Datasheet DS1231S, DS1231 Datasheet (Dallas Semiconductor)

DS1231/S

DS1231/S

Power Monitor Chip

FEATURES

• Warns processor of an impending power failure

• Provides time for an orderly shutdown

• Prevents processor from destroying nonvolatile

memory during power transients

• Automatically restarts processor after power is

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

monitoring

CC

• Replaces power-up reset circuitry

• No external capacitors required

• Optional 16-pin SOIC surface mount package

DESCRIPTION

The DS1231 Power Monitor Chip uses a precise tem­perature-compensated reference circuit which provides
an orderly shutdown and an automatic restart of a pro­cessor-based system. A signal warning of an impending
power failure is generated well before regulated DC
voltages go out of specification by monitoring high volt­age inputs to the power supply regulators. If line isola­tion is required a UL-approved opto-isolator can be di­rectly interfaced to the DS1231. The time for processor

PIN ASSIGNMENT

1

8

IN

2

MODE

3

TOL

4

GND

DS1231 8–Pin DIP

(300 MIL)

See Mech. Drawings

Section

1

NC

2

IN

3

NC

MODE

GND

4
5

NC

6

TOL

7

NC

8

DS1231S 16–Pin SOIC

See Mech. Drawings

(300 MIL)

Section

VCC
NMI

7

RST

6
5

RST

NC

16
15

VCC
NC

14
13

NMI
NC

12

RST

11
10

NC

9

RST

PIN DESCRIPTION

IN – Input
MODE – Selects input pin characteristics
TOL – Selects 5% or 10% V
GND – Ground
RST – Reset (Active High)
RST
NMI
V

CC

– Reset (Active Low, open drain)
– Non–Maskable Interrupt
– +5V Supply

NC – No Connections

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 uncondi­tionally halts the processor to prevent spurious cycles
by enabling Reset as V

falls below a selectable 5 or

CC

10 percent threshold. When power returns, the proces­sor is held inactive until well after power conditions have
stabilized, safeguarding any nonvolatile memory in the
system from inadvertent data changes.

CC

detect

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 monitor­ing the input (Pin 1) and one for monitoring V
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 V

. On power-up, RST and

CC

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 V

CC

curs at VTP-. In this mode Pin 1 is an extremely high im­pedance input allowing for a simple resistor voltage di­vider network to interface with high voltage signals.
When the Mode pin is connected to ground, detection
occurs at V

+. In this mode Pin 1 sources 30 µA of cur-

TP

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

(Pin 8).

CC

, detection oc-

modes of operation the input pin has hysteresis for
noise immunity (Figure 3).

APPLICATION – MODE PIN
CONNECTED TO V

When 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 devel­oped from the AC power line by rectifying and filtering
the AC. Alternatively, a DC voltage level may be se­lected 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 net­work of R1 and R2. Since the IN trip point V
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

CC

- is 2.3

TP

.

.

022698 2/9

POWER MONITOR BLOCK DIAGRAM Figure 1

30A

IN

1

GND

4

V

CC

8

TOL

3

V

CC

TOLERANCE

BIAS

T.C. REFERENCE

NMI

MODE

SELECTION

DIGITAL

SAMPLER

DIGITAL

DELAY

DS1231/S

MODE

2

NMI

7

RST

6

RST

5

POWER-UP RESET Figure 2

DS1231

IN

MODE

TOL

GND

(-5% VCC THRESHOLD)

V

NMI

RST

RST

CC

+5V

NC

MICROPROCESSOR

RST

8051

µP

022698 3/9

DS1231/S

INPUT PIN HYSTERESIS Figure 3

NMI

MODE = GND, VCC = +5V

V

OH

–20

–35

–50

V

TP-

V

OL

NMI

MODE = VCC, VCC = +5V

V

OH

V

OL

V

TP-

TP-

VTP+

VTP+V

V

IN

V

IN

VTP+

NOTE: HYSTERESIS TOLERANCE IS +60 mV

APPLICATION WITH MODE PIN CONNECTED TO VCC Figure 4

VOLTAGE SENSE POINT

IN

MODE

TOL

GND

DS1231

V

NMI

RST

RST

CC

R1

+5V

R2

+5V

2.3

2.5

2.15

2.5

2.0

2.5

TO P

V SENSE 

EXAMPLE: V SENSE = 8 VOLTS AT TRIP POINT AND A

MAXIMUM VOLTAGE OF 17.5V WITH R2 = 10K

THEN 8 

R1  10K

NOTE: RST requires a pull–up resister.

022698 4/9

R1  R2

R2

10K

(-5% VCC THRESHOLD)

X2.3

X2.3

VMAX

R1 = 25K

V SENSE

VTP 

X5.0

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 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 maxi­mum 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

the trip point. This is particularly true if power consump­tion 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.

150V , then a zener diode of 150V will approximately set

AC VOLTAGE MONITOR WITH TRANSFORMER ISOLATION Figure 5

VOLTAGE SENSE POINT

DS1231

+5V

IN

MODE

DC

TOL

GND

V

NMI

RST

RST

CC

-10% V

THRESHOLD

CC

+5V

DC

TO µ P

NOTE: RST requires a pull–up resister.

