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LM77CIM-3 Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LM77CIM-3 Datasheet(HTML) 7 Page - National Semiconductor (TI) |
7 / 17 page Logic Electrical Characteristics (Continued) 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 LOW and THIGH window, or the T_CRIT_A line 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 LOW) and upper (T HIGH) 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 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(THIGH −THYST 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 TLOW +T HYST. 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 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 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 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 tem- perature must cross above T LOW +THYST 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 In- DS100136-5 FIGURE 2. Temperature-to-Digital Transfer Function (Non-linear scale for clarity) www.national.com 7 |
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