DS1231/S
DS1231/S
Power Monitor Chip
FEATURES
PIN ASSIGNMENT
IN
MODE
TOL
GND
1
2
3
4
8
7
6
5
VCC
NMI
RST
RST
•
Warns processor of an impending power failure
•
Provides time for an orderly shutdown
•
Prevents
processor from destroying nonvolatile
memory during power transients
restarts processor after power is
DS1231 8–Pin DIP
(300 MIL)
See Mech. Drawings
Section
NC
IN
NC
MODE
NC
TOL
NC
GND
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
NC
VCC
NC
NMI
NC
RST
NC
RST
•
Automatically
restored
•
Suitable for linear or switching power supplies
•
Adjusts to hold time of the power supply
•
Supplies necessary signals for processor interface
•
Accurate 5% or 10% V
CC
monitoring
•
Replaces power-up reset circuitry
•
No external capacitors required
•
Optional 16-pin SOIC surface mount package
DS1231S 16–Pin SOIC
(300 MIL)
See Mech. Drawings
Section
PIN DESCRIPTION
IN
MODE
TOL
GND
RST
RST
NMI
V
CC
NC
–
–
–
–
–
–
–
–
–
Input
Selects input pin characteristics
Selects 5% or 10% V
CC
detect
Ground
Reset (Active High)
Reset (Active Low, open drain)
Non–Maskable Interrupt
+5V Supply
No Connections
DESCRIPTION
The 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 V
CC
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.
022698 1/9
DS1231/S
OPERATION
The 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 V
CC
(Pin 8).
The V
CC
comparator outputs the signals RST (Pin 5)
and RST (Pin 6) when V
CC
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 V
CC
goes below 4.75
volts. When TOL is connected to V
CC
, the RST and RST
signals become active as V
CC
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 V
CC
. 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
(V
TP
) falls to a level as determined by Mode (Pin 2).
When the Mode pin is connected to V
CC
, detection oc-
curs at V
TP
-. 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 V
TP
+. In this mode Pin 1 sources 30
µA
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 (V
TP
)
is achieved and V
CC
is within nominal limits. In both
modes of operation the input pin has hysteresis for
noise immunity (Figure 3).
APPLICATION – MODE PIN
CONNECTED TO V
CC
When the Mode pin is connected to V
CC
, 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 V
TP
- 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 V
CC
.
022698 2/9
DS1231/S
APPLICATION – MODE PIN CONNECTED TO
GROUND
When the Mode pin is connected to ground, pin 1 is a
current source of 30
µA
with a V
TP
+ 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
µA
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
VOLTAGE SENSE POINT
DS1231
IN
MODE
+5V
DC
TOL
GND
V
CC
NMI
RST
TO
µ
P
-10% V
CC
THRESHOLD
+5V
DC
RST
NOTE: RST requires a pull–up resister.
022698 5/9
MCU
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