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2-Channel Temperature Monitor with Dual

Automatic PWM Fan-Speed Controller

MAX6640

Table 10. Summary of Fan-Drive Options

FIGURE

6

7

8

9

10

DESCRIPTION

High-side PWM drive

Low-side PWM drive

High-side PWM drive with keep-alive supply

High-side linear supply

4-wire fan with PWM speed-control input

PULSE STRETCHING

Yes

No

PWM FREQUENCY

Low

High

PWM POLARITY

Negative

Positive

Negative

Positive

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 MAX6640 is optimized for n

= 1.008, which is the typical value for the Intel庐

Pentium庐 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 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:

铮?/div>

n

1

T

M

=

T

ACTUAL

铮?/div>

n

NOMINAL

铮?/div>

where temperature is measured in Kelvin.

As mentioned above, the nominal ideality factor of the

MAX6640 is 1.008. As an example, assume the

MAX6640 is configured 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:

铮?/div>

n

铮?/div>

1.008

铮?/div>

T

ACTUAL

=

T

M

铮?/div>

NOMINAL

铮?=

T

M

铮?/div>

铮?=

T

M

(1.00599)

1.002

铮?/div>

n

1

铮?/div>

Since 1掳C corresponds to 198.6碌V, series resistance

contributes a temperature offset of:

碌

V

90

掳

C

鈩?/div>

=

0.453

鈩?/div>

碌

V

198.6

掳

C

Assume that the diode being measured has a series

resistance of 3鈩? The series resistance contributes an

offset of:

3

鈩?脳

0.453

掳

C

鈩?/div>

=

1.36

掳

C

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

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

error of -2.13掳C.

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鈩? the total offset can be cal-

culated by adding error due to series resistance with

error due to ideality factor:

1.36掳C - 2.13掳C = -0.77掳C

for a diode temperature of +85掳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 connected to GND

and base connected to DXN. Table 11 lists examples of

discrete transistors that are appropriate for use with the

MAX6640.

Effect of Series Resistance

Series resistance in a sense diode contributes addition-

al errors. For nominal diode currents of 10碌A and

100碌A, change in the measured voltage is:

鈭哣

M

= R

S

(100碌A - 10碌A) = 90碌A x R

S

Table 11. Remote-Sensor Transistor

Manufacturers

MANUFACTURER

Central Semiconductor (USA)

Rohm Semiconductor (USA)

Samsung (Korea)

Siemens (Germany)

MODEL NO.

CMPT3906

SST3906

KST3906-TF

SMBT3906

Intel and Pentium are registered trademarks of Intel Corp.

18

______________________________________________________________________________________

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