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Adding Hysteresis for Smooth Undervoltage and Overvoltage Lockout

– Pinkesh Sachdev, Senior Applications Engineer, ADI

Resistive dividers attenuate high voltages down to a level that low voltage circuits can accommodate without getting overdriven or damaged. In power path control circuits, resistive dividers help set up power supply undervoltage and overvoltage lockout thresholds. Such supply voltage qualification circuits are found in automotive systems, battery-powered portable instruments, and data processing and communication boards. Undervoltage lockout (UVLO) prevents the downstream electronic system from operating with abnormally low power supply voltages, which could cause system malfunction. For example, digital systems can behave erratically or even freeze up when their supply voltage is below specification. When the power supply is a rechargeable battery, undervoltage lockout prevents battery damage due to deep discharge. Overvoltage lockout (OVLO) protects the system from damagingly high supply voltages. Since undervoltage and overvoltage thresholds depend on the system’s valid operating range, resistive dividers are used to set up custom thresholds with the same control circuit. Threshold hysteresis is needed to obtain a smooth and chatter-free lockout function even in the presence of supply noise or resistance. After discussing a simple UVLO/OVLO circuit, this article will present some simple methods for adding threshold hysteresis, which is necessary when the default value is insufficient.

Undervoltage and Overvoltage Lockout Circuit

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