This application note introduces a power-fail comparator for NVRAM supervisory devices, which can deal with issues caused by unexpected power loss and provide early warning functionality. It can monitor the unregulated supply voltage and generate a warning signal when the supply voltage falls below a preset threshold, allowing the system to perform a controlled shutdown sequence.
The document describes a method using an independent bandgap reference comparator to monitor unregulated supply voltage. It details how connecting this supply to the Power-Fail Input (PFI) pin provides several milliseconds of early warning when power is failing, allowing systems to perform necessary operations for a controlled shutdown sequence before the regulated voltage drops below usable levels.
The note provides functional descriptions, circuit diagrams showing the use of external resistors as a voltage divider, and tables with suggested resistor values for different trip points. It also covers system operation sequences during power failure, advantages over traditional monitoring devices, and methods for adding hysteresis to improve noise margin.
| Internal Voltage Reference (VPFI) | 1.25V |
|---|---|
| Recommended Resistor Sum | about 1Mohm |
It provides several milliseconds of early warning that power is failing, allowing a system to perform operations necessary for a controlled shutdown sequence.
It compares the input voltage against an internal reference; if the input falls below 1.25V, the Power-Fail Output goes low.
Hysteresis may be added for additional noise margin if desired by connecting a resistor between PFI and PFO.