022698 5/9

DS1231/S

APPLICATION WITH MODE PIN GROUNDED Figure 6

VOLTAGE SENSE POINT

R1

IF

IC

DS1231

IN

MODE

TOL

-5% VCC THRESHOLD

V

CC

NMI

RST

+5V

DC

TO µ P

GND

VOLTAGE SENSE POINT (TRIP VALUE)  VZ 

IC

CTR 

CTR  CURRENT TRANSFER RATIO

IF

VZ  ZENNER VOLTAGE

EXAMPLE: CTR = 0.2 IC = 30 µA IF = 150 µA

VOLTAGE SENSE POINT = 105 AND
VZ = 100 VOLTS

30

xR1 R133K

0.2

NOTE: RST

THEN 105  100 

requires a pull–up resister.

AC VOLTAGE MONITOR WITH OPTO-ISOLATION Figure 7

DS1231

IN

AC LINE
INPUT

NOTE: RST requires a pull–up resister.

MODE

TOL

GND

RST

V

NMI

RST

RST

IC

XR1

CTR

THRESHOLD

-5% V

CC

+5V

CC

DC

TO µ P

022698 6/9

DS1231/S

ABSOLUTE MAXIMUM RATINGS*

Voltage on VCC Pin Relative to Ground -0.5V to +7.0V
Voltage on I/O Relative to Ground -0.5V to V
Operating Temperature 0°C to 70°C
Operating Temperature (Industrial Version) –40°C to +85°C
Storage Temperature -55°C to +125°C
Soldering Temperature 260°C for 10 seconds

* This is a stress rating only and functional operation of the device at these or any other conditions above those

indicated in the operation sections of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods of time may affect reliability.

CC

+ 0.5V

RECOMMENDED DC OPERATING CONDITIONS (0°C to 70°C)

PARAMETER SYMBOL MIN TYP MAX UNITS NOTES

Supply Voltage V
Input Pin 1 V

CC

IN

4.5 5.0 5.5 V 1
V

CC

V 1

DC ELECTRICAL CHARACTERISTICS (0°C to 70°C; V

= 4.5 to 5.5V)

CC

PARAMETER SYMBOL MIN TYP MAX UNITS NOTES

Low Level @ RST V
Output Voltage @ –500 µA V
Input Leakage I
Output Current @2.4V I
Output Current @0.4V I
Operating Current I
Input Pin 1 (Mode=GND) I
Input Pin 1 (Mode=VCC) I
IN Trip Piont (Mode=GND) V
IN Trip Point (Mode=VCC) V

VCC Trip Point (TOL=GND) V
VCC Trip Point (TOL=VCC) V

OL
OH

IL
OH
OL
CC

C

C
TP
TP

CCTP
CCTP

VCC–0.5V VCC–0.1V V 1, 6

-10 +10 µA 2

1.0 2.0 mA 5

2.0 3.0 mA

0.5 2.0 mA 3

15 25 50 µA

See Figure 3 1

4.50 4.62 4.74 V 1

4.25 4.37 4.49 V 1

CAPACITANCE (T

0.4 V 1

0.1 µA

A

= 25°C)

PARAMETER SYMBOL MIN TYP MAX UNITS NOTES

Input Capacitance C
Output Capacitance C

IN

OUT

5 pF
7 pF

1

022698 7/9

DS1231/S

AC ELECTRICAL CHARACTERISTICS (0°C to 70°C; V

= 5V + 10%)

CC

PARAMETER SYMBOL MIN TYP MAX UNITS NOTES

VTP to NMI Delay t
VCC Slew Rate 4.75-4.25V t
VCC Detect to RST and RST t
VCC Detect to NMI t
VCC Detect to RST and RST t
VCC Slew Rate 4.25-4.75V t

IPD

F

RPD

IPU

RPU

R

300 µs

150 500 1000 ms 4

0 ns

1.1 µs

100 ns
200 µs 4

NOTES:

1. All voltages referenced to ground.
= +5.0 volts with outputs open.

2. V

CC

3. Measured with outputs open.

= 5 µs.

4. t

R

is an open drain output and requires a pull–up resister.

5. RST

6. RST remains within 0.5V of V

on power–down until VCC drops below 2.0V . RST remains within 0.5V of

CC

GND on power–down until VCC drops below 2.0V .

TIMING DIAGRAM: POWER-UP

V

TP

MODE=V

CC

INPUT PIN 1
MODE=GND

022698 8/9

V

NMI

t

R

4.5V

CC

4.25V

4.75V

t

IPU

RST

RST

V

OH

t

RPU

V

OH

V

OL

TIMING DIAGRAM: POWER-DOWN

V

INPUT PIN 1
MODE=V

CC

INPUT PIN 1
MODE=GND

NMI

TP

V

TP

t

IPD

V

OL

DS1231/S

V

RST

RST

t

F

CC

4.75V
V

CCTP

4.25V

t

RPD

RST SLEWS WITH V

V

OH

V

OL

CC

022698 9/9