Datasheet

LM77
www.ti.com SNIS103F –JUNE 1999–REVISED MARCH 2013
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
LOW
and T
HIGH
window, or the T_CRIT_A line for temperatures which exceed
T_CRIT. The lines are programmable for mode and polarity.
TEMPERATURE COMPARISON
LM77 provides a window comparison against a lower (T
LOW
) and upper (T
HIGH
) trip point. A second upper trip
point (T_CRIT) functions as a critical alarm shutdown. Figure 7 depicts the comparison function as well as the
modes of operation.
Status Bits
The internal Status bits operate as follows:
“True”: Temperature above a T
HIGH
or T_CRIT is “true” for those respective bits. A “true” for T
LOW
is
temperature below T
LOW
.
“False”: Assuming temperature has previously crossed above T
HIGH
or T_CRIT, then the temperature must
drop below the points corresponding T
HYST
(T
HIGH
− T
HYST
or T_CRIT − T
HYST
) in order for the condition to be
false. For T
LOW
, assuming temperature has previously crossed below T
LOW
, a “false” occurs when temperature
goes above T
LOW
+ T
HYST
.
The Status bits are not affected by reads or any other actions, and always represent the state of temperature vs.
setpoints.
Hardwire Outputs
The T_CRIT_A hardwire output mirrors the T_CRIT_A flag unless the part is read. When the flag is true, the
T_CRIT_A output is asserted regardless of Interrupt Mode. Reading the LM77 resets the T_CRIT_A output until
the internal conversion is completed. In a typical system, T_CRIT_A is used to immediately shutdown or reset
the system. Thus, once T_CRIT_A asserts the system normally would not be reading the LM77 via the I2C bus.
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
LOW
then INT becomes active. It would stay that way until temperature goes
above T
LOW
+ T
HYST
. However if the user reads the part, the output would 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 (downward) across a setpoint, or
2. Transitioning downward (upward) across a setpoint's corresponding hysteresis (after having exceeded that
setpoint).
For example, if a user never read the part, and temperature went below T
LOW
then INT would become active. It
would 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
temperature must cross above T
LOW
+ T
HYST
to set the output 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 Interrupts just after reprogramming setpoints.
Typically, system Interrupt inputs are masked prior to reprogramming trip points. By doing a read just after
resetting trip points, but prior to unmasking, unexpected Interrupts are prevented.
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