored to prevent the system from resetting by mistake.
Figure 6Whether or not the reset signal is set will depend on the amplitude of the overdrive and its duration.
Manually reset the set period (TMR) and debounce time (TDB).
Reset timeout periods, threshold overdrive vs. duration, and threshold hysteresis address voltage glitches and transients associated with the voltage being monitored, typically the power supply to the system's microcontroller. For burrs caused by mechanical contacts such as switches, manual reset setting cycles and debounce times can help mitigate the effects of voltage transients and glitches.
The Manual Reset Setup Period (TMR) is the time it takes for a manual reset to hold and complete before triggering the reset output. Some monitors have long manual reset setting cycles for enhanced protection of the system. These are common in consumer electronics and have to be held down for a few seconds to reset the system. This approach avoids accidental and unintentional resets, enhancing protection and improving reliability. During the manual reset setting, all short-term transients and glitches generated when the switch is pressed are ignored, as shown in Figure 7a, helping to protect the system from glitches.
The same logic goes for debounce time. As with the settling period, the debounce time (TDB) ignores high-frequency periodic voltage transients when the switch is turned on or off. These high-frequency transients will be considered invalid and will not trigger a reset, as shown in Figure 7b. When the signal exceeds the debounce time, it is considered a valid input signal from a switch or button.
Figure 7Manual reset setup cycle and debounce time plot for a monitor with a long manual reset setup cycle (MAX6444): (a) The manual reset setup cycle (TMR) needs to be completed before the reset signal is valid(b) To be considered a valid input signal, debounce time (TDB) needs to be completed.
Conclusion
Without a voltage monitor, the system is at risk of power outages and failures during voltage transients and glitches. In these cases, the voltage monitor solves the problem by putting the processor into reset mode. All of the parameters discussed above, including reset timeout periods, threshold hysteresis, threshold overdrives, manual reset set cycles, and debounce times, help protect the voltage monitor from faults and transients, enhancing its reliability in monitoring the supply voltage. As a result, the overall system performance can be ensured to be stable and reliable.