Rainbow Electronics LM76 User Manual

January 2000

LM76

±

0.5˚C,±1˚C, 12-Bit + Sign Digital Temperature Sensor
and Thermal Window Comparator with Two-Wire
Interface

LM76

with Two-Wire Interface

±

0.5˚C,

±

1˚C, 12-Bit + Sign Digital Temperature Sensor and Thermal Window Comparator

General Description

The LM76 is a digital temperature sensor and thermal win­dow comparator with an I
accuracy of
fied for a −10˚C to 45˚C temperature range, while for the
LM76CNM the temperature range is 70˚C to 100˚C. The
LM76CHM isspecifiedwith an accuracy
window-comparator architecture of the LM76 eases the de­sign of temperature control systems conforming to the ACPI
(Advanced Configuration and Power Interface) specification
for personal computers. The open-drain Interrupt (INT) out­put becomes active whenever temperature goes outside a
programmable window, while a separate Critical Tempera­ture Alarm (T_CRIT_A) output becomes active when the
temperature exceeds a programmable critical limit. The INT
output can operate in either a comparator or event mode,
while the T_CRIT_A output operates in comparator mode
only.

The host can program both the upper and lower limits of the
window as well as the critical temperature limit. Program­mable hysterisis as well as a fault queue are available to
minimize false tripping. Two pins (A0, A1) are available for
address selection. The sensor powers up with default thresh­olds of 2˚C T
T_CRIT.

The LM76’s 3.3V and 5.0V supply voltage, Serial Bus inter­face, 12-bit + sign output, and full-scale range of over 127˚C
make it ideal for a wide range of applications. These include
thermal management and protection applications in personal
computers, electronic test equipment, office electronics and
bio-medical applications.

±

1˚C. This accuracy for the LM76CHM is speci-

HYST

2

C™Serial Bus interface with an

±

0.5˚C at 25˚C. The

, 10˚C T

LOW

, 64˚C T

HIGH

, and 80˚C

Features

n Window comparison simplifies design of ACPI

compatible temperature monitoring and control.

n Serial Bus interface
n Separate open-drain outputs for Interrupt and Critical

Temperature shutdown

n Shutdown mode to minimize power consumption
n Up to 4 LM76s can be connected to a single bus
n 12-bit + sign output; full-scale reading of over 127˚C

Key Specifications

j

Supply Voltage 3.3V or 5.0V

j

Supply Current operating 250 µA (typ)

450 µA (max)

shutdown 8 µA (max)

j

Temperature

Accuracy

−10˚C to +45˚C
70˚C to 100˚C

j

Resolution 0.0625˚C

+25˚C

±

0.5˚C(max)

±

1.0˚C(max)

±

1.0˚C(max)

Applications

n System Thermal Management
n Personal Computers
n Office Electronics
n HVAC

I2C®is a registered trademark of Philips Corporation.

© 2001 National Semiconductor Corporation DS101015 www.national.com

Simplified Block Diagram

LM76

Connection Diagram

DS101015-1

SO-8

DS101015-2

LM76 See NS Package Number M08A

Ordering Information

Temperature

Order Number Supply Voltage Acurracy

LM76CHM-5 5.0V

LM76CHMX-5 5.0V

LM76CNM-3 3.3V
LM76CNMX-3 3.3V

±

0.5˚C

±

1.0˚C

±

0.5˚C

±

1.0˚C

±

1˚C 70˚C to 100˚C 95 units in Rail

±

1˚C 70˚C to 100˚C 2500 Units on Tape and

Range for

Accuracy

25˚C

−10˚C to 45˚C
25˚C

−10˚C to 45˚C

Transport Media

95 units in Rail

2500 Units on Tape and
Reel

Reel

Pin Description

Label Pin

SDA 1 Serial Bi-Directional Data Line, Open Drain Output,

SCL 2 Serial Bus Clock Input, CMOS Logic Level From Controller I
T_CRIT_A 3 Critical Temperature Alarm, Open Drain Output Pull Up Resistor, Controller Interrupt Line

GND 4 Power Supply Ground Ground
INT 5 Interrupt, Open Drain Output Pull Up Resistor, Controller Interrupt Line
+V

S

A0–A1 7,6 User-Set Address Inputs, TTL Logic Level Ground (Low, “0”) or +V

#

CMOS Logic Level

8 Positive Supply Voltage Input DC Voltage from 3.3V power supply or

Function Typical Connection

Pull Up Resistor, Controller I

2

C Clock Line

or System Hardware Shutdown

5V.

