Datasheet

LM75A

SNOS808O –JANUARY 2000–REVISED MAY 2013

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Figure 11. Timing Diagrams (Continued)

APPLICATION HINTS

To get the expected results when measuring temperature with an integrated circuit temperature sensor like the

LM75A, it is important to understand that the sensor measures its own die temperature. For the LM75A, the best

thermal path between the die and the outside world is through the LM75A's pins. In the VSSOP-8 package, 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 temperatures (unlikely, but possible), they will affect the die temperature, but not as strongly as the GND

pin. In the SOIC-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 LM75A die, the LM75A 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 LM75A 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 LM75A 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 LM75A 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 LM75A or its

connections.

DIGITAL NOISE ISSUES

The LM75A 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 LM75A 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

LM75A—specifically noise with amplitude greater than 400 mV

(the LM75A’s typical hysteresis), overshoot

greater than 300mV above +V

, and undershoot more than 300 mV below GND—may prevent successful serial

communication with the LM75A. Serial bus no-acknowledge is the most common symptom, causing unnecessary

traffic on the bus. Although the serial bus maximum frequency of communication 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.

Product Folder Links: LM75A