LM84
LM84 Diode Input Digital Temperature Sensor with Two-Wire Interface
Literature Number: SNIS108B
LM84
Diode Input Digital Temperature Sensor with Two-Wire
Interface
General Description
The LM84 is a remote diode temperature sensor,
Delta-Sigma analog-to-digital converter, and digital
over-temperature detector with an SMBus™ interface. The
LM84 senses its own temperature as well as the temperature of a target IC with a diode junction, such as a Pentium ®
II processor or a diode connected 2N3904. A diode junction
(semiconductor junction) is required on the target IC’s die. A
host can query the LM84 at any time to read the temperature
of this diode as well as the temperature state of the LM84
itself. A T_CRIT_A interrupt output becomes active when the
temperature is greater than a programmable comparator
limit, T_CRIT.
The host can program as well as read back the state of the
T_CRIT register. Three state logic inputs allow two pins
(ADD0, ADD1) to select up to 9 SMBus address locations for
the LM84. The sensor powers up with default thresholds of
127˚C for T_CRIT.
Features
n Directly senses die temperature of remote ICs
n Senses temperature of remote diodes
n SMBus compatible interface, supports SMBus Timeout
n Register readback capability
n 7 bit plus sign temperature data format
n 2 address select lines enable 9 LM84s to be connected
to a single bus
Key Specifications
j Supply Voltage
3.0V - 3.6V
j Supply Current
1 mA (max)
j Local Temperature Accuracy
± 1.0˚C (typ)
j Remote Diode Temperature Accuracy
± 3˚C (max)
± 5˚C (max)
+60˚C to +100˚C
0˚C to +125˚C
Applications
n
n
n
n
n
System Thermal Management
Personal Computers
Electronic Test Equipment
Office Electronics
HVAC
Simplified Block Diagram
DS100961-1
# Indicates Active Low (”NOT“)
SMBus™ is a trademark of the Intel Corporation.
Pentium ® II processor is a registered trademark of the Intel Corporation.
I2C ® is a registered trademark of the Philips Corporation.
© 2001 National Semiconductor Corporation
DS100961
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LM84 Diode Input Digital Temperature Sensor with Two-Wire Interface
July 2000
LM84
Connection Diagram
QSOP-16
DS100961-2
TOP VIEW
Ordering Information
Order
Number
NS
Package
Number
Transport
Media
LM84BIMQA
MQA16A
(QSOP-16)
LM84BIMQAX
LM84CIMQA
LM84CIMQAX
SMBus
Revision
Level
Noise Filter
on SMBCLK
95 Units in
Rail
1.1
20MHz
MQA16A
(QSOP-16)
2500 Units on
Tape and
Reel
1.1
20MHz
MQA16A
(QSOP-16)
95 Units in
Rail
1.0
Not Available
MQA16A
(QSOP-16)
2500 Units on
Tape and
Reel
1.0
Not Available
Typical Application
DS100961-3
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2
LM84
Pin Descriptions
Label
Pin #
Function
Typical Connection
Manufacturing test pins.
NC
1, 5, 9,
13, 16
VCC
2
D+
3
D−
Positive Supply Voltage
Input
Left floating. PC board traces may be routed
through the pads for these pins. Although, the
components that drive these traces should share
the same supply as the LM84 so that the Absolute
Maximum Voltage at any Pin rating is not violated.
DC Voltage from 3.0V to 3.6V
Diode Current Source
To Diode Anode. Connected to remote discrete
diode or to the diode on the external IC whose die
temperature is being sensed.
4
Diode Return Current
Sink
To Diode Cathode. Must be grounded when not
used.
ADD0–ADD1
10, 6
User-Set SMBus (I2C)
Address Inputs
Ground (Low, “0”), VCC (High, “1”) or open
(“TRI-LEVEL”)
GND
7, 8
Power Supply Ground
Ground
11
Critical Temperature
Alarm, open-drain output
Pull Up Resistor, Controller Interrupt Line or
System Shutdown
From and to Controller, Pull Up Resistor
SMBData
12
SMBus (I2C) Serial
Bi-Directional Data Line,
open-drain output
SMBCLK
14
SMBus (I2C) Clock Input
From Controller
No Connection
Left floating. PC board traces may be routed
through the pads for this pin.