2

(High, “1”)

S

C Data Line

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Pin Description (Continued)

LM76

DS101015-3

FIGURE 1. Typical Application

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Absolute Maximum Ratings (Note 1)

LM76

Supply Voltage −0.3V to 6.5V
Voltage at any Pin −0.3V to (+V
Input Current at any Pin 5mA
Package Input Current (Note 2) 20mA
T_CRIT_A and INT Output Sink

Current 10mA
T_CRIT_A and INT Output

+ 0.3V)

S

Soldering Information, Lead
Temperature

SOP Package (Note 3)

Vapor Phase (60 seconds) 215˚C
Infrared (15 seconds) 220˚C

ESD Susceptibility (Note 4)

Human Body Model 3000V
Machine Model 250V

Voltage 6.5V
Storage Temperature −65˚C to +125˚C

Operating Ratings(Notes 1, 5)

Operating Temperature Range −55˚C to +150˚C
Specified Temperature Range

(Note 6) T

LM76CHM-5 −20˚C to +85˚C
LM76CNM-3 −55˚C to +125˚C

Supply Voltage Range (+V

)(Note 7) +3.0V to +5.5V

S

Temperature-to-Digital Converter Characteristics

Unless otherwise noted, these specifications apply for +VS=+3.3 Vdc±5% for the LM76CNM-3 and for +VS=+5.0 Vdc±10%
for the LM76CHM-5. (Note 7). Boldface limits apply for T

A=TJ=TMIN

wise noted.

Typical

Parameter Conditions

Accuracy (Note 7) T

= −25˚C to +125˚C for

A

(Note 8)

LM76CNM-3

= +70˚C to +100˚C

T

A

T

= −20˚C to +85˚C for

A

LM76CHM-5
T

= −10˚C to +45˚C

A

T

= +25˚C

A

Resolution (Note 10) 13

0.0625

Temperature Conversion

(Note 11) 400 500 1000 ms

Time
Quiescent Current I

2

C Inactive 0.25 mA

2

I

C Active 0.25 0.5 0.45 mA (max)

to T

±

2.5

±

1.5

; all other limits TA=TJ=+25˚C, unless other-

MAX

LM76CNM-3

Limits

(Note 9)

±

1.0

LM76CHM-5

Limits

(Note 9)

±

1.0

±

0.5

to T

MIN

Units

(Limit)

˚C (max)

Bits

˚C

MAX

Shutdown Mode: 5 µA

12 18 µA (max)

T

=+85˚C 8 µA (max)

A

T

=+25˚C 12 µA (max)

A

T

Default Temperature (Notes 13, 14) 2 ˚C

HYST

T

Default Temperature (Note 14) 10 ˚C

LOW

T

Default Temperature (Note 14) 64 ˚C

HIGH

T

Default Temperature (Note 14) 80 ˚C

CRIT

Logic Electrical Characteristics

DIGITAL DC CHARACTERISTICS Unless otherwise noted, these specifications apply for for +VS=+3.3 Vdc±5% for the

LM76CNM-3 and for +V
limits T

=+25˚C, unless otherwise noted.

A=TJ

Symbol Parameter Conditions

V

IN(1)

SDA and SCL Logical “1” Input
Voltage

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=+5.0 Vdc±10% for the LM76CHM-5. . Boldface limits apply for TA=TJ=T

S

Typical

(Note 8)

Limits

(Note 9)

+VSx 0.7 V (min)

+V

+0.3 V (max)

S

MIN

to T

MAX

; all other

Units

(Limit)

Logic Electrical Characteristics (Continued)

DIGITAL DC CHARACTERISTICS Unless otherwise noted, these specifications apply for for +VS=+3.3 Vdc±5% for the

LM76CNM-3 and for +V
limits T

=+25˚C, unless otherwise noted.

