DIGITAL NOISE ISSUES
3.0 Application Hints
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,
ensure that digital lines containing high-speed data commu-
nications cross at right angles to the SDA and SCL lines.
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, the GND pin is directly con-
nected to the die, so the GND pin provides the best thermal
path. If the other pins are at different temperatures (unlikely,
but possible), they will affect the die temperature, 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. Because the pins represent
a good thermal path to the LM75 die, the LM75 will provide
an accurate measurement of the temperature 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 temperature, it will have a small
effect on the measured temperature.
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
common symptom, causing unnecessary traffic on the bus.
The layout procedures mentioned above apply also to the
LM75C. Although the serial bus maximum frequency of com-
munication is only 100 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.
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
insulated 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
often used to insure that moisture cannot corrode the LM75
or its connections.
4.0 Typical Applications
01265812
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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