Datasheet

LM77

SNIS103F –JUNE 1999–REVISED MARCH 2013

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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.

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 general, the pull-up resistor should be as large

as possible. This will minimize any errors due to internal heating of the LM77. The maximum allowable resistance

of the pull up resistor is 90K Ohms based on the Logic “1” Output Leakage Current and a 2 volt high output level.

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 setting bit 4 of the Configuration Register high (see Table 3).

INTERNAL REGISTER STRUCTURE

Figure 8.

The data registers in the LM77 are selected by the Pointer register. At power-up the Pointer is set to “00”; the

location for the Temperature Register. The Pointer register latches the last location it was set to. In Comparator

Interrupt Mode, a read from the LM77 resets the INT output. Placing the device in Shutdown mode resets the

INT and T_CRIT_A outputs. All registers are read and write, except the Temperature register which is read only.

A write to the LM77 will always include the address byte and the Pointer byte. A write to the Configuration

LOW

, T

HIGH

, and T_CRIT registers require two data bytes.

Product Folder Links: LM77