Page 1
LM77
9-Bit + Sign Digital Temperature Sensor and Thermal
Window Comparator with Two-Wire Interface
June 1999
LM77 9-Bit + Sign Digital Temperature Sensor and Thermal Window Comparator with Two-Wire
Interface
General Description
The LM77 is a digital temperature sensor and thermal window comparator with an I
window-comparator architecture of the LM77 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.Thesensor powers up with default thresholds of 2˚C T
T_CRIT.
The LM77’s 3.0V to 5.5V supply voltage range, Serial Bus interface, 9-bit + sign output, and full-scale range of over
128˚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, automotive, and HVAC applications.
HYST
2
C™Serial Bus interface. The
, 10˚C T
LOW
, 64˚C T
, and 80˚C
HIGH
Simplified Block Diagram
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 LM77s can be connected to a single bus
n 9-bit + sign output; full-scale reading of over 128˚C
Key Specifications
n Supply Voltage 3.0V to 5.5V
n Supply Current operating 250 µA (typ)
500 µA (max)
shutdown 5 µA (typ)
n Temperature
Accuracy
−10˚C to 65˚C
−25˚C to 100˚C
−55˚C to 125˚C
±
1.5˚C(max)
±
2˚C(max)
±
3˚C(max)
Applications
n System Thermal Management
n Personal Computers
n Office Electronics
n Electronic Test Equipment
n Automotive
n HVAC
DS100136-1
I2C®is a registered trademark of Philips Corporation.
© 1999 National Semiconductor Corporation DS100136 www.national.com
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Connection Diagram
Ordering Information
Order Number Supply Voltage Supplied As
LM77CIM-3 3.3V
LM77CIMX-3 3.3V 2500 Units on Tape and Reel
LM77CIM-5 5V
LM77CIMX-5 5V 2500 Units on Tape and Reel
Pin Description
SO-8
DS100136-2
LM77 See NS Package Number M08A
Label Pin
SDA 1 Serial Bi-Directional Data Line. Open Drain Output From Controller
SCL 2 Serial Bus Clock Input From Controller
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 Ground (Low, “0”) or +V
#
8 Positive Supply Voltage Input DC Voltage from 3V to 5.5V
Function Typical Connection
or System Hardware Shutdown
S
DS100136-3
FIGURE 1. Typical Application
(High, “1”)
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Page 3
Absolute Maximum Ratings (Note 1)
Supply Voltage −0.3V to 6.5V
Voltage at any Pin −0.3V to (+V
Input Current at any Pin 5 mA
Package Input Current (Note 2) 20 mA
T_CRIT_A and INT Output Sink
Current 10 mA
T_CRIT_A and INT Output
Voltage 6.5V
Storage Temperature −65˚C to +125˚C
+ 0.3V )
S
Soldering Information, Lead
Temperature
SOP and MSOP Package (Note 3)
Vapor Phase (60 seconds) 215˚C
Infrared (15 seconds) 220˚C
ESD Susceptibility (Note 4)
Human Body Model 2500V
Machine Model 250V
Operating Ratings (Notes 1, 5)
Specified Temperature Range T
(Note 6) −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 +V
LM77CIM-3 (Note 7). Boldface limits apply for T
noted.
A
Parameter Conditions
=
Accuracy T
−10˚C to +65˚C
A
=
−25˚C to +100˚C
A
=
T
−55˚C to +125˚C
A
S
=
=
T
J
±
10%for LM77CIM-5 and +V
+5 Vdc
to T
T
MIN
; all other limits T
MAX
Typical
(Note 8)
Resolution (Note 10) 10
0.5
Temperature Conversion Time (Note 11) 70 125 ms
Quiescent Current I
2
C Inactive 0.25 mA
2
I
C Active 0.25 0.5 mA (max)
Shutdown Mode 5 10 µA
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
C
=
+3.3 Vdc
S
=
=
T
+25˚C, unless otherwise
A
J
Limits
(Note 9)
±
1.5
±
2.0
±
3.0
±
10%for
Units
(Limit)
˚C (max)T
Bits
˚C
MIN
to T
MAX
Logic Electrical Characteristics
DIGITAL DC CHARACTERISTICS Unless otherwise noted, these specifications apply for +V
=
and +V
unless otherwise noted.
