ST AN1336 Application note
AN1336
APPLICATION NOTE
Power-Fail C omparator for
NVRAM Supervisory Devices
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) 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
Power-Fail Threshold (V
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 (see
Figure 2 on page 2).
Figure 1. Powe r- Fai l Warning
). This is made possible by the ability of a power supply to continue to function
PFI
) can be generated t
after the supply f alls belo w t he
PFD
PFI
V
PFI
PFO
t
PFD
AI04224
1/9January 2001
AN1336 - APPLICATION NOTE
Figure 2. Supply Hold-Up
AC Input
Regulated Output Voltage
Power-Fail Output
Power-Fail Warning
Supply Hold-up
AI04223
This is a result of the RC time constant inherent to most power supplies (see Figure 3). This time constant
is dominated by capacitors C
power-fa il, w hile C
and C1 will more directly affect the regu lat ed V cc slew r at e. Thus when t he A C input
3
and C3 (C2 is usually quite small). C1 will affect the V
1
slew rate during
UNREG
fails, this capacitance will continue to power t he circuit for several mil liseconds, typically on th e order of
10ms or more.
Figure 3. Typical Power Supply
V
UNREG
REGULATOR
C
C
1
2
C
3
V
CC
2/9
AI042222
AN1336 - APPLICATION NOTE
FUNCTIONAL DESCRIPTION
An independent bandgap reference comparator is used to monitor the unregulated supply voltage by connecting this supply to the Power-Fail Input pin. The RC time constant of the typical power supply will provide several milliseconds of operating voltage before decaying below a usable value. The Power-Fail Input
is constantly compared with an internal voltage reference of 1.25V (see Figure 4). If the input voltage falls
below 1.25V, the Power-Fail Output goes low. When it later goes above 1.25V, the output returns high.
Adding two external resistors (see Figure 5 on pa ge 4) as a v oltage d ivider circuit allows the comparat or
to supervise any voltage above 1.25V. The formula to calculate the trip point voltage of PFI (V
is dependent upon R1 and R2 is:
V
TRIP
PFI
=
R2
Where V
= 1.25V
PFI
V
(R1 + R2)
Figure 4. Powe r- Fai l Comparator C irc ui t
), wh i ch
PFI
PFI
1.25V
+
–
+
PFO
–
AI04221
The sum of both resistors should be about 1Mohm to minimize power consumption and to ensure the current in the PFI pin can be neglected compared with the current through the resistor network. The suggested resistor values are shown below (see Table 1). The tolerance of the resistors should not exceed 1% to
ensure the sensed voltage does not vary too much.
Tabl e 1. Look-up Table f or Diffe rent Tri p Points
R1 (kOhms) R2 (kOhms) Vtrip (V)
750 130 8.5
910 130 10.0
820 100 11.5
820 91 12.5
1100 100 15.0
3/9
Loading...