MAX6658 View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX6658

±1°C, SMBus-Compatible Remote/Local Temperature Sensors with Overtemperature Alarms

Maxim Integrated 

±1°C, SMBus-Compatible Remote/Local Temperature

Sensors with Overtemperature Alarms

surements. The noise can be reduced with careful PC

board layout and proper external noise filtering.

High-frequency EMI is best filtered at DXP and DXN

with an external 2200pF capacitor. Larger capacitor

values can be used for added filtering, but do not

exceed 3300pF because it can introduce errors due to

the rise time of the switched current source.

10MILS

10MILS

GND

10MILS

DXP

MINIMUM

DXN

10MILS

GND

Figure 1. Recommended DXP-DXN PC Traces

PC Board Layout

Follow these guidelines to reduce the measurement

error of the temperature sensors:

1) Place the MAX6657/MAX6658/MAX6659 as close

as is practical to the remote diode. In noisy environ-

ments, such as a computer motherboard, this dis-

tance 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.

2) Do not route the DXP-DXN lines next to the deflec-

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

across fast digital signals, which can easily intro-

duce +30°C error, even with good filtering.

3) Route the DXP and DXN 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-DXN traces (Figure 1).

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

sible to minimize copper/solder thermocouple

effects.

5) When introducing a thermocouple, make sure that

both the DXP and the DXN paths have matching

thermocouples. A copper-solder thermocouple

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

error at DXP-DXN to cause a +1°C measurement

error. Adding a few thermocouples causes a negli-

gible error.

6) Use wide traces. Narrow traces are more inductive

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

and spacings that are recommended in Figure 1 are

not absolutely necessary, as they offer only a minor

improvement in leakage and noise over narrow

traces. Use wider traces when practical.

7) Add a 200Ω resistor in series with VCC for best

noise filtering (see Typical Operating Circuit).

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 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.

Low-Power Standby Mode

Standby mode reduces the supply current to less than

10µA by disabling the ADC. Enter hardware standby

(MAX6659 only) by forcing the STBY pin low, or enter

software standby by setting the RUN/STOP bit to 1 in

the Configuration Byte register. Hardware and software

standbys are very similar—all data is retained in memo-

ry, and the SMB interface is alive and listening for

SMBus commands. The only difference is that in soft-

ware standby mode, the one-shot command initiates a

conversion. With hardware standby, the one-shot com-

mand is ignored. Activity on the SMBus causes the

device to draw extra supply current.

Driving the STBY pin low overrides any software con-

version command. If a hardware or software standby

command is received while a conversion is in progress,

the conversion cycle is interrupted, and the tempera-

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