LM75B,LM75C
LM75B LM75C Digital Temperature Sensor and Thermal Watchdog with
Two-Wire Interface
Literature Number: SNIS153A
LM75B
LM75C
November 2, 2010
Digital Temperature Sensor and Thermal Watchdog with
Two-Wire Interface
General Description
The LM75B and LM75C are industy-standard digital temper-
ature sensors with an integrated Sigma-Delta analog-to-digi-
tal converter and I2C® interface. The LM75 provides 9-bit
digital temperature readings with an accuracy of ±2°C from
-25°C to 100°C and ±3°C over -55°C to 125°C.
Communication is accomplished over a 2-wire interface
which operates up to 400kHz. The LM75 has three address
pins, allowing up to eight LM75 devices to operate on the
same 2-wire bus. The LM75 has a dedicated over-tempera-
ture output (O.S.) with programmable limit and hystersis. This
output has programmable fault tolerance, which allows the
user to define the number of consecutive error conditions that
must occur before O.S. is activated.
The wide temperature and supply range and I2C interface
make the LM75 ideal for a number of applications including
base stations, electronic test equipment, office electronics,
personal computers, and any other system where thermal
management is critical to performance. The LM75B and
LM75C are available in an SOP-8 package or MSOP-8 pack-
age.
Applications
■General System Thermal Management
■Communications Infrastructure
■Electronic Test Equipment
■Environmental Monitoring
Features
■No external components required
■Shutdown mode to minimize power consumption
■Up to eight LM75s can be connected to a single bus
■Power up defaults permit stand-alone operation as
thermostat
■UL Recognized Component (LM75B and LM75C)
Key Specifications
■ Supply Voltage LM75B, LM75C 3.0V to 5.5V
■ Supply Current operating 280 μA (typ)
shutdown 4 μA (typ)
■ Temperature Accuracy −25°C to 100°C ±2°C (max)
−55°C to 125°C ±3°C (max)
Simplified Block Diagram
30099801
I2C® is a registered trademark of Philips Corporation.
© 2010 National Semiconductor Corporation 300998 www.national.com
LM75B LM75C Digital Temperature Sensor and Thermal Watchdog with Two-Wire Interface
Connection Diagram
LM75B, LM75C SOP-8 and Mini MSOP-8
30099802
Pin Descriptions
Label Pin # Function Typical Connection
SDA 1 I2C Serial Bi-Directional Data Line.
Open Drain. From Controller, tied to a pull-up resistor or current source
SCL 2 I2C Clock Input From Controller, tied to a pull-up resistor or current source
O.S. 3 Overtemperature Shutdown.
Open Drain Output Pull–up Resistor, Controller Interrupt Line
GND 4 Power Supply Ground Ground
+VS8 Positive Supply Voltage Input DC Voltage from 3V to 5.5V; 100 nF bypass capacitor with
10 µF bulk capacitance in the near vicinity
A0–A2 7,6,5 User-Set I2C Address Inputs Ground (Low, “0”) or +VS (High, “1”)
Typical Application
30099803
FIGURE 1. Typical Application
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LM75B LM75C
Ordering Information
Order Number Package
Marking
NS Package
Number
Supply
Voltage Transport Media Noise Filter on
SDA and SCL
LM75BIM-3 LM75BIM-3 M08A (SOP-8) 3.3V 95 Units in Rail Yes
LM75BIMX-3 LM75BIM-3 M08A (SOP-8) 3.3V 2500 Units on Tape and Reel Yes
