MAX6642ATT98-T(2003) Просмотр технического описания (PDF) - Maxim Integrated

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MAX6642ATT98-T
(Rev.:2003)

±1°C, SMBus-Compatible Remote/Local Temperature Sensor with Overtemperature Alarm

Maxim Integrated 

±1°C, SMBus-Compatible Remote/Local

Temperature Sensor with Overtemperature Alarm

due to the rise time of the switched current source.

Nearly all noise sources tested cause the temperature

conversion results to be higher than the actual temper-

ature, typically by +1°C to +10°C, depending on the

frequency and amplitude (see the Typical Operating

Characteristics).

PC Board Layout

Follow these guidelines to reduce the measurement

error of the temperature sensors:

1) Connect the thermal-sense diode to the MAX6642

using two traces—one between DXP and the

anode, the other between the MAX6642’s GND and

the cathode. Do not connect the cathode to GND at

the sense diode.

2) Place the MAX6642 as close as is practical to the

remote thermal diode. In noisy environments, such

as a computer motherboard, this distance can be

4in to 8in (typ). This length can be increased if the

worst noise sources are avoided. Noise sources

include CRTs, clock generators, memory buses,

and ISA/PCI buses.

3) Do not route the thermal diode lines next to the

deflection coils of a CRT. Also, do not route the

traces across fast digital signals, which can easily

introduce a 30°C error, even with good filtering.

4) Route the thermal diode traces in parallel and in

close proximity to each other, away from any higher

voltage traces, such as +12VDC. Leakage currents

from PC board contamination must be dealt with

carefully since a 20MΩ leakage path from DXP to

ground causes about +1°C error. If high-voltage

traces are unavoidable, connect guard traces to

GND on either side of the DXP trace (Figure 4).

5) Route through as few vias and crossunders as pos-

sible to minimize copper/solder thermocouple

effects.

6) When introducing a thermocouple, make sure that

both the thermal diode paths have matching ther-

mocouples. A copper-solder thermocouple exhibits

3µV/°C, and it takes about 200µV of voltage error at

DXP to cause a +1°C measurement error. Adding a

few thermocouples causes a negligible error.

7) Use wide traces. Narrow traces are more inductive

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

widths and spacing recommended in Figure 4 are

not absolutely necessary, as they offer only a minor

improvement in leakage and noise over narrow

traces. Use wider traces when practical.

8) Add a 47Ω resistor in series with VCC for best noise

filtering (see the Typical Operating Circuit).

9) Copper cannot be used as an EMI shield; only fer-

rous materials such as steel work well. Placing a

copper ground plane between the DXP-DXN traces

and traces carrying high-frequency noise signals

does not help reduce EMI.

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 GND and

the shield to GND. Leave the shield unconnected at the

remote diode.

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

tance 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 1/2°C.

Thermal Mass and Self-Heating

When sensing local temperature, this device is intend-

ed to measure the temperature of the PC board to

which it is soldered. The leads provide a good thermal

path between the PC board traces and the die. Thermal

conductivity between the die and the ambient air is

poor by comparison, making air temperature measure-

ments impractical. Because the thermal mass of the PC

board is far greater than that of the MAX6642, the

device follows temperature changes on the PC board

with little or no perceivable delay.

When measuring temperature of a CPU or other IC with

an on-chip sense junction, thermal mass has virtually

no effect; the measured temperature of the junction

10 mils

GND

THERMAL DIODE ANODE/DXP

10 mils

THERMAL DIODE CATHODE/GND

GND

Figure 4. Recommended DXP PC Traces

10 mils

MINIMUM

10 mils

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