1.4 TEMPERATURE DATA FORMAT

Temperature data can be read from the Temperature and Set

Point registers; and written to the Set Point registers. Tem-

perature data can be read at any time, although reading

faster than the conversion time of the LM77 will prevent data

from being updated. Temperature data is represented by a

10-bit, two’s complement word with an LSB (Least Signifi-

cant Bit) equal to 0.5˚C:

Temperature

Digital Output

Binary

Hex

+130˚C

01 0000 0100

104h

+125˚C

00 1111 1010

0FAh

+25˚C

00 0011 0010

032h

+0.5˚C

00 0000 0001

001h

0˚C

00 0000 0000

000h

−0.5˚C

11 1111 1111

3FFh

−25˚C

11 1100 1110

3CEh

−55˚C

11 1001 0010

392h

1.5 SHUTDOWN MODE

Shutdown mode is enabled by setting the shutdown bit in the

Configuration register via the Serial Bus. Shutdown mode

reduces power supply current to 5 µA typical. T_CRIT_A is

reset if previously set. Since conversions are stoped during

shutdown, T_CRIT_A and INT will not be operational. The

Serial Bus interface remains active. Activity on the clock and

data lines of the Serial Bus may slightly increase shutdown

mode quiescent current. Registers can be read from and

written to in shutdown mode. The LM77 takes miliseconds to

respond to the shutdown command.

1.6 INT AND T_CRIT_A OUTPUT

The INT and T_CRIT_A outputs are open-drain outputs and

do not have internal pull-ups. A

″high″ level will not be

observed on these pins until pull-up current is provided from

some external source, typically a pull-up resistor. Choice of

resistor value depends on many system factors but, in gen-

eral, the pull-up resistor should be as large as possible. This

will minimize any errors due to internal heating of the LM77.

The maximum resistance of the pull up, based on LM77

specification for High Level Output Current, to provide a 2

volt high level, is 30K ohms.

1.7 FAULT QUEUE

A fault queue of up to 4 faults is provided to prevent false

tripping when the LM77 is used in noisy environments. The 4

faults must occur consecutively to set flags as well as INT

and T_CRIT_A outputs. The fault queue is enabled by set-

ting bit 4 of the Configuration Register high (see Section

1.11).

DS100136-6

Note: Event Interrupt mode is drawn as if the user is reading the part. If the user doesn’t read, the outputs would go low and stay that way until the LM77 is read.

FIGURE 3. Temperature Response Diagram

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