LM75B, LM75C
SNIS153B –JULY 2009–REVISED MARCH 2013
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FUNCTIONAL DESCRIPTION
The LM75 temperature sensor incorporates a band-gap type temperature sensor and 9-bit ADC (Sigma-Delta
Analog-to-Digital Converter). The temperature data output of the LM75 is available at all times via the I2C bus. If
a conversion is in progress, it will be stopped and restarted after the read. A digital comparator is also
incorporated that compares a series of readings, the number of which is user-selectable, to user-programmable
setpoint and hysteresis values. The comparator trips the O.S. output line, which is programmable for mode and
polarity.
The LM75B contains all the functionality of the LM75C, plus two additional features:
1. The LM75B has an integrated low-pass filter on both the SDA and the SCL line. These filters increase
communications reliability in noisy environments.
2. The LM75B also has a bus fault timeout feature. If the SDA line is held low for longer than tTIMEOUT (see
Logic Electrical Characteristics) the LM75B will reset to the IDLE state (SDA set to high impedance) and wait
for a new start condition. The TIMEOUT feature is not functional in Shutdown Mode.
O.S. OUTPUT, TOS AND THYST LIMITS
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.
POWER UP AND POWER DOWN
The LM75 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.
Stand-Alone Thermostat Mode
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.
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
8
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