MAX6692(2002) 데이터 시트보기 (PDF) - Maxim Integrated

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MAX6692
(Rev.:2002)

Precision SMBus-Compatible Remote/Local Temperature Sensors with Overtemperature Alarms

Maxim Integrated 

Precision SMBus-Compatible Remote/Local

Temperature Sensors with Overtemperature Alarms

Effect of Ideality Factor

The accuracy of the remote temperature measurements

depends on the ideality factor (n) of the remote “diode”

(actually a transistor). The MAX6648/MAX6692 are opti-

mized for n = 1.008, which is the typical value for the

Intel® Pentium® III and the AMD Athlon MP model 6. If a

sense transistor with a different ideality factor is used,

the output data is different. Fortunately, the difference

is predictable.

Assume a remote-diode sensor designed for a nominal

ideality factor nNOMINAL is used to measure the tem-

perature of a diode with a different ideality factor n1.

The measured temperature TM can be corrected using:

TM

=

TACTUAL







n1

nNOMINAL







where temperature is measured in Kelvin.

As mentioned above, the nominal ideality factor of the

MAX6648/MAX6692 is 1.008. As an example, assume

you want to use the MAX6648/MAX6692 with a CPU

that has an ideality factor of 1.002.

If the diode has no series resistance, the measured

data is related to the real temperature as follows:

TACTUAL = TM





nNOMINAL

n1





=

TM

 1.008

 1.002

=

TM (1.00599)

For a real temperature of +85°C (358.15 K), the mea-

sured temperature is +82.91°C (356.02 K), which is an

error of -2.13°C.

Effect of Series Resistance

Series resistance in a sense diode contributes addition-

al errors. For nominal diode currents of 10µA and

100µA, change in the measured voltage is:

∆VM = RS(100µA −10µA) = 90µA × RS

Since 1°C corresponds to 198.6µV, series resistance

contributes a temperature offset of:

90 µV

Ω

= 0.453 °C

198.6 µV

Ω

°C

Assume that the diode being measured has a series

Intel and Pentium are registered trademarks of Intel Corp.

resistance of 3Ω. The series resistance contributes an

offset of:

3Ω × 0.453 °C = 1.36°C

Ω

The effects of the ideality factor and series resistance

are additive. If the diode has an ideality factor of 1.002

and series resistance of 3Ω, the total offset can be cal-

culated by adding error due to series resistance with

error due to ideality factor:

1.36°C - 2.13°C = -0.77°C

for a diode temperature of +85°C.

In this example, the effect of the series resistance and

the ideality factor partially cancel each other.

For best accuracy, the discrete transistor should be a

small-signal device with its collector and base connect-

ed together. Table 9 lists examples of discrete transis-

tors that are appropriate for use with the MAX6648/

MAX6692.

Table 9. Remote-Sensor Transistor

Manufacturers

MANUFACTURER

MODEL NO.

Central Semiconductor (USA)

CMPT3904

Rohm Semiconductor (USA)

SST3904

Samsung (Korea)

KST3904-TF

Siemens (Germany)

SMBT3904

Note: Transistors must be diode connected (base shorted to

collector).

The transistor must be a small-signal type with a rela-

tively high forward voltage; otherwise, the A/D input

voltage range can be violated. The forward voltage at

the highest expected temperature must be greater than

0.25V at 10µA, and at the lowest expected tempera-

ture, the forward voltage must be less than 0.95V at

100µA. Large power transistors must not be used.

Also, ensure that the base resistance is less than 100Ω.

Tight specifications for forward current gain (50 < ß

<150, for example) indicate that the manufacturer has

good process controls and that the devices have con-

sistent VBE characteristics.

ADC Noise Filtering

The integrating ADC used has good noise rejection for

low-frequency signals such as 60Hz/120Hz power-sup-

ply hum. In noisy environments, high-frequency noise

reduction is needed for high-accuracy remote mea-

12 ______________________________________________________________________________________

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