MAX6640 datasheet(19/21 Pages) MAXIM | 2-Channel Temperature Monitor with Dual Automatic PWM Fan-Speed Controller

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

tively high forward voltage; otherwise, the ADC 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 temperature,

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 100

Ω. Tight speci-

fications for forward current gain (50 < ﬂ < 150, for

example) indicate that the manufacturer has good

process controls and that the devices have consistent

VBE characteristics.

ADC Noise Filtering

The integrating ADC has inherently good noise rejec-

tion, especially of low-frequency signals such as

60Hz/120Hz power-supply hum. Micropower operation

places constraints on high-frequency noise rejection.

Lay out the printed-circuit board (PCB) carefully with

proper external noise filtering for high-accuracy remote

measurements in electrically noisy environments.

Filter high-frequency electromagnetic interference

(EMI) at DXP and DXN with an external 2200pF capaci-

tor connected between the two inputs. This capacitor

can be increased to about 3300pF (max), including

cable capacitance. A capacitance higher than 3300pF

introduces errors due to the rise time of the switched-

current source.

Twisted Pairs and Shielded Cables

For remote-sensor distances longer than 8in, or in par-

ticularly noisy environments, a twisted pair is recom-

mended. Its practical length is 6ft to 12ft (typ) before

noise becomes a problem, as tested in a noisy elec-

tronics laboratory. For longer distances, the best solu-

tion is a shielded twisted pair like that used for audio

microphones. For example, Belden #8451 works well

for distances up to 100ft in a noisy environment.

Connect the twisted pair to DXP and DXN and the

shield to ground, and leave the shield’s remote end

unterminated. Excess capacitance at DXN or DXP limits

practical remote-sensor distances (see the

Typical

Operating Characteristics).

For very long cable runs, the cable’s parasitic capaci-

tance often provides noise filtering, so the recommend-

ed 2200pF capacitor can often be removed or reduced

in value. Cable resistance also affects remote-sensor

accuracy. A 1

Ω series resistance introduces about

+1/2°C error.

PCB Layout Checklist

1) Place the MAX6640 as close as practical to the

remote diode. In a noisy environment, such as a

computer motherboard, this distance can be 4in to

8in, or more, as long as the worst noise sources

(such as CRTs, clock generators, memory buses,

and ISA/PCI buses) are avoided.

2) Do not route the DXP/DXN lines next to the deflection

coils of a CRT. Also, do not route the traces across a

fast memory bus, which can easily introduce +30°C

error, even with good filtering. Otherwise, most noise

sources are fairly benign.

3) Route the DXP and DXN traces parallel and close to

each other, away from any high-voltage traces such

as +12VDC. Avoid leakage currents from PCB cont-

amination. A 20M

Ω leakage path from DXP ground

causes approximately +1°C error.

4) Connect guard traces to GND on either side of the

DXP/DXN traces. With guard traces, placing routing

near high-voltage traces is no longer an issue.

5) Route as few vias and crossunders as possible to

minimize copper/solder thermocouple effects.

6) When introducing a thermocouple, make sure that

both the DXP and the DXN paths have matching

thermocouples. In general, PCB-induced thermo-

couples are not a serious problem. A copper solder

thermocouple exhibits 3µV/°C, and it takes approxi-

mately 200µV of voltage error at DXP/DXN to cause

a +1°C measurement error, so most parasitic ther-

mocouple errors are swamped out.

7) Use wide traces. Narrow traces are more inductive

and tend to pick up radiated noise. The 10-mil widths

and spacings recommended are not absolutely nec-

essary (as they offer only a minor improvement in

leakage and noise), but use them where practical.

8) Placing an electrically clean copper ground plane

between the DXP/DXN traces and traces carrying

high-frequency noise signals helps reduce EMI.

2-Channel Temperature Monitor with Dual

Automatic PWM Fan-Speed Controller

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