MAX6692YMSA+【PDF 12/16 页】IC SENSOR REMOTE SMBUS 8SOIC

III and

the AMD Athlon MP model 6. If a sense transistor with a

different ideality factor is used, the output data is differ-

ent. Fortunately, the difference is predictable.

Assume a remote-diode sensor designed for a nominal

ideality factor n

NOMINAL

is used to measure the tem-

perature of a diode with a different ideality factor n

1

.

The measured temperature T

M

can be corrected using:

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:

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

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

error of -2.13癈.

Effect of Series Resistance

Series resistance in a sense diode contributes addition-

al errors. For nominal diode currents of 10礎 and

100礎, change in the measured voltage is:

Since 1癈 corresponds to 198.6礦, series resistance

contributes a temperature offset of:

Assume that the diode being measured has a series

resistance of 3? The series resistance contributes an

offset of:

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 3? the total offset can be cal-

culated by adding error due to series resistance with

error due to ideality factor:

1.36癈 - 2.13癈 = -0.77癈

for a diode temperature of +85癈.

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.

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 10礎, and at the lowest expected tempera-

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

100礎. Large power transistors must not be used.

Also, ensure that the base resistance is less than 100?

Tight specifications for forward current gain (50 < ?/DIV>

<150, for example) indicate that the manufacturer has

good process controls and that the devices have con-

sistent V

BE

characteristics.

Operation with 45nm Substrate PNPs

Small transistor geometries and specialized processes

can affect temperature measurement accuracy.

Parasitic series resistance can be higher, which

increases the measured temperature value. Beta may

3

0 453

1 36

?/DIV>

=

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.

C

90

198 6

0 453

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V

C

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.

擵 R

A

A R

M S

S

=

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(

)

100

10

90

T

n

T

ACTUAL

M

NOMINAL

M

=

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=

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=

.

( .

)

1

1 008

1 002

1 00599

T T

n

M

ACTUAL

NOMINAL

=

?/DIV>

1

Intel and Pentium are registered trademarks of Intel Corp.

MANUFACTURER

MODEL NO.

Central Semiconductor (USA)

CMPT3904

Rohm Semiconductor (USA)

SST3904

Samsung (Korea)

KST3904-TF

Siemens (Germany)

SMBT3904

Table 9. Remote-Sensor Transistor

Manufacturers

Note: Transistors must be diode connected (base shorted to

collector).

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