LM75BIMM-3 T01B MUA08A (MSOP-8) 3.3V 1000 Units on Tape and Reel Yes
LM75BIMMX-3 T01B MUA08A (MSOP-8) 3.3V 3500 Units on Tape and Reel Yes
LM75BIM-5 LM75BIM-5 M08A (SOP-8) 5V 95 Units in Rail Yes
LM75BIMX-5 LM75BIM-5 M08A (SOP-8) 5V 2500 Units on Tape and Reel Yes
LM75BIMM-5 T00B MUA08A (MSOP-8) 5V 1000 Units on Tape and Reel Yes
LM75BIMMX-5 T00B MUA08A (MSOP-8) 5V 3500 Units on Tape and Reel Yes
LM75CIM-3 LM75CIM-3 M08A (SOP-8) 3.3V 95 Units in Rail Not Available
LM75CIMX-3 LM75CIM-3 M08A (SOP-8) 3.3V 2500 Units on Tape and Reel Not Available
LM75CIMM-3 T01C MUA08A (MSOP-8) 3.3V 1000 Units on Tape and Reel Not Available
LM75CIMMX-3 T01C MUA08A (MSOP-8) 3.3V 3500 Units on Tape and Reel Not Available
LM75CIM-5 LM75CIM-5 M08A (SOP-8) 5V 95 Units in Rail Not Available
LM75CIMX-5 LM75CIM-5 M08A (SOP-8) 5V 2500 Units on Tape and Reel Not Available
LM75CIMM-5 T00C MUA08A (MSOP-8) 5V 1000 Units on Tape and Reel Not Available
LM75CIMMX-5 T00C MUA08A (MSOP-8) 5V 3500 Units on Tape and Reel Not Available
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LM75B LM75C
Absolute Maximum Ratings (Note 1)
Supply Voltage Pin (+VS) −0.3V to 6.5V
Voltage at A0, A1and A2 Pins −0.3V to (+VS + 0.3V) and
must be ≤ 6.5V
Voltage at OS, SCL and SDA Pins −0.3V to 6.5V
Input Current at any Pin (Note 2) 5 mA
Package Input Current (Note 2) 20 mA
Storage Temperature −65°C to +150°C
ESD Susceptibility (Note 4) LM75B LM75C
Human Body Model 2500V 1500V
Machine Model 250V 100V
O.S. Output Sink Current 10 mA
O.S. Output Voltage 6.5V
Operating Ratings
Specified Temperature Range TMIN to TMAX
(Note 5) −55°C to +125°C
Supply Voltage Range (+VS)
LM75B, LM75C +3.0V to +5.5V
Soldering process must comply with National
Semiconductor's Reflow Temperature Profile specifications.
Refer to www.national.com/packaging.(Note 3)
Temperature-to-Digital Converter Characteristics
Unless otherwise noted, these specifications apply for: +VS = +5 Vdc for LM75BIM-5, LM75BIMM-5, LM75CIM-5, and
LM75CIMM-5; and +VS = +3.3 Vdc for LM75BIM-3, LM75BIMM-3, LM75CIM-3, and LM75CIMM-3 (Note 6). Boldface limits apply
for TA = TJ = TMIN to TMAX; all other limits TA = TJ = +25°C, unless otherwise noted.
Parameter Conditions Typical
(Note 12)
Limits
(Note 7)
Units
(Limit)
Accuracy TA = −25°C to +100°C ±2.0 °C (max)
TA = −55°C to +125°C ±3.0
Resolution 9 Bits
Temperature Conversion Time (Note 8) 100 300 ms (max)
Quiescent Current
LM75B
I2C Inactive 0.25 0.5 mA (max)
Shutdown Mode, +VS = 3V 4 μA
Shutdown Mode, +VS = 5V 6 μA
LM75C
I2C Inactive 0.25 1.0 mA (max)
Shutdown Mode, +VS = 3V 4 μA
Shutdown Mode, +VS = 5V 6 μA
O.S. Output Saturation Voltage IOUT = 4.0 mA 0.8 V (max)
O.S. Delay (Note 10) 1Conversion (min)
6Conversions (max)
TOS Default Temperature (Note 11) 80 °C
THYST Default Temperature (Note 11) 75 °C
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LM75B LM75C
Logic Electrical Characteristics
DIGITAL DC CHARACTERISTICS
Unless otherwise noted, these specifications apply for +VS = +5 Vdc for LM75BIM-5, LM75BIMM-5, LM75CIM-5, and LM75CIMM-5;
and +VS = +3.3 Vdc for LM75BIM-3, LM75BIMM-3, LM75CIM-3, and LM75CIMM-3 (Note 6). Boldface limits apply for TA = TJ =
TMIN to TMAX; all other limits TA = TJ = +25°C, unless otherwise noted.