+3.3 Vdc
S
±
10%for LM77CIM-3. Boldface limits apply for T
Symbol Parameter Conditions
V
V
V
V
I
I
C
IN(1)
IN(0)
IN(1)
IN(0)
IN(1)
IN(0)
IN
SDA and SCL Logical “1” Input
Voltage
SDA and SCL Logical “0” Input
Voltage
A0 and A1 Logical “1” Input
Voltage
A0 and A1 Logical “0” Input
Voltage
Logical “1” Input Current V
Logical “0” Input Current V
=
+V
IN
=
0V −0.005 −1.0 µA (max)
IN
Capacitance of All Digital Inputs 20 pF
S
=
=
T
J
Typical
(Note 8)
MIN
to T
T
A
0.005 1.0 µA (max)
=
±
10%for LM77CIM-5
+5 Vdc
S
; all other limits T
MAX
Limits
(Note 9)
+VSx 0.7 V (min)
+V
+0.3 V (max)
S
−0.3 V (min)
+V
x 0.3 V (max)
S
2.0 V (min)
+V
+0.3 V (max)
S
−0.3 V (min)
0.8 V (max)
=
=
T
+25˚C,
A
J
Units
(Limit)
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Page 4
Logic Electrical Characteristics (Continued)
DIGITAL DC CHARACTERISTICS Unless otherwise noted, these specifications apply for +V
=
and +V
unless otherwise noted.
+3.3 Vdc
S
±
10%for LM77CIM-3. Boldface limits apply for T
Symbol Parameter Conditions
I
OH
V
OL
High Level Output Current V
Low Level Output Voltage I
T_CRIT_A Output Saturation
Voltage
=
+V
OH
=
3mA 0.4 V (max)
OL
=
I
4.0 mA
OUT
(Note 12)
T_CRIT_A Delay 1 Conversions
t
OF
Output Fall Time C
=
400 pF 250 ns (max)
L
=
I
3mA
O
S
=
=
T
J
Typical
(Note 8)
MIN
to T
T
A
=
+5 Vdc
S
; all other limits T
MAX
Limits
(Note 9)
10 µA (max)
0.8 V (max)
±
10%for LM77CIM-5
=
=
T
+25˚C,
A
J
Units
(Limit)
(max)
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Page 5
Logic Electrical Characteristics (Continued)
SERIAL BUS DIGITAL SWITCHING CHARACTERISTICS Unless otherwise noted, these specifications apply for +V
±
10%for LM77CIM-5, +V
Vdc
wise specified. Boldface limits apply for T
The switching characteristics of the LM77 fully meet or exceed the published specifications of the I
rameters are the timing relationship between SCL and SDA signal related to the LM77. They are not the I2C bus specifications.
Symbol Parameter Conditions
t
1
t
2
t
3
t
4
t
5
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: SeeAN-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: LM77 θ
Note 6: While the LM77 has a full-scale-range in excess of 128˚C, prolonged operation at temperatures above 125˚C is not recommended.
Note 7: Both part numbers of the LM77 will operate properly over the +V
curacy at their nominal supply voltage. Accuracy will typically degrade 1˚C/V of variation in +V
Note 8: Typicals are at T
Note 9: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 10: 9 bits + sign, two’s complement
Note 11: Thisspecification is provided only to indicate how often temperature data is updated. The LM77 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.
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)
(thermal resistance, junction-to-ambient) when attached to a printed circuit board with 2 oz. foil is 200˚C/W.
JA
A
and T_CRIT setpoints (e.g.: if T
HIGH
=
+3.3 Vdc
S
) at any pin exceeds the power supplies (V
I
=
25˚C and represent most likely parametric norm.
