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DS1231-20-DS1231-35-DS1231-50-DS1231S-20
Power Monitor
DS1231/S
DS1231/SPower Monitor Chip
022698 1/9
FEATURESWarns processor of an impending power failureProvides time for an orderly shutdownPrevents processor from destroying nonvolatile
memory during power transientsAutomatically restarts processor after power is
restoredSuitable for linear or switching power suppliesAdjusts to hold time of the power supplySupplies necessary signals for processor interfaceAccurate 5% or 10% VCC monitoringReplaces power-up reset circuitryNo external capacitors requiredOptional 16-pin SOIC surface mount package
PIN ASSIGNMENTMODE
GNDRST
VCC
NMI
RSTNC
TOL
DS1231S 16–Pin SOIC
(300 MIL)
See Mech. Drawings
Section
VCC
NMI
RST
RST
MODE
TOL
GND
DS1231 8–Pin DIP
(300 MIL)
See Mech. Drawings
Section59
PIN DESCRIPTION–Input
MODE–Selects input pin characteristics
TOL–Selects 5% or 10% VCC detect
GND–Ground
RST–Reset (Active High)
RST–Reset (Active Low, open drain)
NMI–Non–Maskable Interrupt
VCC–+5V Supply–No Connections
DESCRIPTIONThe DS1231 Power Monitor Chip uses a precise tem-
perature-compensated reference circuit which provides
an orderly shutdown and an automatic restart of a pro-
cessor-based system. A signal warning of an impending
power failure is generated well before regulated DC
voltages go out of specification by monitoring high volt-
age inputs to the power supply regulators. If line isola-
tion is required a UL-approved opto-isolator can be di-
rectly interfaced to the DS1231. The time for processor
shutdown is directly proportional to the available
hold-up time of the power supply. Just before the
hold-up time is exhausted, the Power Monitor uncondi-
tionally halts the processor to prevent spurious cycles
by enabling Reset as VCC falls below a selectable 5 or
10 percent threshold. When power returns, the proces-
sor is held inactive until well after power conditions have
stabilized, safeguarding any nonvolatile memory in the
system from inadvertent data changes.
DS1231/S
022698 2/9
OPERATIONThe DS1231 Power Monitor detects out-of-tolerance
power supply conditions and warns a processor-based
system of impending power failure. The main elements
of the DS1231 are illustrated in Figure 1. As shown, the
DS1231 actually has two comparators, one for monitor-
ing the input (Pin 1) and one for monitoring VCC (Pin 8).
The VCC comparator outputs the signals RST (Pin 5)
and RST (Pin 6) when VCC falls below a preset trip level
as defined by TOL (Pin 3).
When TOL is connected to ground, the RST and RST
signals will become active as VCC goes below 4.75
volts. When TOL is connected to VCC, the RST and RST
signals become active as VCC goes below 4.5 volts. The
RST and RST signals are excellent control signals for a
microprocessor, as processing is stopped at the last
possible moments of valid VCC. On power-up, RST and
RST are kept active for a minimum of 150 ms to allow the
power supply to stabilize (see Figure 2).
The comparator monitoring the input pin produces the
NMI signal (Pin 7) when the input threshold voltage
(VTP) falls to a level as determined by Mode (Pin 2).
When the Mode pin is connected to VCC, detection oc-
curs at VTP-. In this mode Pin 1 is an extremely high im-
pedance input allowing for a simple resistor voltage di-
vider network to interface with high voltage signals.
When the Mode pin is connected to ground, detection
occurs at VTP+. In this mode Pin 1 sources 30 mA of cur-
rent allowing for connection to switched inputs, such as
a UL-approved opto-isolator. The flexibility of the input
pin allows for detection of power loss at the earliest point
in a power supply system, maximizing the amount of
time allotted between NMI and RST. On power-up, NMI
is released as soon as the input threshold voltage (VTP)
is achieved and VCC is within nominal limits. In both
modes of operation the input pin has hysteresis for
noise immunity (Figure 3).
APPLICATION – MODE PIN
CONNECTED TO VCCWhen the Mode pin is connected to VCC, pin 1 is a high
impedance input. The voltage sense point and the level
of voltage at the sense point are dependent upon the
application (Figure 4). The sense point may be devel-
oped from the AC power line by rectifying and filtering
the AC. Alternatively, a DC voltage level may be se-
lected which is closer to the AC power input than the
regulated +5-volt supply, so that ample time is provided
for warning before regulation is lost.
Proper operation of the DS1231 requires a maximum
voltage of 5 volts at the input (Pin 1), which must be
derived from the maximum voltage at the sense point.
This is accomplished with a simple voltage divider net-
work of R1 and R2. Since the IN trip point VTP- is 2.3
volts (using the -20 device), and the maximum allowable
voltage on pin 1 is 5 volts, the dynamic range of voltage
at the sense point is set by the ratio of 2.3/5.0=.46 min.
