Datasheet

LM87
SNAS034J –APRIL 2000–REVISED MARCH 2013 www.ti.com
Fans do not start reliably at reduced voltages, so operation at a reduced voltage should be preceded by a brief
(typically 1 second) excursion to full operating voltage, then reduce the voltage. Most fans do not operate at all
below 5 to 7 volts. At those lower voltages the fan will simply consume current, dissipate power, and not operate,
and such conditions should be avoided.
The output of the amplifier can be configured to provide a high or low side pass transistor. A high side pass
transistor simplifies the coupling of tachometer outputs to the tachometer inputs of the LM87 since the fan
remains grounded. Low side drive will require AC coupling along with clamping at the LM87 input to prevent
negative excursions.
A typical circuit for fan drive is shown in Figure 16.
TEMPERATURE MEASUREMENT SYSTEM
The LM87 temperature sensor(s) and ADC produce 8-bit two's-complement temperature data. One internal diode
junction temperature, and up to two external junction temperatures can be monitored. A digital comparator
compares the temperature data to the user-programmable High, Low, and Hardware Limit setpoints, and
Hysteresis values.
(Non-Linear Scale for Clarity)
Figure 11. 8-bit Temperature-to-Digital Transfer Function
Temperature Data Format
Temperature data can be read from the Temperature, T
HIGH
setpoint, T
LOW
setpoint, and Hardware Temperature
limit registers; and written to the T
HIGH
setpoint, T
LOW
setpoint, and Hardware Temperature limit registers. T
HIGH
setpoint, T
LOW
setpoint, Hardware Temperature Limit, and Temperature data is represented by an 8-bit, two's
complement word with an LSB (Least Significant Bit) equal to 1°C:
Temperature Digital Output
Binary Hex
+125°C 0111 1101 7Dh
+25°C 0001 1001 19h
+1.0°C 0000 0001 01h
+0°C 0000 0000 00h
−1.0°C 1111 1111 FFh
−25°C 1110 0111 E7h
−40°C 1101 1000 D8h
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