T_CRIT_A
NC
15
3
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LM84
Absolute Maximum Ratings (Note 1)
Supply Voltage
Voltage at Any Pin:
NC (Pins 1,5,9), ADD0, ADD1, D+
All other pins (except D−)
D− Input Current
Input Current at All Other Pins (Note
2)
Package Input Current (Note 2)
SMBData, T_CRIT_A Output Sink
Current
Output Voltage
Storage Temperature
Soldering Information, Lead Temperature
QSOP Package (Note 3)
Vapor Phase (60 seconds)
Infrared (15 seconds)
ESD Susceptibility (Note 4)
Human Body Model
Machine Model
−0.3V to 6.0V
−0.3V to
(VCC + 0.3V)
−0.3V to 6.0V
± 1 mA
215˚C
220˚C
2500V
250V
Operating Ratings
5 mA
20 mA
(Note 1) and (Note 5)
Specified Temperature Range
LM84
Supply Voltage Range (VCC)
10 mA
6.0V
−65˚C to +150˚C
TMIN to TMAX
0˚C to +125˚C
+3.0V to +3.6V
Temperature-to-Digital Converter Characteristics
Unless otherwise noted, these specifications apply for VCC =+3.0 Vdc to +3.6 Vdc. Boldface limits apply for TA = TJ = TMIN
to TMAX; all other limits TA = TJ =+25˚C, unless otherwise noted.
Parameter
Conditions
Typical
Limits
Units
(Note 6)
(Note 7)
(Limit)
±1
Local Temperature Error (Note 8)
˚C
Remote Temperature Error using
Pentium Diode (Note 8) and (Note
9)
+60˚C ≤TA ≤ +100˚C,
VCC = 3.3 Vdc
±3
0˚C ≤ TA ≤ +125˚C,
VCC = 3.3 Vdc
±5
˚C (max)
Remote Temperature Error using
Diode Connected 2N3904 (Note 8)
and (Note 9)
+60˚C ≤TA ≤ +100˚C,
VCC = 3.3 Vdc
+1, −5
˚C (max)
0˚C ≤ TA ≤ +125˚C,
VCC = 3.3 Vdc
+3, −7
˚C (max)
Resolution
8
Bits
1
Temperature Conversion Time
(Note 11)
Quiescent Current (Note 10)
SMBus (I2C Inactive)
˚C (max)
˚C
120
145
ms
0.500
1
mA (max)
(D+ − D−)=+ 0.65V; high
level
160
µA (max)
50
µA (min)
Low level
16
µA (max)
5
µA (min)
T_CRIT_A Output Saturation
Voltage
IOUT = 3.0 mA
0.4
Power-On Reset Threshold
On VCC input, falling
edge
2.2
1.2
Local and Remote T_CRIT Default
Temperature
(Note 12)
D− Source Voltage
Diode Source Current
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0.7
+127
4
V
V (max)
V (max)
V (min)
˚C
Symbol
Parameter
Conditions
Typical
LM84B
LM84C
Units
(Note 6)
Limits
(Note 7)
Limits
(Note 7)
(Limit)
SMBData, SMBCLK
VIN(1)
Logical “1” Input Voltage
2.1
1.4
V (min)
VIN(0)
Logical “0”Input Voltage
0.8
0.6
V (max)
IIN(1)
Logical “1” Input Current
VIN = VCC
0.005
1.0
1.0
µA (max)
IIN(0)
Logical “0” Input Current
VIN = 0V
−0.005
−1.0
−1.0
µA (max)
ADD0, ADD1
VIN(1)
Logical “1” Input Voltage
VCC
1.6
1.6
V (min)
VIN(0)
Logical “0”Input Voltage
GND
0.5
0.5
V (max)
IIN(1)
Logical “1” Input Current
VIN = VCC
50
600
600
µA (max)
IIN(0)
Logical “0” Input Current
VIN = 0V
50
600
600
µA (max)
ALL DIGITAL INPUTS
CIN
Input Capacitance
20
pF
ALL DIGITAL OUTPUTS
IOH
High Level Output Current
VOH = VCC
100
100
µA (max)
VOL
SMBus Low Level Output
Voltage
IOL = 3 mA
IOL = 6 mA
0.4
0.6
0.4
0.6
V (max)
5
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LM84
Logic Electrical Characteristics
DIGITAL DC CHARACTERISTICS
Unless otherwise noted, these specifications apply for VCC =+3.0 to 3.6 Vdc. Boldface limits apply for TA = TJ = TMIN to
TMAX; all other limits TA = TJ =+25˚C, unless otherwise noted.
