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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
(PFI) to monitor the unregulated supply voltage, a power fail output (
PFO ) can be generated t
PFD
after
the supply falls below the power-fail threshold (V
PFI
). This is made possible by the ability of a power
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
V
PFI
t
PFD
AI04224
March 2014 DocID007594 Rev 2 1/10
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