Symbol Parameter Conditions Typical
(Note 12)
Limits
(Note 7)
Units
(Limit)
VIN(1) Logical “1” Input Voltage +VS × 0.7 V (min)
+VS + 0.3 V (max)
VIN(0) Logical “0” Input Voltage −0.3 V (min)
+VS × 0.3 V (max)
IIN(1) Logical “1” Input Current VIN = +VS0.005 1.0 μA (max)
IIN(0) Logical “0” Input Current VIN = 0V −0.005 −1.0 μA (max)
CIN All Digital Inputs 5 pF
IOH High Level Output Current LM75B VOH = 5V 10 μA (max)
LM75C VOH = 5V 100 μA (max)
VOL Low Level Output Voltage IOL = 3 mA 0.4 V (max)
tOF Output Fall Time CL = 400 pF
IO = 3 mA
250 ns (max)
I2C DIGITAL SWITCHING CHARACTERISTICS
Unless otherwise noted, these specifications apply for VS = +5 Vdc for LM75BIM-5, LM75BIMM-5, LM75CIM-5, and LM75CIMM-5;
and +VS = +3.3 Vdc for LM75BIM-3, LM75BIMM-3, LM75CIM-3, and LM75CIMM-3CL (load capacitance) on output lines = 80 pF
unless otherwise specified. Boldface limits apply for TA = TJ = TMIN to TMAX; all other limits TA = TJ = +25°C, unless otherwise
noted.
Symbol Parameter Conditions Typical
(Note 12)
Limits
(Note 7,
Note 14)
Units
(Limit)
t1SCL (Clock) Period 2.5 μs (min)
t2Data in Set-Up Time to SCL High 100 ns (min)
t3Data Out Stable after SCL Low 0ns (min)
t4SDA Low Set-Up Time to SCL Low (Start Condition) 100 ns (min)
t5SDA High Hold Time after SCL High (Stop Condition) 100 ns (min)
tTIMEOUT SDA Time Low for Reset of Serial Interface (Note 13)LM75B 75
325
ms (min)
ms (max)
LM75C Not Applicable
30099804
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.
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LM75B LM75C
Note 2: When the input voltage (VI) at any pin exceeds the power supplies (VI < GND or VI > +VS) 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.
Note 3: Reflow temperature profiles are different for lead-free and non-lead-free packages.
Note 4: Human body model, 100 pF discharged through a 1.5 kΩ resistor. Machine model, 200 pF discharged directly into each pin. The Charged Device Model
(CDM) is a specified circuit characterizing an ESD event that occurs when a device acquires charge through some triboelectric (frictional) or electrostatic induction
processes and then abruptly touches a grounded object or surface.
Note 5: LM75 θJA (thermal resistance, junction-to-ambient) when attached to a printed circuit board with 2 oz. foil similar to the one shown in Figure 3 is summarized
in the table below:
Device Number NS Package
Number
Thermal
Resistance (θJA)
LM75BIM-3, LM75BIM-5, LM75CIM-3, LM75CIM-5 M08A 200°C/W
LM75BIMM-3, LM75BIMM-5, LM75CIMM-3, LM75CIMM-5 MUA08A 250°C/W
Note 6: All part numbers of the LM75 will operate properly over the +VS supply voltage range of 3V to 5.5V. The devices are tested and specified for rated accuracy
at their nominal supply voltage. Accuracy will typically degrade 1°C/V of variation in +VS as it varies from the nominal value.
Note 7: Limits are guaranteed to National's AOQL (Average Outgoing Quality Level).