LM84
Logic Electrical Characteristics
(Continued)
SMBus DIGITAL SWITCHING CHARACTERISTICS
Unless otherwise noted, these specifications apply for VCC =+3.0 Vdc to +3.6 Vdc, CL (load capacitance) on output lines = 80
pF. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = +25˚C, unless otherwise noted.
The switching characteristics of the LM84 fully meet or exceed the published specifications of the SMBus or I2C bus. The following parameters are the timing relationships between SMBCLK and SMBData signals related to the LM84. They are not necessarily the I2C or SMBus bus specifications.
Symbol
Parameter
fSMB
SMBus Clock Frequency
tLOW
SMBus Clock Low Time
Conditions
Typical
Limits
Units
(Note 6)
(Note 7)
(Limit)
400
10
kHz (max)
kHz (min)
10% to 10%
1.3
25
µs (min)
ms (max)
25
ms (max)
0.6
µs (min)
tLOWSEXT Cumulative Clock Low Extend Time
tHIGH
SMBus Clock High Time
90% to 90%
tR;SMB
SMBus Rise Time
10% to 90%
1
tF;SMB
SMBus Fall Time
90% to 10%
0.3
tOF
Output Fall Time
CL = 400 pF
IO = 3 mA
tTIMEOUT
µs
µs
250
ns (max)
SMBData and SMBCLK Time Low for
Reset of Serial Interface (Note 13)
25
40
ms (min)
ms (max)
t1
SMBCLK (Clock) Period
2.5
µs (min)
t2,
tSU;DAT
Data In Setup Time to SMBCLK High
100
ns (min)
t 3,
tHD;DAT
Data Out Stable after SMBCLK Low
0
0.9
ns (min)
µs (max)
t4,
tHD;STA
SMBData Low Setup Time to SMBCLK
Low
100
ns (min)
t 5,
tSU;STO
SMBData High Delay Time after
SMBCLK High (Stop Condition Setup)
100
ns (min)
t 6,
tSU;STA
SMBus Start-Condition Setup Time
0.6
µs (min)
tBUF
SMBus Free Time
1.3
µs (min)
SMBus Communication
DS100961-4
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6
LM84
Logic Electrical Characteristics
(Continued)
SMBus TIMEOUT
DS100961-13
7
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LM84
Logic Electrical Characteristics
(Continued)
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 (VI) at any pin exceeds the power supplies (VI < GND or VI > VCC), the current at that pin should be limited to 5 mA. The 20 mA
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.
Parasitics and or ESD protection circuitry are shown in the figure below for the LM84’s pins. The nominal breakdown voltage of the zener D3 is 6.5V. Care should
be taken not to forward bias the parasitic diode, D1, present on pins: NC pins 1,5 and 9, D+, ADD1 and ADD0. Doing so by more than 50 mV may corrupt a
temperature or voltage measurement.
Pin Name
NC (pins 1, 5, 9)
D1
D2
x
x
x
D−
ADD0, ADD1
x
D4
Pin Name
D1
D2
D3
x
T_CRIT_A
x
x
SMBData
x
x
x
x
NC (pin 13)
x
x
x
x
x
x
VCC
D+
D3
x
SMBCLK
NC (pin 16)
D4
x
x
Note: An x indicates that the diode exists.
DS100961-8
FIGURE 1. ESD Protection Input Structure
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: Thermal resistance of the QSOP-16 package is TBD ˚C/W, junction-to-ambient when attached to a printed circuit board with 2 oz. foil.
