Voltage Supervisors

Overview

Voltage supervisors (a.k.a. reset ICs¹) are ICs that monitor a voltage and output a digital logic level which changes state if the voltage is above or below a threshold. They are typically used for monitoring supply rails and turning ON devices once the voltage reaches an operating level (or looking at it another way, turning OFF devices when the voltage falls below a safe level).

ERROR: fig-apx803l-voltage-supervisor-functional-block-diagram REF NOT FOUND shows the functional block diagram for a typical simple voltage supervisor.

Simple voltage supervisors are very similar to comparators, except for a few key differences:

1. Voltage supervisors are usually designed to operated from very low voltages, such that when the voltage drops, they remain on and keep the microcontroller in reset when the voltage is not high enough to keep the microcontroller operating correctly.
2. Voltage supervisors usually have built-in delays that keeps the MCU in reset for a minimum amount of time even if the voltage only drops below what is considered valid for a much shorter period of time.
3. Voltage supervisors usually have built-in voltage references. Whilst a traditional comparator does not, many comparator ICs also come with internal voltage references.

Most voltage supervisors have a \(\overline{RESET}\) output which is active low (i.e. drive low to reset, drive high to not reset), to align with the industry-standard \(\overline{RESET}\) input present on most MCUs.

Voltage supervisor ICs can come in families which include preset threshold voltages from about 1.2V to up to 5.0V. They also come in versions which have adjustable thresholds, at the expense of one more pin.

How They Work

Internally, voltage supervisors have a few core components. One is a stable voltage reference to generate a fixed and known voltage. This is fed into a comparator. The measured analogue voltage is fed to the other input of the comparator.

References