Note 8: The conversion-time specification is provided to indicate how often the temperature data is updated. The LM75 can be accessed at any time and reading
the Temperature Register will yield result from the last temperature conversion. When the LM75 is accessed, the conversion that is in process will be interrupted
and it will be restarted after the end of the communication. Accessing the LM75 continuously without waiting at least one conversion time between communications
will prevent the device from updating the Temperature Register with a new temperature conversion result. Consequently, the LM75 should not be accessed
continuously with a wait time of less than 300 ms.
Note 9: 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 10: O.S. Delay is user programmable up to 6 “over limit” conversions before O.S. is set to minimize false tripping in noisy environments.
Note 11: Default values set at power up.
Note 12: Typicals are at TA = 25°C and represent most likely parametric norm.
Note 13: Holding the SDA line low for a time greater than tTIMEOUT will cause the LM75B to reset SDA to the IDLE state of the serial bus communication (SDA
set High).
Note 14: Timing specifications are tested at the bus input logic levels (Vin(0)=0.3xVA for a falling edge and Vin(1)=0.7xVA for a rising edge) when the SCL and
SDA edge rates are similar.
30099805
FIGURE 2. Temperature-to-Digital Transfer Function (Non-linear scale for clarity)
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LM75B LM75C
30099806
FIGURE 3. Printed Circuit Board Used for Thermal Resistance Specifications
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LM75B LM75C
Typical Performance Characteristics
Static Quiescent Current vs Temperature (LM75C)
30099816
Dynamic Quiescent Current vs Temperature (LM75C)
30099817
Accuracy vs Temperature (LM75C)
30099818
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LM75B LM75C
1.0 Functional Description
The LM75 temperature sensor incorporates a band-gap type
temperature sensor and 9-bit ADC (Sigma-Delta Analog-to-
Digital Converter). The temperature data output of the LM75
is available at all times via the I2C bus. If a conversion is in
progress, it will be stopped and restarted after the read. A
digital comparator is also incorporated that compares a series
of readings, the number of which is user-selectable, to user-
programmable setpoint and hysteresis values. The compara-
tor trips the O.S. output line, which is programmable for mode
and polarity.
The LM75B contains all the functionality of the LM75C, plus
two additional features:
1. The LM75B has an integrated low-pass filter on both the
SDA and the SCL line. These filters increase
communications reliability in noisy environments.
2. The LM75B also has a bus fault timeout feature. If the
SDA line is held low for longer than tTIMEOUT (see
specification) the LM75B will reset to the IDLE state (SDA
set to high impedance) and wait for a new start condition.
The TIMEOUT feature is not functional in Shutdown
Mode.
1.1 O.S. OUTPUT, TOS AND THYST LIMITS
In Comparator mode the O.S. Output behaves like a thermo-
stat. The output becomes active when temperature exceeds
the TOS limit, and leaves the active state when the tempera-
ture drops below the THYST limit. In this mode the O.S. output
can be used to turn a cooling fan on, initiate an emergency
system shutdown, or reduce system clock speed. Shutdown
mode does not reset O.S. state in a comparator mode.
In Interrupt mode exceeding TOS also makes O.S. active but
O.S. will remain active indefinitely until reset by reading any
register via the I2C interface. Once O.S. has been activated
by crossing TOS, then reset, it can be activated again only by
Temperature going below THYST. Again, it will remain active
indefinitely until being reset by a read. Placing the LM75 in
shutdown mode also resets the O.S. Output.
1.2 POWER UP AND POWER DOWN
The LM75 always powers up in a known state. The power up
default conditions are:
1. Comparator mode
2. TOS = 80°C
3. THYST = 75°C
4. O.S. active low
5. Pointer = “00”
When the supply voltage is less than about 1.7V, the LM75 is
considered powered down. As the supply voltage rises above
the nominal 1.7V power up threshold, the internal registers
are reset to the power up default values listed above.
