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SNIS128C – AUGUST 2002 – REVISED MARCH 2013
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 Remote High (RHS), R emote
Low (RLS) and Remote T_CRIT (RCS) user-programmable temperature limit registers. Activation 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
occurs when the temperature is above the T_CRIT setpoint. T_CRIT_A remains activated until the temperature
goes below the setpoint calculated by T_CRIT ? TH. The hysteresis register impacts both the remote
temperature and local temperature 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 nonideality error, discussed further in DIODE NONIDEALITY . The remote temperature reading
reported is adjusted by subtracting from or adding to the actual temperature reading the value placed in the
offset registers.
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 process the busy bit (D7) in the Status register (02h) is
high. These conversions are addressed in a round robin sequence. The conversion rate may be modified by the
Conversion 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
2000
1800
1600
140
0
1200
1000
800
600
400
0.01
0.1
1.0
10
100
CONVERSION RATE (Hz)
Figure 3. Conversion Rate Effect on Power Supply Current
8
Product Folder Links: LM89
Copyright ? 2002–2013, Texas Instruments Incorporated
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