A=TJ

Symbol Parameter Conditions

V

IN(0)

SDA and SCL Logical “0” Input
Voltage

V

IN(HYST)

SDA and SCL Digital Input
Hysteresis

V

IN(1)

A0 and A1 Logical “1” Input
Voltage

V

IN(0)

A0 and A1 Logical “0” Input
Voltage

I
I
C
I
V

IN(1)
IN(0)

IN

OH

OL

Logical “1” Input Current VIN=+V
Logical “0” Input Current VIN= 0V −0.005 −1.0 µA (max)
Capacitance of All Digital Inputs 20 pF
High Level Output Current VOH=+V
Low Level Output Voltage IOL=3mA 0.4 V (max)
T_CRIT_A Output Saturation

Voltage
T_CRIT_A Delay 1 Conversions

t

OF

Output Fall Time CL= 400 pF 250 ns (max)

SERIAL BUS DIGITAL SWITCHING CHARACTERISTICS Unless otherwise noted, these specifications apply for +VS=+3.3

±

Vdc

5% for the LM76CNM-3 and for +VS=+5.0 Vdc±10% for the LM76CHM-5, CL (load capacitance) on output lines = 80
pF unless otherwise specified. Boldface limits apply for T
wise noted.
The switching characteristics of the LM76 fully meet or exceed the published specifications of the I
rameters are the timing relationship between SCL and SDA signal related to the LM76. They are not the I

Symbol Parameter Conditions

t

1

t

2

t

3

t

4

t

5

SCL (Clock) Period 2.5 µs(min)
Data in Set-Up Time to SCL High 100 ns(min)
Data Out Stable after SCL Low 0 ns(min)
SDA Low Set-Up Time to SCL Low (Start Condition) 100 ns(min)
SDA High Hold Time after SCL High (Stop Condition) 100 ns(min)

=+5.0 Vdc±10% for the LM76CHM-5. . Boldface limits apply for TA=TJ=T

S

Typical

(Note 8)

Limits

(Note 9)

−0.3 V (min)

+V

x 0.3 V (max)

S

500 250 mV (min)

2.0 V (min)

+V

+0.3 V (max)

S

−0.3 V (min)

0.8 V (max)

0.005 1.0 µA (max)

10 µA (max)

0.8 V (max)

I

OUT

S

S

= 4.0 mA

(Note 12)

I

=3mA

O

to T

A=TJ=TMIN

; all other limits TA=TJ= +25˚C, unless other-

MAX

2

C bus. The following pa-

Typical

(Note 8)

MIN

to T

MAX

; all other

Units

(Limit)

(max)

2

C bus specifications.

Limits

(Note 9)

LM76

Units

(Limit)

DS101015-4

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Logic Electrical Characteristics (Continued)

LM76

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur.DC and AC electrical specifications do not apply when operating

the device beyond its rated operating conditions.
Note 2: When the input voltage (V

maximum package input current rating limits the number of pins that can safely exceed the power supplies with an input current of 5 mA to four.
Note 3: See AN-450 “Surface Mounting Methods and Their Effect on Product Reliability” or the section titled “Surface Mount” found in a current National

Semiconductor Linear Data Book for other methods of soldering surface mount devices.

Note 4: Human body model, 100 pF discharged through a 1.5 kΩ resistor. Machine model, 200 pF discharged directly into each pin.
Note 5: LM76 θ
Note 6: While the LM76 has a full-scale-range in excess of 128˚C, prolonged operation at temperatures above 125˚C is not recommended.
Note 7: The LM76 will operate properly over the +V

specified for rated accuracy at the nominal supply voltage of 3.3V.Accuracyof the LM76CNM-3 will degrade 0.2˚C for a
The LM76CHM-5 is tested and specified for a rated accuracy at the nominal supply voltage of 5.0V.Accuracy of the LM76CHM-5 will degrade 0.08˚C for a
variation in +V

Note 8: Typicals are at T
Note 9: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 10: 12 bits + sign, two’s complement
Note 11: This specification is provided only to indicate how often temperature data is updated. The LM76 can be read at any time without regard to conversion state

(and will yield last conversion result). If a conversion is in process it will be interrupted and restarted after the end of the read.
Note 12: For best accuracy, minimize output loading. Higher sink currents can affect sensor accuracy with internal heating. This can cause an error of 0.64˚C at full

rated sink current and saturation voltage based on junction-to-ambient thermal resistance.
Note 13: Hysteresis value adds to the T

subtracts from the T
discussion of the function of hysteresis refer to

Note 14: Default values set at power up.

(thermal resistance, junction-to-ambient) when attached to a printed circuit board with 2 oz. foil is 200˚C/W.

JA

from the nominal value.

