Datasheet

LM75A
www.ti.com SNOS808O –JANUARY 2000–REVISED MAY 2013
FUNCTIONAL DESCRIPTION
The LM75A 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 LM75A is available at all times via the I
2
C 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 LM75A has an integrated low-pass filter on both the SDA and the SCL line. These filters increase
communications reliability in noisy environments.
The LM75A also has a bus fault timeout feature. If the SDA line is held low for longer than t
TIMEOUT
(see
specification) the LM75A 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, T
OS
AND T
HYST
LIMITS
In Comparator mode the O.S. Output behaves like a thermostat. The output becomes active when temperature
exceeds the T
OS
limit, and leaves the active state when the temperature drops below the T
HYST
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 T
OS
also makes O.S. active but O.S. will remain active indefinitely until reset by
reading any register via the I
2
C interface. Once O.S. has been activated by crossing T
OS
, then reset, it can be
activated again only by Temperature going below T
HYST
. Again, it will remain active indefinitely until being reset
by a read. Placing the LM75A in shutdown mode also resets the O.S. Output.
POWER UP AND POWER DOWN
The LM75A always powers up in a known state. The power up default conditions are:
1. Comparator mode
2. T
OS
= 80°C
3. T
HYST
= 75°C
4. O.S. active low
5. Pointer = “00”
When the supply voltage is less than about 1.7V, the LM75A 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 LM75A is not connected to the I
2
C 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 +V
S
for better noise immunity. Any of these pins
may also be tied high separately through a 10k pull-up resistor.
I
2
C BUS INTERFACE
The LM75A operates as a slave on the I
2
C bus, so the SCL line is an input (no clock is generated by the LM75A)
and the SDA line is a bi-directional serial data path. According to I
2
C bus specifications, the LM75A has a 7-bit
slave address. The four most significant bits of the slave address are hard wired inside the LM75A 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 +V
S
for a high, (1).
Therefore, the complete slave address is:
1 0 0 1 A2 A1 A0
MSB LSB
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