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, THIGH setpoint, TLOW setpoint, and Hardware Temperature

limit registers; and written to the THIGH setpoint, TLOW setpoint, and Hardware Temperature limit registers. THIGH

setpoint, TLOW 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

20

Submit Documentation Feedback

Copyright © 2000–2013, Texas Instruments Incorporated

Product Folder Links: LM87