S

HIGH

) at any pin exceeds the power supplies (V

I

supply voltage range of 3V to 5.5V for the LM76CNM-3 and the LM76CHM-5. The LM76CNM-3 is tested and

S

= 25˚C and represent most likely parametric norm.

A

setpoint value (e.g.: if T

and T_CRIT setpoints (e.g.: if T

LOW

Section 1.1

HIGH

LOW

setpoint = 64˚C, and hysteresis = 2˚C, then actual hysteresis point is 64−2 = 62˚C). For a detailed

, TEMPERATURE COMPARISON, and

<

GND or V

I

setpoint = 10˚C, and hysteresis = 2˚C, then actual hysteresis point is 10+2 = 12˚C); and

>

+VS) the current at that pin should be limited to 5 mA. The 20 mA

I

±

1% variation in +VSfrom the nominal value.

Figure 3

.

Electrical Characteristics

Continued

±

1%

FIGURE 2. Temperature-to-Digital Transfer Function (Non-linear scale for clarity)

1.0 Functional Description

The LM76 temperature sensor incorporates a band-gap type
temperature sensor, 13-bit ADC, and a digital comparator
with user-programmable upper and lower limit values. The
comparator activates either the INT line for temperatures
outside the T
for temperatures which exceed T_CRIT. The lines are pro­grammable for mode and polarity.

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LOW

and T

window, or the T_CRIT_A line

HIGH

DS101015-5

1.1 TEMPERATURE COMPARISON

LM76 provides a window comparison against a lower (T
and upper (T

) trip point. A second upper trip point

HIGH

(T_CRIT) functions as a critical alarm shutdown.

LOW

Figure 3

depicts the comparison function as well as the modes of
operation.

1.1.1 STATUS BITS

The internal Status bits operate as follows:

)

1.0 Functional Description (Continued)

“True”: Temperature above a T
those respective bits. A “true” for T
T

.

LOW

“False”: Assuming temperature has previously crossed
above T

or T_CRIT, then the temperature must drop

HIGH

below the points corresponding T
T_CRIT − T
T

, assuming temperature has previously crossed below

LOW

T

, a “false” occurs when temperature goes above T

LOW

+T

HYST

.

) in order for the condition to be false. For

HYST

The Status bits are not affected by reads or any other
actions, and always represent the state of temperature vs.
setpoints.

1.1.2 HARDWIRE OUTPUTS

The T_CRIT_A hardwire output mirrors the T_CRIT_A flag,
when the flag is true, the T_CRIT_A output is asserted at all
times regardless of mode. Reading the LM76 has no effect
on the T_CRIT_A output, although the internal conversion is
restarted.

The behavior of the INT hardwire output is as follows:
Comparator Interrupt Mode (Default): User reading part

resets output until next measurement completes. If condition
is still true, output is set again at end of next conversion
cycle. For example, if a user never reads the part, and
temperature goes below T

LOW

would stay that way until temperature goes above T
T

. However if the user reads the part, the output would

HYST

be reset. At the end of the next conversion cycle, if the
condition is true, it is set again. If not, it remains reset.

Event Interrupt Mode: User reading part resets output
until next condition ″event″ occurs (in other words, output is
only set once for a true condition, if reset by a read, it
remains reset until the next triggering threshold has been
crossed). Conversely, if a user never read the part, the
output would stay set indefinitely after the first event that set
the output.An “event” for Event Interrupt Mode is defined as:

1. Transitioning upward across a setpoint, or

2. Transitioning downward across a setpoint’s correspond-

ing hysteresis (after having exceeded that setpoint).

For example, if a user never read the part, and temperature
went below T

then INT would become active. It would

LOW

stay that way forever if a user never read the part.
However if the user read the part, the output would be reset.

Even if the condition is true, it will remain reset. The tem­perature must cross above T

LOW+THYST

again.
In either mode, reading any register in the LM76 restarts the

conversion. This allows a designer to know exactly when the
LM76 begins a comparison. This prevents unnecessary In­terrupts just after reprogramming setpoints. Typically, sys­tem Interrupt inputs are masked prior to reprogramming trip
points. By doing a read just after resetting trip points, but
prior to unmasking, unexpected Interrupts are prevented.

or T_CRIT is “true” for

HIGH

is temperature below

LOW

HYST(THIGH

−T

HYST

LOW

then INT becomes active. It

LOW

to set the output

or

LM76

Avoid programming setpoints so close that their hysteresis
values overlap. An example would be that with a T
of 2˚C then setting T