LOW
±
10%for LM77CIM-3, CL (load capacitance) on output lines=80 pF unless other-
=
=
T
A
setpoint value (e.g.: if T
HIGH
Section 1.1
, TEMPERATURE COMPARISON, and
to T
T
J
MIN
LOW
setpoint=64˚C, and hysteresis=2˚C, then actual hysteresis point is 64−2=62˚C). For a detailed
=
; all other limits T
MAX
<
I
supply voltage range of 3V to 5.5V . The devices are tested and specified for rated ac-
S
setpoint=10˚C, and hysteresis=2˚C, then actual hysteresis point is 10+2=12˚C); and
GND or V
>
+VS) the current at that pin should be limited to 5 mA. The 20 mA
I
as it varies from the nominal value.
S
Figure 3
=
T
+25˚C, unless otherwise noted.
A
J
.
2
C bus. The following pa-
Typical
(Note 8)
DS100136-4
Limits
(Note 9)
S
=
+5
Units
(Limit)
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Page 6
Logic Electrical Characteristics (Continued)
FIGURE 2. Temperature-to-Digital Transfer Function (Non-linear scale for clarity)
1.0 Functional Description
The LM77 temperature sensor incorporates a band-gap type
temperature sensor, 10-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.
1.1 TEMPERATURE COMPARISON
LM77 provides a window comparison against a lower (T
and upper (T
(T_CRIT) functions as a critical alarm shutdown.
depicts the comparison function as well as the modes of operation.
1.1.1 STATUS BITS
The internal Status bits operate as follows:
“True”: Temperature above a T
those respective bits. A “true” for T
T
.
LOW
“False”: Assuming temperature has previously crossed
above T
HIGH
low 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
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 LM77 has no effect
on the T_CRIT_A output, although the internal conversion is
restarted.
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and T
LOW
) trip point. A second upper trip point
HIGH
window, or the T_CRIT_A line
HIGH
or T_CRIT is “true” for
HIGH
is temperature below
LOW
LOW
Figure 3
or T_CRIT, then the temperature must drop be-
) in order for the condition to be false. For
HYST
HYST(THIGH−THYST
)
or
LOW
DS100136-5
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
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 con-
then INT becomes active. It
LOW
LOW
dition is true, it is set again. If not, it remains reset.
Event Interrupt Mode: User reading part resets output un-
til 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 corresponding hysteresis (after having exceeded that setpoint).
For example, if a user never read the part, and temperature
went below T
stay that way forever if a user never read the part.
then INT would become active. It would
LOW
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
again.
LOW+THYST
to set the output
In either mode, reading any register in the LM77 restarts the
conversion. This allows a designer to know exactly when the
LM77 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.
Avoid programming setpoints so close that their hysteresis
values overlap. An example would be that with a T
HYST
value
+
Page 7
1.0 Functional Description (Continued)
of 2˚C then setting T
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.
1.2 DEFAULT SETTINGS
The LM77 always powers up in a known state. LM77 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
HYST
set to 2˚C
5. T
6. INT and T_CRIT_A active low
7. Pointer set to “00”; Temperature Register
The LM77 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 LM77 registers will reset again when the
power supply drops below the voltage plotted in this curve.
HIGH
and T
LOW
set to 64˚C. If T
HIGH
to within 4˚C of each
is set
LOW
Average Power on Reset Voltage
vs Temperature
DS100136-18
1.3 SERIAL BUS INTERFACE
The LM77 operates as a slave on the Serial Bus, so the SCL
line is an input (no clock is generated by the LM77) and the
SDA line is a bi-directional serial data line. According to Serial Bus specifications, the LM77 has a 7-bit slave address.
The five most significant bits of the slave address are hard
wired inside the LM77 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
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Page 8
1.0 Functional Description (Continued)
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 LM77 is read.
DS100136-6
FIGURE 3. Temperature Response Diagram
1.4 TEMPERATURE DATA FORMAT
Temperature data can be read from theTemperatureand 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 LM77 will prevent data
from being updated. Temperature data is represented by a
10-bit, two’s complement word with an LSB (Least Significant Bit) equal to 0.5˚C:
Temperature Digital Output
Binary Hex
+130˚C 01 0000 0100 104h
+125˚C 00 1111 1010 0FAh
+25˚C 00 0011 0010 032h
+0.5˚C 00 0000 0001 001h
0˚C 00 0000 0000 000h
−0.5˚C 11 1111 1111 3FFh
−25˚C 11 1100 1110 3CEh
−55˚C 11 1001 0010 392h
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 re-
set 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 LM77 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 LM77.
