LM87
Test Circuit
10099506
FIGURE 3. Digital Output Load Test Circuitry
Typical Performance Characteristics
DAC Power Supply Sensitivity
10099536
Functional Description
1.0 GENERAL DESCRIPTION
The LM87 provides 7 analog inputs, an internal junction type
temperature sensor, two remote junction temperature sens-
ing channels, a Delta-Sigma ADC (Analog-to-Digital Con-
verter), a DAC output, 2 fan speed counters, WATCHDOG
registers, and a variety of inputs and outputs on a single
chip. A two wire SMBus Serial Bus interface is included. The
LM87 performs power supply, temperature, fan control and
fan monitoring for personal computers.
The analog inputs are useful for monitoring several power
supplies present in a typical computer. The LM87 includes
internal resistor dividers that scale external Vccp1, Vccp2,
+2.5V, +5.0 V, +12 V and internal +3.3V power supply volt-
ages to a 3/4 scale nominal ADC output. Two additional
inputs, +AIN1 and +AIN2 (2.5V full scale) are input directly
with no resistive dividers. The LM87 ADC continuously con-
verts the scaled inputs to 8-bit digital words. Measurement of
negative voltages (such as -5 V and -12 V power supplies)
can be accommodated with an external resistor divider ap-
plied to the +AIN1 or +AIN2 inputs. Internal and external
temperature is converted to 8-bit two’s-complement digital
words with a 1 ˚C LSB.
Fan inputs measure the period of tachometer pulses from
the fans, providing a higher count for lower fan speeds. The
fan inputs are Schmitt-Trigger digital inputs with an accept-
able range of 0 V to V
+
and a transition level of approxi-
mately V
+
/2. Full scale fan counts are 255 (8-bit counter) and
this represents a stopped or very slow fan. Nominal speeds,
based on a count of 153, are programmable from 1100 to
8
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