1.2.1 Stand-Alone Thermostat Mode
If the LM75 is not connected to the I2C bus on power up, it will
act as a stand-alone thermostat with the power up default
conditions listed above. It is optional, but recommended, to
connect the address pins (A2, A1, A0) and the SCL and SDA
pins together and to a 10k pull-up resistor to +VS for better
noise immunity. Any of these pins may also be tied high sep-
arately through a 10k pull-up resistor.
1.3 I2C BUS INTERFACE
The LM75 operates as a slave on the I2C bus, so the SCL line
is an input (no clock is generated by the LM75) and the SDA
line is a bi-directional serial data path. According to I2C bus
specifications, the LM75 has a 7-bit slave address. The four
most significant bits of the slave address are hard wired inside
the LM75 and are “1001”. The three least significant bits of
the address are assigned to pins A2–A0, and are set by con-
necting these pins to ground for a low, (0); or to +VS for a high,
(1).
Therefore, the complete slave address is:
1001A2 A1 A0
MSB LSB
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LM75B LM75C
30099807
Note 15: These interrupt mode resets of O.S. occur only when LM75 is read or placed in shutdown. Otherwise, O.S. would remain active indefinitely for any
event.
FIGURE 4. O.S. Output Temperature Response Diagram
1.4 TEMPERATURE DATA FORMAT
Temperature data can be read from the Temperature, TOS Set
Point, and THYST Set Point registers; and written to the TOS
Set Point, and THYST Set Point registers. Temperature data is
represented by a 9-bit, two's complement word with an LSB
(Least Significant Bit) equal to 0.5°C:
Temperature Digital Output
Binary Hex
+125°C 0 1111 1010 0FAh
+25°C 0 0011 0010 032h
+0.5°C 0 0000 0001 001h
0°C 0 0000 0000 000h
−0.5°C 1 1111 1111 1FFh
−25°C 1 1100 1110 1CEh
−55°C 1 1001 0010 192h
1.5 SHUTDOWN MODE
Shutdown mode is enabled by setting the shutdown bit in the
Configuration register via the I2C bus. Shutdown mode re-
duces power supply current significantly. See specified qui-
escent current specification in the electrical tables. In Interrupt
mode O.S. is reset if previously set and is undefined in Com-
parator mode during shutdown. The I2C interface remains
active. Activity on the clock and data lines of the I2C bus may
slightly increase shutdown mode quiescent current. TOS,
THYST, and Configuration registers can be read from and writ-
ten to in shutdown mode.
For the LM75B, the TIMEOUT feature is turned off in Shut-
down Mode.
1.6 FAULT QUEUE
A fault queue of up to 6 faults is provided to prevent false
tripping of O.S. when the LM75 is used in noisy environments.
The number of faults set in the queue must occur consecu-
tively to set the O.S. output.
1.7 COMPARATOR/INTERRUPT MODE
As indicated in the O.S. Output Temperature Response Dia-
gram, Figure 4, the events that trigger O.S. are identical for
either Comparator or Interrupt mode. The most important dif-
ference is that in Interrupt mode the O.S. will remain set
indefinitely once it has been set. To reset O.S. while in Inter-
rupt mode, perform a read from any register in the LM75.
1.8 O.S. OUTPUT
The O.S. output is an open-drain output and does not have
an internal pull-up. A “high” level will not be observed on this
pin 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 LM75. The maximum re-
sistance of the pull up, based on LM75 specification for High
Level Output Current, to provide a 2V high level, is 30 kΩ.
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LM75B LM75C
1.9 O.S. POLARITY
The O.S. output can be programmed via the configuration
register to be either active low (default mode), or active high.
In active low mode the O.S. output goes low when triggered
exactly as shown on the O.S. Output Temperature Response
Diagram, Figure 4. Active high simply inverts the polarity of
the O.S. output.
