LM87 datasheet(17/32 Pages) NSC

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Functional Description (Continued)

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 condi-

tions 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 tachom-

eter 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 13.

8.0 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 tem-

peratures can be monitored. A digital comparator compares

the temperature data to the user-programmable High, Low,

and Hardware Limit setpoints, and Hysteresis values.

8.1 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, TLOW setpoint,

and Hardware Temperature limit registers. T

HIGH setpoint,

LOW setpoint, Hardware Temperature Limit, and Tempera-

ture 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

Temperature

Digital Output

Binary

Hex

+0˚C

0000 0000

00h

−1.0˚C

1111 1111

FFh

−25˚C

1110 0111

E7h

−40˚C

1101 1000

D8h

8.2 Internal Temperature Measurement

The LM87 internal temperature is monitored using a junction

type temperature sensor.

8.3 Remote Temperature Measurement

The LM87 monitors the temperature of remote semiconduc-

tor devices using the p-n junction temperature sensing prin-

cipal. Up to two remote IC, diode or bipolar transistor tem-

peratures can be monitored. The remote measurement

channels have been optimized to measure the remote diode

of a Pentium II processor. A discrete diode or bipolar tran-

sistor can also be used to sense the temperature of external

objects or ambient air. The 2N3904 NPN transistor base

emitter junction performs well in this type of application.

When using a 2N3904, the collector should be connected to

the base to provide a device that closely approximates the

characteristics of the Pentium II PNP monitoring diode.

When using two external 2N3904 sensors, the D− inputs

should be connected together. This provides the best pos-

sible accuracy by compensating for differences between the

2N3904 and Pentium II sensors.

During each conversion cycle, the remote monitoring inputs

perform an external diode fault detection sequence. If the D+

input is shorted to V

CC or floating then the temperature

reading will be +127˚C, and bit 6 or bit 7 of Interrupt Status

temperature reading will be 0˚C and bit 6 or 7 of Interrupt

Status Register 2 will not be set.

8.4 Accuracy Effects of Diode Non-Ideality Factor

The technique used in today’s remote temperature sensors

is to measure the change in V

BE at two different operating

points of a diode. For a bias current ratio of N:1, this differ-

ence is given as:

where:

•

η is the non-ideality factor of the process the diode is

manufactured on,

•

q is the electron charge,

•

k is the Boltzmann’s constant,

•

N is the current ratio,

•

T is the absolute temperature in ˚K.

The temperature sensor then measures

∆V

BE and converts

to digital data. In this equation, k and q are well defined

universal constants, and N is a parameter controlled by the

temperature sensor. The only other parameter is

η, which

depends on the diode that is used for measurement. Since

∆V

BE is proportional to both

η and T, the variations in η

cannot be distinguished from variations in temperature.

Since the non-ideality factor is not controlled by the tempera-

ture sensor, it will directly add to the inaccuracy of the

sensor. For the Pentium II Intel specifies a ±1% variation in

10099524

(Non-Linear Scale for Clarity)

FIGURE 8. 8-bit Temperature-to-Digital Transfer

Function

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