lm77cimmx-5 National Semiconductor Corporation, lm77cimmx-5 Datasheet - Page 7

• Manufacturer Part Number: lm77cimmx-5
• Description: 9-bit Sign Digital Temperature Sensor And Thermal Window Comparator With Two-wire Interface
• Manufacturer: National Semiconductor Corporation

Logic Electrical Characteristics
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
(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.
LOW
LOW
LOW
HYST
.
, assuming temperature has previously crossed below
, a “false” occurs when temperature goes above T
.
HIGH
HYST
LOW
or T_CRIT, then the temperature must drop
HIGH
) in order for the condition to be false. For
and T
) trip point. A second upper trip point
FIGURE 2. Temperature-to-Digital Transfer Function (Non-linear scale for clarity)
HIGH
window, or the T_CRIT_A line
HIGH
LOW
HYST
or T_CRIT is “true” for
is temperature below
(T
HIGH
− T
HYST
(Continued)
LOW
LOW
or
)
7
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-
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-
HYST
ing hysteresis (after having exceeded that setpoint).
. However if the user reads the part, the output would
LOW
then INT would become active. It would
DS100136-5
LOW
LOW
then INT becomes active. It
+ T
HYST
to set the output
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LOW
+