HIGH

and T

to within 4˚C of each

LOW

other will violate this restriction. To be more specific, with
T

set to 2˚C assume T

HYST

equal to, or higher than 60˚C this restriction is violated.

set to 64˚C. If T

HIGH

1.2 DEFAULT SETTINGS

The LM76 always powers up in a known state. LM76 power
up default conditions are:

1. Comparator Interrupt Mode

2. T

3. T

LOW
HIGH

set to 10˚C

set to 64˚C

4. T_CRIT set to 80˚C

5. T

HYST

set to 2˚C

6. INT and T_CRIT_A active low

7. Pointer set to “00”; Temperature Register
The LM76 registers will always reset to these default values

when the power supply voltage is brought up from zero volts
as the supply crosses the voltage level plotted in the follow­ing curve. The LM76 registers will reset again when the
power supply drops below the voltage plotted in this curve.

Average Power on Reset Voltage
vs Temperature

+

DS101015-18

1.3 SERIAL BUS INTERFACE

The LM76 operates as a slave on the Serial Bus, so the SCL
line is an input (no clock is generated by the LM76) and the
SDA line is a bi-directional serial data line. According to
Serial Bus specifications, the LM76 has a 7-bit slave ad­dress. The five most significant bits of the slave address are
hard wired inside the LM76 and are “10010”. The two least
significant bits of the address are assigned to pins A1–A0,
and are set by connecting these pins to ground for a low,(0);
or to +V

for a high, (1).

S

Therefore, the complete slave address is:

10010A1A0

MSB LSB

HYST

LOW

value

is set

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1.0 Functional Description (Continued)

LM76

DS101015-6

Note: Event Interrupt mode is drawn as if the user is reading the part. If the user doesn’t read, the outputs would go low and stay that way until the LM76 is read.

FIGURE 3. Temperature Response Diagram

1.4 TEMPERATURE DATA FORMAT

Temperature data can be read from the Temperature and Set
Point registers; and written to the Set Point registers. Tem­perature data can be read at any time, although reading
faster than the conversion time of the LM76 will prevent data
from being updated. Temperature data is represented by a
13-bit, two’s complement word with an LSB (Least Signifi­cant Bit) equal to 0.0625˚C:

Temperature Digital Output

Binary Hex

+130˚C 0 1000 0 010 0000 08 20h
+125˚C 0 0111 1101 0000 07 D0h

+80˚C 0 0101 1010 0000 05 90h
+64˚C 0 0100 0000 0000 04 00h
+25˚C 0 0001 1001 0000 01 90h
+10˚C 0 0000 1010 0000 00 A0h

+2˚C 0 0000 0010 0000 00 20h

+0.0625˚C 0 0000 0000 0001 00 01h

0˚C 00 0000 0000 00 00h

−0.0625˚C 1 1111 1111 1111 1F FFh

−25˚C 1 1110 0111 0000 1E 70h

−55˚C 1 1100 1001 0000 1C 90h

1.5 SHUTDOWN MODE

Shutdown mode is enabled by setting the shutdown bit in the
Configuration register via the Serial Bus. Shutdown mode
reduces power supply current to 5 µA typical. T_CRIT_A is
reset if previously set. Since conversions are stoped during
shutdown, T_CRIT_A and INT will not be operational. The
Serial Bus interface remains active. Activity on the clock and
data lines of the Serial Bus may slightly increase shutdown
mode quiescent current. Registers can be read from and
written to in shutdown mode. The LM76 takes miliseconds to
respond to the shutdown command.

1.6 INT AND T_CRIT_A OUTPUT

The INT and T_CRIT_A outputs are open-drain outputs and
do not have internal pull-ups. A ″high″ level will not be
observed on these pins until pull-up current is provided from
some external source, typically a pull-up resistor. Choice of
resistor value depends on many system factors but, in gen­eral, the pull-up resistor should be as large as possible. This
will minimize any errors due to internal heating of the LM76.
The maximum resistance of the pull up, based on LM76
specification for High Level Output Current, to provide a 2
volt high level, is 30K ohms.

1.7 FAULT QUEUE

A fault queue of up to 4 faults is provided to prevent false
tripping when the LM76 is used in noisy environments. The 4
faults must occur consecutively to set flags as well as INT

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1.0 Functional Description (Continued)

and T_CRIT_A outputs. The fault queue is enabled by set­ting bit 4 of the Configuration Register high (see Section

1.11).