The maximum resistance of the pull up, based on LM77
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 LM77 is used in noisy environments. The 4
faults must occur consecutively to set flags as well as INT
and T_CRIT_A outputs. The fault queue is enabled by setting bit 4 of the Configuration Register high (see Section
1.11).
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Page 9
1.0 Functional Description (Continued)
1.8 INTERNAL REGISTER STRUCTURE
There are four data registers in the LM77, 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 LM77 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 Temperature register
which is read only.
A write to the LM77 will always include the address byte and
the Pointer byte. A write to the Configuration register requires one data byte, while the T
registers require two data bytes.
LOW,THIGH
Reading the LM77 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 Temperatureregister because it will be the data most frequently read from the
LM77), then the read can simply consist of an address byte,
, and T_CRIT
DS100136-7
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 LM77 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 LM77 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 LM77.
FIGURE 4. Inadvertent 8-Bit Read from 16-Bit Register where D7 is Zero (“0”)
DS100136-8
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Page 10
1.0 Functional Description (Continued)
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
0 0 1 Configuration (Read/Write)
010T
0 1 1 T_CRIT (Read/Write)
100T
101T
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 Sign Sign Sign MSB Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
D0–D2: Status Bits
D3–D15: Temperature Data. One LSB=0.5˚C. Two’s complement format.
1.11 CONFIGURATION REGISTER
(Read/Write):
D7 D6 D5 D4 D3 D2 D1 D0
0 0 0 Fault Queue INT Polarity T_CRIT_A
D0: Shutdown - When set to 1 the LM77 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 Sign Sign Sign MSB Bit7 Bit6 Bit5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 X X X
D0–D2: Undefined
D3–D15: T
T
T
HYST,TLOW,THIGH
=
2˚C.
HYST
is subtracted from T
HYST
or T_CRIT TripTemperatureData. Power up default is T
, and T_CRIT, and added to T
HIGH
Avoid programming setpoints so close that their hysteresis values overlap. See
default)
HYST
LOW
HIGH
(Read/Write)
(Read/Write)
(Read/Write)
.
LOW
Polarity
=
LOW
Section 1.1
CRIT HIGH LOW
INT Mode Shutdown
10˚C, T
=
64˚C, T_CRIT=80˚C,
HIGH
.
Status Bits
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Page 11
2.0 I2C Timing Diagrams
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
DS100136-11
Typical 1-Byte Read from Configuration Register with Preset Pointer
DS100136-9
DS100136-10
Typical Pointer Set Followed by Immediate Read from Configuration Register
Configuration Register Write
Comparison Register Write
FIGURE 6. Timing Diagrams
DS100136-12
DS100136-13
DS100136-14
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Page 12
3.0 Application Hints
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 LM77 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_Acan 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 LM77’s status bits and determines
that T
ing. The system then reprograms the temperature limits to a
was exceeded, indicating that temperature is ris-
HIGH
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 LM77 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 LM77, 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.
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Page 13
3.0 Application Hints (Continued)
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 LM77 is read.
DS100136-15
FIGURE 7. Temperature Response Diagram for ACPI Implementation
4.0 Typical Applications
DS100136-16
FIGURE 8. Typical Application
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Page 14
4.0 Typical Applications (Continued)
FIGURE 9. Remote HVAC temperature sensor communicates via 3 wires, including thermostat signals.
DS100136-17
DS100136-19
FIGURE 10. ACPI Compatible Terminal Alarm Shutdown. By powering the LM77 from auxilary output of the power
supply, a non-functioning overheated computer can be powered down to preserve as much of the system as
possible.
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Page 15
Physical Dimensions inches (millimeters) unless otherwise noted
8-Lead (0.150" Wide) Molded Small Outline Package (SOP), JEDEC
Order Number LM77CIM-3, LM77CIMX-3, LM77CIM-5 or LM77CIMX-5
NS Package Number M08A
LM77 9-Bit + Sign Digital Temperature Sensor and Thermal Window Comparator with Two-Wire
Interface
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