Note 6: Typicals are at TA = 25˚C and represent most likely parametric norm.
Note 7: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 8: The Temperature Error specification does not include an additional error of ± 1˚C, caused by the quantization error.
Note 9: The Temperature Error will vary less than ± 1.0˚C for a variation in VCC of 3V to 3.6V from the nominal of 3.3V.
Note 10: Quiescent current will not increase substantially with an active SMBus.
Note 11: This specification is provided only to indicate how often temperature data is updated. The LM84 can be read at any time without regard to conversion state
(and will yield last conversion result).
Note 12: Default values set at power up.
Note 13: Holding the SMBData and/or SMBCLK lines Low for a time interval greater than tTIMEOUT will cause the LM84 to reset SMBData and SMBCLK to the IDLE
state of an SMBus communication (SMBCLK and SMBData set High).
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LM84
Logic Electrical Characteristics
(Continued)
DS100961-5
FIGURE 2. Temperature-to-Digital Transfer Function (Non-linear scale for clarity)
1.0 Functional Description
The LM84 temperature sensor incorporates a band-gap type
temperature sensor using a Local or Remote diode and an
8-bit ADC (Delta-Sigma Analog-to-Digital Converter). The
LM84 is compatible with the serial SMBus and I2C interfaces. Digital comparators compare Local and Remote readings to user-programmable setpoints (LT_CRIT and
RT_CRIT). Activation of the T_CRIT_A output indicates that
a temperature reading is greater than the limit preset in a
T_CRIT register.
1.1 T_CRIT_A OUTPUT, T_CRIT LIMITS
T_CRIT_A is activated when the Local temperature reading
is greater than the limit preset in the local critical temperature
setpoint register (LT_CRIT) or when the Remote temperature reading is greater than the limit preset in the remote
critical temperature setpoint register (RT_CRIT), as shown in
Figure 3. The T_CRIT_A mask bit (bit 7 of the Configuration
Register) when set will disable the T_CRIT_A output.
DS100961-6
FIGURE 3. T_CRIT_A Temperature Response Diagram
1.2 POWER-ON RESET DEFAULT STATES
LM84 always powers up to these known default states:
1. Local Temperature set to 0˚C
2. Remote Temperature set to 0˚C until the LM84 senses a
diode present or open circuit on the D+ and D− input
pins.
3. Status Register set to 00h.
4. Command Register set to 00h; T_CRIT_A enabled.
The Status Register can be read to determine which event
caused the alarm. A bit in the Status Register is set high to
indicate T_CRIT temperature alarm, see Section 1.8.3.
Local and remote temperature diodes are sampled alternately by the A/D converter. The T_CRIT_A output and the
Status Register flags are updated at the completion of a
conversion, which takes approximately 60 ms. T_CRIT_A
and the Status Register flags are reset only after the Status
Register is read and if the temperature is below the setpoint.
5.
9
Local and Remote T_CRIT set to 127˚C
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LM84
1.0 Functional Description
minimize any local temperature reading errors due to self
heating of the LM84. The maximum resistance of the pull-up,
based on LM84 specification for High Level Output Current,
to provide a 2V high level, is 30 kΩ.
(Continued)
1.3 SMBus INTERFACE
The LM84 operates as a slave on the SMBus, so the
SMBCLK line is an input (no clock is generated by the LM84)
and the SMBData line is bi-directional. According to SMBus
specifications, the LM84 has a 7-bit slave address. Bit 4 (A3)
of the slave address is hard wired inside the LM84 to a 1.
The remainder of the address bits are controlled by the
address select pins ADD1 and ADD0, and are set by connecting these pins to ground for a low, (0) , to VCC for a high,
(1), or left floating (TRI-LEVEL).
Therefore, the complete slave address is:
A6
A5
A4
A3
A2
A1
1.6 DIODE FAULT DETECTION
Before each remote conversion the LM84 goes through an
external diode fault detection sequence. If the D+ input is
shorted to VCC or floating then the temperature reading will
be +127˚C, bit 2 (OPEN) of the Status Register will be set. If
the Remote T_CRIT setpoint is set to less than +127˚C then
bit 4 (RTCRIT) of the Status Register will be set which will
activate the T_CRIT_A output, if enabled. If D+ is shorted to
GND or D−, the temperature reading will be 0˚C and bit 2 of
the Status Register will not be set.
