Supervisor
A voltage supervisor monitors one or more supply rails and holds the system in reset or issues a flag the moment any rail falls out of specification. It guarantees that a microcontroller or FPGA only executes code when its supply is in the valid operating range.
An internal band-gap reference is compared to a resistor-divided sample of the monitored rail. When the rail drops below the threshold, the comparator output changes state and the device drives the nRESET pin low (or asserts an interrupt) for a fixed or adjustable timeout before releasing.
In plain terms
A hall monitor for voltage: it stands at the rail's door and rings an alarm the instant the voltage drops below the acceptable minimum, ensuring the microcontroller never runs on a supply too low to operate reliably.
Why designers use it
- Prevent microcontrollers from executing garbage code when the supply droops during brownout.
- Sequence multiple rails — hold the processor in reset until all supply rails are valid.
- Monitor battery voltage and warn firmware before a hard shutdown.
- Detect brown-out earlier than the MCU's internal BOD which typically only triggers at very low voltages.
Best for
- MCU reset
- Rail sequencing
- Battery monitor
Key specifications
- Threshold accuracy: ±0.5 % – ±2 %
- Reset delay: 1 ms – 1 s
- Quiescent current: 1 µA – 50 µA
- Supply: 0.7 V – 36 V
- Output: Open-drain or push-pull
When not to use it
- Where the MCU's built-in brown-out detector meets the system's requirements — an external supervisor adds cost and pins.
Common mistakes
- Picking a threshold too close to the normal operating voltage, so power-supply ripple triggers spurious resets.
- Forgetting the power-on delay — the supervisor holds reset for a programmable time after the rail is valid, which can confuse debug sessions.
Where you will find it
- An automotive ECU uses a voltage supervisor to hold the microcontroller in reset during engine-start: when the starter motor drags the 12 V bus down to 6 V, the supervisor asserts reset immediately, preventing the processor from executing corrupted instructions and writing garbage to EEPROM.
- A spacecraft's command-and-data-handling computer uses a supervisory IC on each of its eight supply rails: if any rail falls out of spec due to a radiation-induced latch-up, the supervisor resets the affected card within 10 ms, preventing the card from sending corrupted telemetry to the ground station.
- A gas meter's ultrasonic measurement IC uses a voltage supervisor with a manual-reset pin tied to a magnetic reed switch: a field technician can trigger a safe restart by holding a magnet to the meter's exterior without opening the sealed enclosure.
A short history
A voltage supervisor monitors a power supply rail and produces an output once a preset threshold is crossed. When voltage drops below that threshold, it can assert a reset signal, holding a microcontroller in reset or disabling another device until proper voltage returns, helping prevent system errors during brownout conditions and improving overall reliability.
Good to know
- A supervisor IC is the silent heroes of every microcontroller board: it watches Vcc and yanks the RESET pin low any time the rail dips below the safe operating threshold.
- Without a supervisor, an MCU coming out of a brown-out can land in an unknown state — the textbook cause of 'random' field failures.
- Multi-rail supervisors with windowed thresholds (under and over) protect FPGAs whose I/O can latch up if rails come up out of order.