Undervoltage Lockout (UVLO)
It is the circuit that shuts the driver down when the supply voltage falls below a set threshold and keeps it off until the voltage recovers. Its purpose is to prevent the power switches from failing to turn fully on under insufficient supply and overheating as a result. A hysteresis is usually added to the threshold.
If sufficient voltage is not applied to the gate of a power MOSFET it cannot turn fully on and behaves like a resistor. In this region the loss grows quickly; a supply a few volts short can cause destruction within seconds. Undervoltage lockout prevents exactly this.
In the field it engages most often in the following scenarios:
- Weak battery: When high current is drawn at start-up, the voltage collapses across the battery's internal resistance.
- Thin or long cable: The voltage drop along the line brings the voltage read at the driver terminal below the threshold.
- Loose terminal: Increased contact resistance creates a noticeable drop under load.
For diagnosis, measure the voltage not at the battery terminal but at the driver's supply terminal and while the motor is at full load. The difference between the two measurements gives the line loss directly. Thanks to the hysteresis the system does not keep switching on and off at the threshold boundary; even so, frequent lockout shows that the supply path is inadequate.
The context in which this term is used: DC Motor Driver Selection Guide