LM75A datasheet(7/21 Pages) TI1

部品番号	LM75A

部品情報	LM75A Digital Temperature Sensor and Thermal Watchdog with Two-Wire Interface

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LM75A

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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 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 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 tTIMEOUT (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, 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 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. 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 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 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.

I

2C BUS INTERFACE

The LM75A operates as a slave on the I2C 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 I2C 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 +VS for a high, (1).

Therefore, the complete slave address is:

1

0

1

A2

A1

A0

MSB

LSB

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