www.analog.com POWER SUPPLY DESIGN 13 www.power-mag.com Issue 1 2024 Power Electronics Europe The role of voltage supervisors for system power reliability Voltage supervisors add reliability by monitoring the power supply failures and putting a microcontroller in reset mode to prevent system error and malfunction. By Noel Tenorio, Product Applications Engineer, Analog Devices Power supply imperfections such as noise, voltage glitches, and transients can lead to false and nuisance resets that can affect system behaviour. Voltage supervisors address factors that can trigger false and nuisance resets to improve system performance and reliability. Applications that compute and process data requiring FPGAs, microprocessors, DSPs and microcontrollers depend on safe and reliable operations. These devices tax power supply requirements as they are only allowed to operate at a certain range of power supply tolerance. Voltage supervisors can act immediately to put the system in reset mode when an unexpected failure from the power supply arises, such as under-voltage or over-voltage. Monitoring voltages in power supply rails always comes with some nuisances that can trigger unwanted false reset outputs. These are power supply noises, voltage transients, and glitches that can come from the power supply circuit itself. System glitches Power supplies have inherent imperfections. There are always noise artefacts coupled on the DC that can come from the power supply circuit component itself, noise from other power supplies, and other noise generated from the system. These problems can be worse if the DC power supply is a switch mode power supply (SMPS). SMPS produces switching ripple that is coherently related to the switching frequency. They also produce high frequency switching transients that occur during switching transitions. These transitions are caused by the fast on and off switching of the power MOSFETs. Figure 1 shows an application circuit in which the MAX705 supervisor is used to monitor any failure in the output of the switching regulator, which is the voltage supply of the microcontroller. Aside from the steady-state operation noise artefacts, there are also scenarios in the power supply where voltage transients are more pronounced. During startup, a voltage output over-shoot is usually observed related to the feedback-loop response of the power supply and is followed by voltage ringing for some time until it reaches stability. This ringing can be worse if the feedback loop compensation values are not optimised. Voltage overshoot and under-shoot can also be observed during transient or dynamic loading. In the applications, there are times when the load needs more current to execute complex processes, which leads to a voltage under-shoot. On the other hand, reducing the load instantly or at a fast ramp rate will give a voltage over-shoot. There are also short-duration voltage glitches that can occur to the power supply due to external factors. Figure 2 shows an illustration of the different voltage transients and glitches that can be present on a Figure 1: The MAX705 supervisor is used to monitor a switching regulator output, which is the input voltage supply of a microcontroller. Figure 2: Voltage transients and glitches that can be observed on a supply voltage in different scenarios.
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