MAX6664AEE+T【PDF 20/24 页】IC TEMP MON FAN CNTRL 16-QSOP

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Temperature Monitors and

PWM Fan Controllers

20 ______________________________________________________________________________________

Twisted-Pair and Shielded Cables

Use a twisted-pair cable to connect the remote sensor

for remote-sensor distances longer than 8in, or in very

noisy environments. Twisted-pair cable lengths can be

between 6ft and 12ft before noise introduces excessive

errors. For longer distances, the best solution is a

shielded twisted pair like that used for audio micro-

phones. For example, Belden 8451 works well for dis-

tances up to 100ft in a noisy environment. At the

device, connect the twisted pair to DXP and DXN and

the shield to GND. Leave the shield unconnected at the

remote sensor. For very long cable runs, the cables

parasitic capacitance often provides noise filtering, so

the 2200pF capacitor can often be removed or reduced

in value.

Cable resistance also affects remote-sensor accuracy.

For every 1& of series resistance, the error is approxi-

mately 0.5癈.

PC Board Layout Checklist

" Place the MAX6653/MAX6663/MAX6664 close to

the remote-sense junction.

" Keep traces away from high voltages (+12V bus).

" Keep traces away from fast data buses and CRTs.

" Use recommended trace widths and spacings.

" Place a ground plane under the traces.

" Use guard traces flanking DXP and DXN and con-

necting to GND.

" Place the noise filter and the 0.1礔 V

bypass

capacitors close to the MAX6653/MAX6663/

MAX6664.

Remote Diode Considerations

The accuracy of the remote temperature measurements

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

(actually a transistor). The MAX6653/MAX6663/

MAX6664 are optimized for n = 1.008, which is the typi-

cal value for the Intel Pentium III. A thermal diode on

the substrate of an IC is normally a PNP with its collec-

tor grounded. DXP should be connected to the anode

(emitter) and DXN should be connected to the cathode

(base) of this PNP.

When the remote-sensing diode is a discrete transistor,

its collector and base should be connected together.

Table 16 lists examples of discrete transistors that are

appropriate for use with the MAX6653/MAX6663/

MAX6664.

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

Table 16. Remote-Sensor Transistor

Manufacturers

Note: Discrete transistors must be diode connected (base

shorted to collector).

MANUFACTURER

MODEL NO.

Central Semiconductor (USA)

CMPT3904

Rohm Semiconductor (USA)

SST3904

Samsung (Korea)

KST3904-TF

Siemens (Germany)

SMBT3904

Zetex (England)

FMMT3904CT-ND

Addr(H)

READ/WRITE POR STATE

DESCRIPTION

0011 1000 Device ID

0100 1101 Manufacturer ID

R/W

1000 0000

THERM behavior/revision:

Bit [7]: THERM behavior:

1: enable THERM as an output.

0: disable THERM as an output.

Bits [3:0] revision number.

*For MAX6663 bit 7 has to be 1 all the time.

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