1.0 Functional Description

The LM77 temperature sensor incorporates a band-gap type

temperature sensor, 10-bit ADC, and a digital comparator

with user-programmable upper and lower limit values. The

comparator activates either the INT line for temperatures

outside the T

for temperatures which exceed T_CRIT. The lines are pro-

grammable for mode and polarity.

1.1 TEMPERATURE COMPARISON

LM77 provides a window comparison against a lower (T

and upper (T

trip point. A second upper trip point

(T_CRIT) functions as a critical alarm shutdown.

Figure 3

depicts the comparison function as well as the modes of

operation.

1.1.1 STATUS BITS

The internal Status bits operate as follows:

“True”:

Temperature above a T

those respective bits. A “true” for T

T

“False”:

Assuming temperature has previously crossed

above T

below the points corresponding T

T_CRIT − T

T

T

+T

The Status bits are not affected by reads or any other

actions, and always represent the state of temperature vs.

setpoints.

1.1.2 HARDWIRE OUTPUTS

The T_CRIT_A hardwire output mirrors the T_CRIT_A flag,

when the flag is true, the T_CRIT_A output is asserted at all

times regardless of mode. Reading the LM77 has no effect

on the T_CRIT_A output, although the internal conversion is

restarted.

The behavior of the INT hardwire output is as follows:

Comparator Interrupt Mode (Default):

User reading part

resets output until next measurement completes. If condition

is still true, output is set again at end of next conversion

cycle. For example, if a user never reads the part, and

temperature goes below T

would stay that way until temperature goes above T

T

be reset. At the end of the next conversion cycle, if the

condition is true, it is set again. If not, it remains reset.

Event Interrupt Mode:

User reading part resets output

until next condition

″event″ occurs (in other words, output is

only set once for a true condition, if reset by a read, it

remains reset until the next triggering threshold has been

crossed). Conversely, if a user never read the part, the

output would stay set indefinitely after the first event that set

the output. An “event” for Event Interrupt Mode is defined as:

1.

Transitioning upward across a setpoint, or

2.

Transitioning downward across a setpoint’s correspond-

ing hysteresis (after having exceeded that setpoint).

For example, if a user never read the part, and temperature

went below T

stay that way forever if a user never read the part.

However if the user read the part, the output would be reset.

Even if the condition is true, it will remain reset. The tem-

perature must cross above T

again.

In either mode, reading any register in the LM77 restarts the

conversion. This allows a designer to know exactly when the

LM77 begins a comparison. This prevents unnecessary In-

DS100136-5

FIGURE 2. Temperature-to-Digital Transfer Function (Non-linear scale for clarity)

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