1.10 INTERNAL REGISTER STRUCTURE
30099808
There are four data registers in the LM75B and LM75C se-
lected by the Pointer register. At power-up the Pointer is set
to “000”; the location for the Temperature Register. The Point-
er register latches whatever the last location it was set to. In
Interrupt Mode, a read from the LM75, or placing the device
in shutdown mode, resets the O.S. output. All registers are
read and write, except the Temperature register which is a
read only.
A write to the LM75 will always include the address byte and
the Pointer byte. A write to the Configuration register requires
one data byte, and the TOS and THYST registers require two
data bytes.
Reading the LM75 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 regis-
ter because it will be the data most frequently read from the
LM75), then the read can simply consist of an address byte,
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 will ac-
complish a read.
The first data byte is the most significant byte with most sig-
nificant bit first, permitting only as much data as necessary to
be read to determine temperature condition. For instance, if
the first four bits of the temperature data indicates an overtem-
perature condition, the host processor could immediately take
action to remedy the excessive temperatures. At the end of a
read, the LM75 can accept either Acknowledge or No Ac-
knowledge 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 LM75 to stop in a state where the SDA
line is held low as shown in Figure 5. This can prevent any
further bus communication until at least 9 additional clock cy-
cles have occurred. Alternatively, the master can issue clock
cycles until SDA goes high, at which time issuing a “Stop”
condition will reset the LM75.
30099809
FIGURE 5. Inadvertent 8-Bit Read from 16-Bit Register where D7 is Zero (“0”)
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LM75B LM75C
1.11 POINTER REGISTER (Selects which registers will be
read from or written to):
P7 P6 P5 P4 P3 P2 P1 P0
0 0 0 0 0 Register Select
P0-P1: Register Select:
P2 P1 P0 Register
0 0 0 Temperature (Read only) (Power-up default)
0 0 1 Configuration (Read/Write)
0 1 0 THYST (Read/Write)
0 1 1 TOS (Read/Write)
P3–P7: Must be kept zero.
1.12 TEMPERATURE REGISTER (Read Only):
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
MSB Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 LSB X X X X X X X
D0–D6: Undefined. D7–D15: Temperature Data. One LSB = 0.5°C. Two's complement format.
1.13 CONFIGURATION REGISTER (Read/Write):
D7 D6 D5 D4 D3 D2 D1 D0
0 0 0 Fault Queue O.S. Polarity Cmp/Int Shutdown
Power up default is with all bits “0” (zero).
D0: Shutdown: When set to 1 the LM75 goes to low power shutdown mode.
D1: Comparator/Interrupt mode: 0 is Comparator mode, 1 is Interrupt mode.
D2: O.S. Polarity: 0 is active low, 1 is active high. O.S. is an open-drain output under all conditions.
D3–D4: Fault Queue: Number of faults necessary to detect before setting O.S. output to avoid false tripping due to noise. Faults
are determind at the end of a conversion. See specified temperature conversion time in the electrical tables.
D4 D3 Number of Faults
0 0 1 (Power-up default)
0 1 2
1 0 4
1 1 6
D5–D7: These bits are used for production testing and must be kept zero for normal operation.
1.14 THYST AND TOS REGISTER (Read/Write):
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
MSB Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 LSB X X X X X X X
D0–D6: Undefined D7–D15: THYST Or TOS Trip Temperature Data. Power up default is TOS = 80°C, THYST = 75°C
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LM75B LM75C
2.0 I2C Timing Diagrams
30099810
FIGURE 6. Timing Diagram
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LM75B LM75C
30099811
FIGURE 7. Timing Diagrams (Continued)
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LM75B LM75C
2.0 Application Hints
To get the expected results when measuring temperature with
an integrated circuit temperature sensor like the LM75, it is
important to understand that the sensor measures its own die
temperature. For the LM75, the best thermal path between
the die and the outside world is through the LM75's pins. In
the MSOP-8 package for the LM75B and LM75C, the GND
pin is directly connected to the die, so the GND pin provides
the best thermal path. If the other pins are at different tem-
peratures (unlikely, but possible), they will affect the die tem-
perature, but not as strongly as the GND pin. In the SO-8
package, none of the pins is directly connected to the die, so
they will all contribute similarly to the die temperature. Be-
cause the pins represent a good thermal path to the LM75 die,
the LM75 will provide an accurate measurement of the tem-
perature of the printed circuit board on which it is mounted.