A0
MSB
LSB
and is selected as follows:
Address Select Pin State
ADD0
LM84 SMBus
Slave Address
ADD1
A6:A0 binary
0
0
001 1000
0
TRI-LEVEL
001 1001
0
1
001 1010
TRI-LEVEL
0
010 1001
TRI-LEVEL
TRI-LEVEL
010 1010
TRI-LEVEL
1
010 1011
1
0
100 1100
1
TRI-LEVEL
100 1101
1
1
100 1110
The LM84 latches the state of the address select pins during
the first read or write on the SMBus. Changing the state of
the address select pins after the first read or write to any
device on the SMBus will not change the slave address of
the LM84.
1.4 TEMPERATURE DATA FORMAT
Temperature data can be read from the Local Temperature,
Remote Temperature, and T_CRIT setpoint registers. Temperature data can only be written to the T_CRIT setpoint
registers. Temperature data is represented by an 8-bit, two’s
complement byte with an LSB (Least Significant Bit) equal to
1˚C:
Temperature
Digital Output
Binary
Hex
+125˚C
0111 1101
7Dh
+25˚C
0001 1001
19h
+1˚C
0000 0001
01h
0˚C
0000 0000
00h
−1˚C
1111 1111
FFh
−25˚C
1110 0111
E7h
−55˚C
1100 1001
C9h
1.5 OPEN-DRAIN OUTPUTS
SMBData and T_CRIT_A outputs are open-drain 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
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10
LM84
1.0 Functional Description
(Continued)
1.7 COMMUNICATING with the LM84
DS100961-9
1.7.1 SMBus TIMEOUT
The LM84 SMBus interface circuitry will be reset to the
SMBus idle state if the SMBData or SMBCLK lines are held
low for more than 40 ms. The LM84 may or may not reset the
state SMBData or SMBCLK if either of these lines are held
low between 25 ms and 40 ms. Holding SMBData or SMBCLK low for less than or equal to 25 ms will not reset the
interface circuitry. The LM84 has a built-in internal timer to
guarantee that the interface is reset if the SMBData line were
to get stuck low. This can commonly occur when the master
is reset while the slave is transmitting low. This enhancement to the SMBus TIMEOUT specification ensures error
free performance even in remote systems where complete
power supply shutdown, for reset, is a nuisance. This would
have to occur since many cost effective temperature sensors
such as the LM84 do not have a pin dedicated for reset.
There are 10 data registers in the LM84, selected by the
Command Register. At power-up the Command Register is
set to “00”, the location for the Read Local Temperature
Register. The Command Register latches whatever the last
location it was set to. Reading the Status Register resets
T_CRIT_A. All registers are predefined as read only or write
only. Read and write registers with the same function contain
mirrored data.
A Write to the LM84 will always include the address byte and
the command byte. A write to any register requires one data
byte.
Reading the LM84 can take place either of two ways:
1. If the location latched in the Command Register is correct (most of the time it is expected that the Command
Register will point to one of the Read Temperature Registers because that will be the data most frequently read
from the LM84), the read can simply consist of an address byte, followed by retrieving the data byte.
2. If the Command Register needs to be set, then an
address byte, command byte, repeat start, and another
address byte will accomplish a read.
The data byte has the most significant bit first. At the end of
a read, the LM84 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).
11
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LM84
1.0 Functional Description
(Continued)
1.8 LM84 REGISTERS
1.8.1 COMMAND REGISTER
Selects which registers will be read from or written to. Data for this register should be transmitted during the Command Byte of
the SMBus write communication.
