3.0 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, 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.

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.

3.1 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,

ensure that digital lines containing high-speed data commu-

nications 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

300 mV above +V

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 k

Ω resistor should be placed in

series with the SCL line, placed as close as possible to the

SCL pin on the LM75C. This 5 k

Ω 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.

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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