There is a less efficient thermal path between the plastic
package and the LM75 die. If the ambient air temperature is
significantly different from the printed circuit board tempera-
ture, it will have a small effect on the measured temperature.
In probe-type applications, the LM75 can be mounted inside
a sealed-end metal tube, and can then be dipped into a bath
or screwed into a threaded hole in a tank. As with any IC, the
LM75 and accompanying wiring and circuits must be kept in-
sulated and dry, to avoid leakage and corrosion. This is
especially true if the circuit may operate at cold temperatures
where condensation can occur. Printed-circuit coatings and
varnishes such as Humiseal and epoxy paints or dips are of-
ten used to insure that moisture cannot corrode the LM75 or
its connections.
2.1 DIGITAL NOISE ISSUES
The LM75B features an integrated low-pass filter on both the
SCL and the SDA digital lines to mitigate the effects of bus
noise. Although this filtering makes the LM75B communica-
tion robust in noisy environments, good layout practices are
always recommended. Minimize noise coupling by keeping
digital traces away from switching power supplies. Also, en-
sure that digital lines containing high-speed data communi-
cations cross at right angles to the SDA and SCL lines.
Excessive noise coupling into the SDA and SCL lines on the
LM75C-specifically noise with amplitude greater than
400 mVpp (the LM75’s typical hysteresis), overshoot greater
than 300 mV above +Vs, and undershoot more than 300 mV
below GND-may prevent successful serial communication
with the LM75C. Serial bus no-acknowledge is the most com-
mon symptom, causing unnecessary traffic on the bus. The
layout procedures mentioned above apply also to the LM75C.
Although the serial bus maximum frequency of communica-
tion is only 400 kHz, care must be taken to ensure proper
termination within a system with long printed circuit board
traces or multiple parts on the bus. Resistance can be added
in series with the SDA and SCL lines to further help filter noise
and ringing. A 5 kΩ resistor should be placed in series with
the SCL line, placed as close as possible to the SCL pin on
the LM75C. This 5 kΩ resistor, with the 5 pF to 10 pF stray
capacitance of the LM75 provides a 6 MHz to 12 MHz low
pass filter, which is sufficient filtering in most cases.
3.0 Typical Applications
30099812
When using the two-wire interface: program O.S. for active high and connect O.S. directly to Q2's gate.
FIGURE 8. Simple Fan Controller, Interface Optional
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LM75B LM75C
30099814
FIGURE 9. Simple Thermostat, Interface Optional
30099815
FIGURE 10. Temperature Sensor with Loudmouth Alarm (Barking Watchdog)
www.national.com 16
LM75B LM75C
Physical Dimensions inches (millimeters) unless otherwise noted
8-Lead (0.150″ Wide) Molded Small Outline Package (SOP), JEDEC
Order Number LM75CIM-3, LM75CIMX-3, LM75CIM-5, LM75CIMX-5,
LM75BIM-3, LM75BIMX-3, LM75BIM-5, or LM75BIMX-5
NS Package Number M08A
8-Lead Molded Mini Small Outline Package (MSOP)
(JEDEC REGISTRATION NUMBER M0-187)
Order Number LM75CIMM-3, LM75CIMMX-3, LM75CIMM-5, LM75CIMMX-5,
LM75BIMM-3, LM75BIMMX-3,LM75BIMM-5, or LM75BIMMX-5
NS Package Number MUA08A
17 www.national.com
LM75B LM75C
LM75B LM75C Digital Temperature Sensor and Thermal Watchdog with Two-Wire Interface
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