P7
P6
P5
P4
0
0
0
0
P3
P2
P1
P0
Command Select
P0-P7: Command Select:
Command Select Address
Power On Default State
< P7:P0 > hex
< D7:D0 > binary
Register Name
Register Function
< D7:D0 > decimal
00h
0000 0000
0
RLT
Read Local Temperature
01h
0000 0000
0
RRT
Read Remote Temperature
02h
0000 0000
0
RS
Read Status
03h
0000 0000
0
RC
Read Configuration
04h
0000 0000
0
RMID
Manufacturers ID
05h
0111 1111
127
RLCS
Read Local T_CRIT Setpoint
07h
0111 1111
127
RRCS
Read Remote T_CRIT
Setpoint
09h
0000 0000
0
WC
Write Configuration
0Bh
0111 1111
127
WLCS
Write Local T_CRIT Setpoint
0Dh
0111 1111
127
WRCS
Write Remote T_CRIT
Setpoint
1.8.2 LOCAL and REMOTE TEMPERATURE REGISTERS
(Read Only Address 00h and 01h):
D7
D6
D5
D4
D3
D2
D1
D0
MSB
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
LSB
D7–D0: Temperature Data. One LSB = 1˚C. Two’s complement format.
1.8.3 STATUS REGISTER
(Read Only Address 02h):
D7
D6
D5
D4
D3
D2
D1
D0
0
LTCRIT
0
RTCRIT
0
OPEN
0
0
Power up default is with all bits “0” (zero).
D2: OPEN: When set to 1 indicates a Remote Diode disconnect.
D4: RTCRIT: When set to 1 indicates an RT_CRIT alarm.
D6: LTCRIT: When set to 1 indicates an LT_CRIT alarm.
D7, D5, D3, D1–D0: These bits are always set to 0.
1.8.4 Manufacturers ID Register
(Read Address 04h) Default value 00h.
1.8.5 CONFIGURATION REGISTER
(Read Address 03h /Write Address 09h):
D7
D6
D5
D4
D3
D2
D1
D0
T_CRIT_A
mask
0
0
0
0
0
0
0
Power up default is with all bits “0” (zero).
D7: T_CRIT_A mask: When set to 1 T_CRIT_A interrupts are masked.
D6–D0: These bits are always set to 0. A write of 1 will return a 0 when read.
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12
LM84
1.0 Functional Description
(Continued)
1.8.6 LOCAL AND REMOTE T_CRIT REGISTERS
(Read/Write):
D7
D6
D5
D4
D3
D2
D1
D0
MSB
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
LSB
D7–D0: RT_CRIT and LT_CRIT setpoint temperature data. Power up default is LT_CRIT = RT_CRIT = 127˚C.
2.0 SMBus Timing Diagrams
DS100961-10
(a) Serial Bus Write to the internal Command Register followed by a the Data Byte
DS100961-11
(b) Serial Bus Write to the internal Command Register
DS100961-12
(c) Serial Bus Read from a Register with the internal Command Register preset to desired value.
FIGURE 4. Serial Bus Timing Diagrams
13
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LM84
3.0 Application Hints
The LM84 can be applied easily in the same way as other
integrated-circuit temperature sensors, and its remote diode
sensing capability allows it to be used in new ways as well.
It can be soldered to a printed circuit board, and because the
path of best thermal conductivity is between the die and the
pins, its temperature will effectively be that of the printed
circuit board lands and traces soldered to the LM84’s pins.
This presumes that the ambient air temperature is almost the
same as the surface temperature of the printed circuit board;
if the air temperature is much higher or lower than the
surface temperature, the actual temperature of the of the
LM84 die will be at an intermediate temperature between the
surface and air temperatures. Again, the primary thermal
conduction path is through the leads, so the circuit board
temperature will contribute to the die temperature much
more strongly than will the air temperature.
To measure temperature external to the LM84’s die, use a
remote diode. This diode can be located on the die of a
target IC, allowing measurement of the IC’s temperature,
independent of the LM84’s temperature. The LM84 has been
optimized to measure the remote diode of a Pentium II
processor as shown in Figure 5. A discrete diode can also be
used to sense the temperature of external objects or ambient
air. Remember that a discrete diode’s temperature will be
affected, and often dominated, by the temperature of its
leads.
where:
•
• q is the electron charge,
• k is the Boltzmann’s constant,
• N is the current ratio,
• T is the absolute temperature in ˚K.
