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LM89CIMMX Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LM89CIMMX Datasheet(HTML) 7 Page - National Semiconductor (TI) |
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7 / 20 page  1.0 Functional Description The LM89 temperature sensor incorporates a delta V BE based temperature sensor using a Local or Remote diode and a 10-bit plus sign ADC (Delta-Sigma Analog-to-Digital Converter). The LM89 is compatible with the serial SMBus version 2.0 two-wire interface. Digital comparators compare the measured Local Temperature (LT) to the Local High (LHS), Local Low (LLS) and Local T_CRIT (LCS) user- programmable temperature limit registers. The measured Remote Temperature (RT) is digitally compared to the Re- mote High (RHS), Remote Low (RLS) and Remote T_CRIT (RCS) user-programmable temperature limit registers. Acti- vation of the ALERT output indicates that a comparison is greater than the limit preset in a T_CRIT or HIGH limit register or less than the limit preset in a LOW limit register. The T_CRIT_A output responds as a true comparator with built in hysteresis. The hysteresis is set by the value placed in the Hysteresis register (TH). Activation of T_CRIT_A oc- curs when the temperature is above the T_CRIT setpoint. T_CRIT_A remains activated until the temperature goes be- low the setpoint calculated by T_CRIT − TH. The hysteresis register impacts both the remote temperature and local tem- perature readings. The LM89 may be placed in a low power consumption (Shutdown) mode by setting the RUN/STOP bit found in the Configuration register. In the Shutdown mode, the LM89’s SMBus interface remains while all circuitry not required is turned off. The Local temperature reading and setpoint data registers are 8-bits wide. The format of the 11-bit remote temperature data is a 16-bit left justified word. Two 8-bit registers, high and low bytes, are provided for each setpoint as well as the temperature reading. Two offset registers (RTOLB and RTOHB) can be used to compensate for non_ideality error, discussed further in Section 4.1 DIODE NON-IDEALITY. The remote temperature reading reported is adjusted by subtracting from or adding to the actual temperature reading the value placed in the offset registers. 1.1 CONVERSION SEQUENCE The LM89 takes approximately 31.25 ms to convert the Local Temperature (LT), Remote Temperature (RT), and to update all of its registers. Only during the conversion pro- cess the busy bit (D7) in the Status register (02h) is high. These conversions are addressed in a round robin se- quence. The conversion rate may be modified by the Con- version Rate Register (04h). When the conversion rate is modified a delay is inserted between conversions, the actual conversion time remains at 31.25ms. Different conversion rates will cause the LM89 to draw different amounts of supply current as shown in Figure 2. 1.2 THE ALERT OUTPUT The LM89’s ALERT pin is an active-low open-drain output that is triggered by a temperature conversion that is outside the limits defined by the temperature setpoint registers. Re- set of the ALERT output is dependent upon the selected method of use. The LM89’s ALERT pin is versatile and will accommodate three different methods of use to best serve the system designer: as a temperature comparator, as a temperature based interrupt flag, and as part of an SMBus ALERT system. The three methods of use are further de- scribed below. The ALERT and interrupt methods are differ- ent only in how the user interacts with the LM89. Each temperature reading (LT and RT) is associated with a T_CRIT setpoint register (LCS, RCS), a HIGH setpoint reg- ister (LHS and RHS) and a LOW setpoint register (LLS and RLS). At the end of every temperature reading, a digital comparison determines whether that reading is above its HIGH or T_CRIT setpoint or below its LOW setpoint. If so, the corresponding bit in the STATUS REGISTER is set. If the ALERT mask bit is not high, any bit set in the STATUS REGISTER, with the exception of Busy (D7) and OPEN (D2), will cause the ALERT output to be pulled low. Any temperature conversion that is out of the limits defined by the temperature setpoint registers will trigger an ALERT. Addi- tionally, the ALERT mask bit in the Configuration register must be cleared to trigger an ALERT in all modes. 1.2.1 ALERT Output as a Temperature Comparator When the LM89 is implemented in a system in which it is not serviced by an interrupt routine, the ALERT output could be used as a temperature comparator. Under this method of use, once the condition that triggered the ALERT to go low is no longer present, the ALERT is de-asserted (Figure 3). For example, if the ALERT output was activated by the compari- son of LT > LHS, when this condition is no longer true the ALERT will return HIGH. This mode allows operation without software intervention, once all registers are configured dur- ing set-up. In order for the ALERT to be used as a tempera- ture comparator, bit D0 (the ALERT configure bit) in the FILTER and ALERT CONFIGURE REGISTER (xBF) must be set high. This is not the power-on-default state. 20041539 FIGURE 2. Conversion Rate Effect on Power Supply Current www.national.com 7 |
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