MAX6653/MAX6663/MAX6664
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 cable’s
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 1Ω of series resistance, the error is approxi-
mately 0.5°C.
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 0.1µF V
CC
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).
0: disable THERM as an output.
*For MAX6663 bit 7 has to be 1 all the time.