This ratio determines the maximum deviation between
the maximum voltage at the sense point and the actual
voltage which will generate NMI.
Having established the desired ratio, and confirming
that the ratio is greater than .46 and less than 1, the
proper values for R1 and R2 can be determined by the
equation as shown in Figure 4. A simple approach to
solving this equation is to select a value for R2 which is
high enough impedance to keep power consumption
low, and solve for R1. Figure 5 illustrates how the
DS1231 can be interfaced to the AC power line when
the mode pin is connected to VCC.
DS1231/S
022698 3/9
POWER MONITOR BLOCK DIAGRAM Figure 1NMI
MODE
SELECTION
30A
TOLERANCE
BIAS
DIGITAL
SAMPLER
DIGITAL
DELAY
T.C. REFERENCE
GND
TOL
VCC
RST
MODE
NMI
RST
VCC
POWER-UP RESET Figure 2DS1231
VCC
NMI
RST
RST
MODE
TOL
GND
MICROPROCESSOR
+5V
RST
(-5% VCC THRESHOLD)
8051mP
DS1231/S
022698 4/9
INPUT PIN HYSTERESIS Figure 3NMI
VOH
VTP+VIN
VOL
MODE = GND, VCC = +5V
NMI
VOH
VIN
VOL
MODE = VCC, VCC = +5V
VTP-
VTP+
NOTE: HYSTERESIS TOLERANCE IS +60 mVVTP-
VTP+VTP-
APPLICATION WITH MODE PIN CONNECTED TO VCC Figure 4DS1231
VCC
NMI
RST
RST
MODE
TOL
GND
+5V
+5V
VOLTAGE SENSE POINTPSENSER1R2X2.3VMAXVSENSE
VTPX5.0
THEN8R110K
10KX2.3R1 = 25K
EXAMPLE:V SENSE = 8 VOLTS AT TRIP POINT AND A
MAXIMUM VOLTAGE OF 17.5V WITH R2 = 10K
(-5% VCC THRESHOLD)
NOTE: RST requires a pull–up resister.
DS1231/S
022698 5/9
APPLICATION – MODE PIN CONNECTED TO
GROUNDWhen the Mode pin is connected to ground, pin 1 is a
current source of 30 mA with a VTP+ of 2.5 volts. Pin 1 is
held below the trip point by a switching device like an
opto-isolator as shown in Figure 6. Determination of the
sense point has the same criteria as discussed in the
previous application. However, determining component
values is significantly different. In this mode, the maxi-
mum dynamic range of the sense point versus desired
trip voltage is primarily determined by the selection of a
zener diode. As an example, if the maximum voltage at
the sense point is 200V and the desired trip point is
150V, then a zener diode of 150V will approximately set
the trip point. This is particularly true if power consump-
tion on the high voltage side of the opto-isolator is not an
issue. However, if power consumption is a concern,
then it is desirable to make the value of R1 high. As the
value of R1 increases, the effect of the LED current in
the opto-isolator starts to affect the IN trip point. This can
be seen from the equation shown in Figure 6. R1 must
also be low enough to allow the opto-isolator to sink the
30 mA of collector current required by pin 1 and still have
enough resistance to keep the maximum current
through the opto-isolator’s LED within data sheet limits.
Figure 7 illustrates how the DS1231 can be interfaced to
the AC power line when the mode pin is grounded.
AC VOLTAGE MONITOR WITH TRANSFORMER ISOLATION Figure 5+5VDC
MODE
TOL
GND
VCC
NMI
RST
RST
-10% VCC THRESHOLD
TO m P
DS1231
+5VDC
VOLTAGE SENSE POINT
NOTE: RST requires a pull–up resister.
DS1231/S
022698 6/9
APPLICATION WITH MODE PIN GROUNDED Figure 6MODE
TOL
GND
VCC
NMI
RST
RST
TO m P
DS1231
+5VDC
VOLTAGE SENSE POINT
EXAMPLE:CTR = 0.2 IC = 30 mA IF = 150 mA
VOLTAGE SENSE POINT = 105 AND
VZ = 100 VOLTS
-5% VCC THRESHOLD
VOLTAGESENSEPOINT(TRIPVALUE)VZIC
CTRXR1
CTRICCTRCURRENTTRANSFERRATIOZENNERVOLTAGE
THEN10510030
0.2xR1R133K
NOTE: RST requires a pull–up resister.
AC VOLTAGE MONITOR WITH OPTO-ISOLATION Figure 7MODE
TOL
GND
VCC
NMI
RST
RST
-5% VCC THRESHOLD
TO m P
DS1231
+5VDC
AC LINE
INPUT
NOTE: RST requires a pull–up resister.