1.8 INTERNAL REGISTER STRUCTURE

LM76

There are four data registers in the LM76, selected by the
Pointer register. At power-up the Pointer is set to “00”; the
location for the Temperature Register. The Pointer register
latches the last location it was set to. In Interrupt Mode, a
read from the LM76 resets the INT output. Placing the device
in Shutdown mode resets the INT and T_CRIT_A outputs. All
registers are read and write, except the Temperatureregister
which is read only.

A write to the LM76 will always include the address byte and
the Pointer byte. A write to the Configuration register re­quires one data byte, while the T

LOW,THIGH

, and T_CRIT

registers require two data bytes.
Reading the LM76 can take place either of two ways: If the

location latched in the Pointer is correct (most of the time it is
expected that the Pointer will point to the Temperature reg­ister because it will be the data most frequently read from the
LM76), then the read can simply consist of an address byte,

DS101015-7

followed by retrieving the corresponding number of data
bytes. If the Pointer needs to be set, then an address byte,
pointer byte, repeat start, and another address byte plus
required number of data bytes will accomplish a read.

The first data byte is the most significant byte with most
significant bit first, permitting only as much data as neces­sary to be read to determine the temperature condition. For
instance, if the first four bits of the temperature data indi­cates a critical condition, the host processor could immedi­ately take action to remedy the excessive temperature. At
the end of a read, the LM76 can accept either Acknowledge
or No Acknowledge from the Master (No Acknowledge is
typically used as a signal for the slave that the Master has
read its last byte).

An inadvertent 8-bit read from a 16-bit register, with the D7
bit low,can cause the LM76 to stop in a state where the SDA
line is held low as shown in

Figure 4

. This can prevent any
further bus communication until at least 9 additional clock
cycles have occurred. Alternatively, the master can issue
clock cycles until SDA goes high, at which time issuing a
“Stop” condition will reset the LM76.

FIGURE 4. Inadvertent 8-Bit Read from 16-Bit Register where D7 is Zero (“0”)

DS101015-8

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1.0 Functional Description (Continued)

LM76

1.9 POINTER REGISTER

(Selects which registers will be read from or written to):

P7 P6 P5 P4 P3 P2 P1 P0

00000 Register Select

P0–P2: Register Select:

P2 P1 P0 Register

0 0 0 Temperature (Read only) (Power-up

default)
0 0 1 Configuration (Read/Write)
010T

HYST

(Read/Write)
0 1 1 T_CRIT (Read/Write)
100T
101T

(Read/Write)

LOW

(Read/Write)

HIGH

P3–P7: Must be kept zero.

1.10 TEMPERATURE REGISTER

(Read Only):

D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0

Sign MSB Bit10Bit 9 Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

CRIT HIGH LOW

D0–D2: Status Bits
D3–D15: Temperature Data. One LSB = 0.0625˚C. Two’s complement format.

Status Bits

1.11 CONFIGURATION REGISTER

(Read/Write):

D7 D6 D5 D4 D3 D2 D1 D0

0 0 0 Fault Queue INT Polarity T_CRIT_A

INT Mode Shutdown

Polarity
D0: Shutdown - When set to 1 the LM76 goes to low power shutdown mode. Power up default of “0”.
D1: Interrupt mode-0isComparator Interrupt mode, 1 is Event Interrupt mode. Power up default of “0”.
D2, D3: T_CRIT_A and INT Polarity-0isactive low, 1 is active high. Outputs are open-drain. Power up default of “0”

D4: Fault Queue - When set to 1 the Fault Queu is enabled,
see

Section 1.7

. Power up default of “0”.

D5–D7: These bits are used for production testing and must be kept zero for normal operation.

1.12 T

HYST,TLOW,THIGH

AND T_CRIT_A REGISTERS

(Read/Write):

D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0

Sign MSB Bit10Bit 9 Bit 8 Bit7 Bit6 Bit5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 X X X

D0–D2: Undefined
D3–D15: T

T

HYST

T

HYST

HYST,TLOW,THIGH

= 2˚C.
is subtracted from T

Avoid programming setpoints so close that their hysteresis values overlap. See

or T_CRIT Trip Temperature Data. Power up default is T

, and T_CRIT, and added to T

HIGH

LOW

.