The temperature sensor then measures ∆VBE and converts
to digital data. In this equation, k and q are well defined
universal constants, and N is a parameter controlled by the
temperature sensor. The only other parameter is η, which
depends on the diode that is used for measurement. Since
∆VBE is proportional to both η and T, the variations in η
cannot be distinguished from variations in temperature.
Since the non-ideality factor is not controlled by the temperature sensor, it will directly add to the inaccuracy of the
sensor. For the Pentium II Intel specifies a ± 1% variation in
η from part to part. As an example, assume a temperature
sensor has an accuracy specification of ± 3˚C at room temperature of 25˚C and the process used to manufacture the
diode has a non-ideality variation of ± 1%. The resulting
accuracy of the temperature sensor at room temperature will
be:
TACC = ± 3˚C + ( ± 1% of 298˚K) = ± 6˚C.
The additional inaccuracy in the temperature measurement
caused by η, can be eliminated if each temperature sensor is
calibrated with the remote diode that it will be paired with.
3.2 PCB LAYOUT for MINIMIZING NOISE
In a noisy environment, such as a processor mother board,
layout considerations are very critical. Noise induced on
traces running between the remote temperature diode sensor and the LM84 can cause temperature conversion errors.
The following guidelines should be followed:
1. Place a 0.1 µF power supply bypass capacitor as close
as possible to the VCC pin and the recommended 2.2 nF
capacitor as close as possible to the D+ and D− pins.
Make sure the traces to the 2.2 nF capacitor are
matched.
2. Ideally, the LM84 should be placed within 10 cm of the
Processor diode pins with the traces being as straight,
short and identical as possible.
3. Diode traces should be surrounded by a GND guard ring
to either side, above and below if possible. This GND
guard should not be between the D+ and D− lines. In the
event that noise does couple to the diode lines it would
be ideal if it is coupled common mode. That is equally to
the D+ and D− lines.(See Figure 6)
4. Avoid routing diode traces in close proximity to power
supply switching or filtering inductors.
5. Avoid running diode traces close to or parallel to high
speed digital and bus lines. Diode traces should be kept
at least 2 cm. apart from the high speed digital traces.
6. If it is necessary to cross high speed digital traces, the
diode traces and the high speed digital traces should
cross at a 90 degree angle.
DS100961-16
Pentium Temperature vs LM84 Temperature Reading
Most silicon diodes do not lend themselves well to this
application. It is recommended that a 2N3904 transistor
base emitter junction be used with the collector tied to the
base.
A diode connected 2N3904 approximates the junction available on a Pentium microprocessor for temperature measurement. Therefore, the LM84 can sense the temperature of this
diode effectively.
3.1 ACCURACY EFFECTS OF DIODE NON-IDEALITY
FACTOR
The technique used in today’s remote temperature sensors
is to measure the change in VBE at two different operating
points of a diode. For a bias current ratio of N:1, this difference is given as:
www.national.com
η is the non-ideality factor of the process the diode is
manufactured on,
14
7.
with the sense diode. For the Pentium II this would be
pin A14.
(Continued)
The ideal place to connect the LM84’s GND pin is as
close as possible to the Processors GND associated
DS100961-15
FIGURE 6. Recommended Diode Trace Layout
Noise on the digital lines, overshoot greater than VCC and
undershoot less than GND, may prevent successful SMBus
communication with the LM84. SMBus no acknowledge is
the most common symptom, causing unnecessary traffic on
the bus. Although, the SMBus maximum frequency of com-
munication is rather low (400 kHz max) care still needs to be
taken to ensure proper termination within a system with
multiple parts on the bus and long printed circuit board
traces.
4.0 Typical Applications
DS100961-17
Using a Diode Connected 2N3904 as a Remote Temperture Sensing Element
15
www.national.com
LM84
3.0 Application Hints
LM84 Diode Input Digital Temperature Sensor with Two-Wire Interface
Physical Dimensions
inches (millimeters) unless otherwise noted
16-Lead QSOP Package
Order Number LM84BIMQA, LM84BIMQAX, LM84CIMQA or LM84CIMQAX
NS Package Number MQA16
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DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
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