Under-Voltage Lockout — Protecting Systems from Brownout Damage

Brownout is less dramatic than a short circuit but potentially more damaging. When a circuit operates below its minimum supply voltage, strange things happen: logic gates sit at indeterminate levels, microcontrollers may execute random instructions, flash memory write cycles can corrupt data, and linear regulators operate outside their specification in ways that can produce output voltages that damage downstream loads.

The classic failure mode: a LiPo-powered robot continues running as the battery discharges, getting slower and less reliable, until the voltage drops low enough that the microcontroller starts behaving erratically. By this point, the ESC may have received corrupted commands, the flash may have been written in an undefined state, and the battery may have been damaged by deep discharge.

Under-voltage lockout (UVLO) prevents this by cutting power cleanly when the supply voltage falls below a defined threshold.

UVLO in regulators and ICs

Most modern linear and switching regulators include built-in UVLO. The regulator monitors its own input voltage and shuts down its output if the input drops below a threshold. This protects the regulator's downstream load from operating in an undefined voltage region.

Most microcontrollers also have internal brownout detection that resets the processor if the supply falls below a threshold. This is configurable in many microcontrollers (the STM32, for example, has a programmable voltage detector with configurable thresholds). The reset prevents erratic operation but doesn't disconnect power from the system.

For a complete solution, you want UVLO at the system level — a circuit that monitors the battery and disconnects the load before the battery is damaged and before any subsystem operates below specification.

Discrete UVLO circuit

A simple UVLO can be built with a comparator or voltage supervisor IC, a voltage divider to set the threshold, and a P-channel FET or load switch to disconnect the load.

The supervisor IC monitors the divided input voltage. When the input drops below the threshold, the supervisor's output toggles and turns off the FET. Hysteresis ensures the circuit doesn't oscillate around the threshold — once it trips off, it stays off until the voltage recovers to a higher level (typically set 5–10% above the shutoff threshold).

Common supervisor ICs: TLV803, MAX809, MCP101. These are small SOT-23 packages with minimal external components. An external FET scales the current capability to whatever the load requires.

Setting thresholds for LiPo

For a single-cell (1S) LiPo (nominal 3.7V, fully charged 4.2V, minimum safe discharge 3.0V):

UVLO trip threshold: 3.0–3.2V. Below 3.0V, the cell is in the overdischarge region where capacity is permanently lost and fire risk increases.

Recovery threshold: 3.4–3.5V (with hysteresis above the trip threshold to prevent oscillation).

For 2S (7.4V nominal): scale proportionally. Trip at 6.0–6.4V.

For microcontroller supplies specifically: set the UVLO above the microcontroller's minimum operating voltage, not just the battery minimum. An ESP32 requires minimum 3.0V at its supply input. If the battery is discharging and passes through this range, a UVLO set at 3.0V on the battery doesn't guarantee the regulated supply is above 3.0V when the regulator dropout is considered.