= 10˚C, T

LOW

Section 1.1

= 64˚C, T_CRIT = 80˚C,

HIGH

.

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2.0 I2C Timing Diagrams

DS101015-9

Typical 2-Byte Read From Preset Pointer Location Such as Temp or Comparison Registers

Typical Pointer Set Followed by Immediate Read for 2-Byte Register such as Temp or Comparison Registers

LM76

DS101015-10

DS101015-11

Typical 1-Byte Read from Configuration Register with Preset Pointer

Typical Pointer Set Followed by Immediate Read from Configuration Register

Configuration Register Write

Comparison Register Write

FIGURE 6. Timing Diagrams

DS101015-12

DS101015-13

DS101015-14

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3.0 Application Hints

LM76

The temperature response graph in
cal application designed to meet ACPI requirements. In this
type of application, the temperature scale is given an arbi­trary value of ″granularity″, or the window within which tem­perature notification events should occur. The LM76 can be
programmed to the window size chosen by the designer,and
will issue interrupts to the processor whenever the window
limits have been crossed. The internal flags permit quick
determination of whether the temperature is rising or falling.

The T_CRIT limit would typically use its separate output to
activate hardware shutdown circuitry separate from the pro­cessor. This is done because it is expected that if tempera­ture has gotten this high that the processor may not be
responding. The separate circuitry can then shut down the
system, usually by shutting down the power supply.

Note that the INT and T_CRIT_A outputs are separate, but
can be wire-or’d together.Alternatively the T_CRIT_A can be
diode or’d to the INT line in such a way that a T_CRIT_A
event activates the INT line, but an INT event does not
activate the T_CRIT_A line. This may be useful in the event
that it is desirable to notify both the processor and separate
T_CRIT_A shutdown circuitry of a critical temperature alarm
at the same time (maybe the processor is still working and
can coordinate a graceful shutdown with the separate shut­down circuit).

Figure 7

depicts a typi-

To implement ACPI compatible sensing it is necessary to
sense whenever the temperature goes outside the window,
issue an interrupt, service the interrupt, and reprogram the
window according to the desired granularity of the tempera­ture scale. The reprogrammed window will now have the
current temperature inside it, ready to issue an interrupt
whenever the temperature deviates from the current window.

To understand this graph, assume that at the left hand side
the system is at some nominal temperature. For the 1st
event temperature rises above the upper window limit,
T

, causing INT to go active. The system responds to the

HIGH

interrupt by querying the LM76’s status bits and determines
that T
rising. The system then reprograms the temperature limits to
a value higher by an amount equal to the desired granularity.
Note that in Event Interrupt Mode, reprogramming the limits
has caused a second, known, interrupt to be issued since
temperature has been returned within the window. In Com­parator Interrupt Mode, the LM76 simply stops issuing inter­rupts.

The 2nd event is another identical rise in temperature. The
3rd event is typical of a drop in temperature. This is one of
the conditions that demonstrates the power of the LM76, as
the user receives notification that a lower limit is exceeded in
such a way that temperature is dropping.

The Critical Alarm Event activates the separate T_CRIT_A
output. Typically, this would feed circuitry separate from the
processor on the assumption that if the system reached this
temperature, the processor might not be responding.

was exceeded, indicating that temperature is

HIGH

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3.0 Application Hints (Continued)

LM76

DS101015-15

Note: Event Interrupt mode is drawn as if the user is reading the part. If the user doesn’t read, the outputs would go low and stay that way until the LM76 is read.

FIGURE 7. Temperature Response Diagram for ACPI Implementation

4.0 Typical Applications

DS101015-16

FIGURE 8. Typical Application

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4.0 Typical Applications (Continued)

LM76

FIGURE 9. ACPI Compatible Terminal Alarm Shutdown. By powering the LM76 from auxilary output of the power

supply, a non-functioning overheated computer can be powered down to preserve as much of the system as

DS101015-19

possible.

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Physical Dimensions inches (millimeters) unless otherwise noted

LM76

with Two-Wire Interface

±

0.5˚C,

±

1˚C, 12-Bit + Sign Digital Temperature Sensor and Thermal Window Comparator

8-Lead (0.150" Wide) Molded Small Outline Package (SOP), JEDEC

Order Number LM76CNM-3, LM76CNMX-3, LM76CHM-5 or LM76CHM-5X

NS Package Number M08A

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