LM75A

SNOS808P – JANUARY 2000 – REVISED DECEMBER 2014

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7.5.2 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:

Digital Output

Temperature

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

7.5.3 Shutdown Mode

Shutdown mode is enabled by setting the shutdown bit in the Configuration register via the I2C bus. Shutdown

mode reduces power supply current significantly. See specified quiescent current specification in the electrical

tables. In Interrupt mode O.S. is reset if previously set and is undefined in Comparator 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 written to in

shutdown mode.

For the LM75A, the TIMEOUT feature is turned off in Shutdown Mode.

7.5.4 Fault Queue

A fault queue of up to 6 faults is provided to prevent false tripping of O.S. when the LM75A is used in noisy

environments. The number of faults set in the queue must occur consecutively to set the O.S. output.

7.5.5 Comparator and Interrupt Mode

As indicated in the O.S. Output Temperature Response Diagram, Figure 7, the events that trigger O.S. are

identical for either Comparator or Interrupt mode. The most important difference is that in Interrupt mode the O.S.

will remain set indefinitely once it has been set. To reset O.S. while in Interrupt mode, perform a read from any

register in the LM75A.

7.5.6 O.S. Output

The O.S. output is an open-drain output and does not have an internal pullup. A “high” level will not be observed

on this pin until pullup current is provided from some external source, typically a pullup resistor. Choice of

resistor value depends on many system factors but, in general, the pullup resistor should be as large as possible.

This will minimize any errors due to internal heating of the LM75A. The maximum resistance of the pullup, based

on LM75A specification for High Level Output Current, to provide a 2-V high level, is 30 k

Ω.

7.5.7 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 7. Active high simply inverts the polarity of the O.S. output.

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