AN1336

Application note

Power-fail comparator for NVRAM supervisory devices

Introduction

Dealing with unexpected power loss

Inadvertent or unexpected loss of power can cause a number of system level problems. Memory loss,

uncontrolled program status and indeterminate processor state are just a few of the issues which can

occur during catastrophic power failure. Power-fail recovery is critical for applications created to perform

machine control or instrumentation monitoring, therefore knowing the state of the operating system at

the time of power loss is very important.

The function of the power-fail comparator is to provide several milliseconds of early warning that power

is failing. This advance warning (see Figure 1: "Power-fail warning") will allow a system to perform

operations necessary to prepare for a controlled shutdown sequence. By using a special power-fail input

supply to continue to function and to provide output power for a period of time after the input power to

the power supply has failed. This facility enables the power supply to ride through missing half cycles or

missing cycles in an AC supply (Figure 2: "Supply hold-up").

Figure 1: Power-fail warning

PFO

PFI

VPFI

tPFD

AI04224

March 2014

DocID007594 Rev 2

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