LM75B, LM75C

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SNIS153D – JULY 2009 – REVISED OCTOBER 2015

7.4 Device Functional Modes

In Comparator mode the O.S. Output behaves like a thermostat. The output becomes active when temperature

exceeds the TOS limit, and leaves the active state when the temperature drops below the THYST limit. In this mode

the O.S. output can be used to turn a cooling fan on, initiate an emergency system shutdown, or reduce system

clock speed. Shutdown mode does not reset O.S. state in a comparator mode.

In Interrupt mode exceeding TOS also makes O.S. active but O.S. will remain active indefinitely until reset by

reading any register via the I2C interface. Once O.S. has been activated by crossing TOS, then reset, it can be

activated again only by Temperature going below THYST. Again, it will remain active indefinitely until being reset

by a read. Placing the LM75 in shutdown mode also resets the O.S. Output.

The LM75B always powers up in a known state. The power up default conditions are:

1. Comparator mode

2. TOS = 80°C

3. THYST = 75°C

4. O.S. active low

5. Pointer = “00”

When the supply voltage is less than about 1.7V, the LM75 is considered powered down. As the supply voltage

rises above the nominal 1.7V power up threshold, the internal registers are reset to the power up default values

listed above.

If the LM75 is not connected to the I2C bus on power up, it will act as a stand-alone thermostat with the power up

default conditions listed above. It is optional, but recommended, to connect the address pins (A2, A1, A0) and

the SCL and SDA pins together and to a 10k pull-up resistor to +VS for better noise immunity. Any of these pins

may also be tied high separately through a 10k pull-up resistor.

7.5 Programming

7.5.1

I2C Bus Interface

The LM75 operates as a slave on the I2C bus, so the SCL line is an input (no clock is generated by the LM75)

and the SDA line is a bi-directional serial data path. According to I2C bus specifications, the LM75 has a 7-bit

slave address. The four most significant bits of the slave address are hard wired inside the LM75 and are “1001”.

The three least significant bits of the address are assigned to pins A2–A0, and are set by connecting these pins

to ground for a low, (0); or to +VS for a high, (1).

Therefore, the complete slave address is:

1

0

0

1

A2

A1

A0

MSB

LSB

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Product Folder Links: LM75B LM75C