LM75AIM/NOPB National Semiconductor, LM75AIM/NOPB Datasheet - Page 15

• Manufacturer Part Number: LM75AIM/NOPB
• Description: IC TEMP SENSOR WATCHDOG 8-SOIC
• Manufacturer: National Semiconductor

Specifications of LM75AIM/NOPB

• Function: Temp Sensor, Watchdog
• Topology: ADC (Sigma Delta), Comparator, Register Bank
• Sensor Type: Internal
• Sensing Temperature: -55°C ~ 125°C
• Output Type: I²C™
• Output Alarm: Yes
• Output Fan: Yes
• Voltage - Supply: 2.7 V ~ 5.5 V
• Operating Temperature: -55°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• For Use With: OM6285 - EVAL BOARD I2C-2002-1A568-4002 - DEMO BOARD I2C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM75AIM

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.
3.0 Typical Applications
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
15
2.1 DIGITAL NOISE ISSUES
The LM75A and LM75B feature an integrated low-pass filter
on both the SCL and the SDA digital lines to mitigate the ef-
fects of bus noise. Although this filtering makes the LM75A
and LM75B communication robust in noisy environments,
good layout practices are always recommended. Minimize
noise coupling by keeping digital traces away from switching
power supplies. Also, ensure that digital lines containing high-
speed data communications 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 mV
than 300 mV above +V
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.
pp
(the LM75’s typical hysteresis), overshoot greater
s
, and undershoot more than 300 mV
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