MAX6690MEE+ Datasheet MAX6690MEE+, MAX6690MEE+T | P7-P9

MAX6690

2°C Accurate Remote/Local Temperature

Sensor with SMBus Serial Interface

_______________________________________________________________________________________ 7

Figure 1. Functional Diagram

REMOTE

MUX

LOCAL

REMOTE TEMPERATURE

DATA REGISTER

HIGH-TEMPERATURE THRESHOLD

(REMOTE T

HIGH

)

LOW-TEMPERATURE THRESHOLD

(REMOTE T

LOW

)

DIGITAL COMPARATOR

(REMOTE)

LOCAL TEMPERATURE

DATA REGISTER

HIGH-TEMPERATURE THRESHOLD

(LOCAL T

HIGH)

LOW-TEMPERATURE THRESHOLD

(LOCAL T

LOW

)

DIGITAL COMPARATOR

(LOCAL)

COMMAND BYTE

(INDEX) REGISTER

SMBDATA

SMBCLK

ADDRESS

DECODER

READ WRITE

CONTROL

LOGIC

SMBus

ADD1ADD0STBY

STATUS BYTE REGISTER

CONFIGURATION

BYTE REGISTER

CONVERSION RATE

ALERT RESPONSE

ADDRESS REGISTER

SELECTED VIA

SLAVE ADD = 0001 100

ADC

DIODE

FAULT

DXP

DXN

GND

ALERT

MAX6690

2°C Accurate Remote/Local Temperature

Sensor with SMBus Serial Interface

8 _______________________________________________________________________________________

measure ambient temperature; when measuring local

temperature, it senses the temperature of the PC board

to which it is soldered. The leads provide a good ther-

mal path between the PC board traces and the

MAX6690’s die. Thermal conductivity between the

MAX6690’s die and the ambient air is poor by compari-

son. Because the thermal mass of the PC board is far

greater than that of the MAX6690, the device follows

temperature changes on the PC board with little or no

perceivable delay.

When measuring temperature with discrete remote sen-

sors, the use of smaller packages, such as SOT23s,

yields the best thermal response times. Take care to

account for thermal gradients between the heat source

and the sensor, and ensure that stray air currents

across the sensor package do not interfere with mea-

surement accuracy. When measuring the temperature

of a CPU or other IC with an on-chip sense junction,

thermal mass has virtually no effect; the measured tem-

perature of the junction tracks the actual temperature

within a conversion cycle.

Self-heating does not significantly affect measurement

accuracy. Remote-sensor self-heating due to the diode

current source is negligible. For the local diode, the

worst-case error occurs when autoconverting at the

fastest rate and simultaneously sinking maximum cur-

rent at the ALERT output. For example, at an 8Hz rate

and with ALERT sinking 1mA, the typical power dissi-

pation is V

x 450µA + 0.4V x 1mA. Package theta J-

A is about 150°C/Ω, so with V

= 5V and no copper

PC board heat sinking, the resulting temperature rise is:

ΔT = 2.7mW x 150°C/W = 0.4°C

Even with these contrived circumstances, it is difficult

to introduce significant self-heating errors.

ADC Noise Filtering

The ADC is an integrating type with inherently good

noise rejection, especially of low-frequency signals such

as 60Hz/120Hz power-supply hum. Micropower opera-

tion places constraints on high-frequency noise rejection;

therefore, careful PC board layout and proper external

noise filtering are required for high-accuracy remote

measurements in electrically noisy environments.

High-frequency EMI is best filtered at DXP and DXN with

an external 2200pF capacitor. This value can be

increased to about 3300pF (max), including cable

capacitance. Capacitance >3300pF introduces errors

due to the rise time of the switched current source.

Nearly all noise sources tested cause the ADC measure-

ments to be higher than the actual temperature, typically

by +1°C to +10°C, depending on the frequency and

amplitude (see

Typical Operating Characteristics

PC Board Layout

1) Place the MAX6690 as close as practical to the

remote diode. In a noisy environment, such as a

computer motherboard, this distance can be 4in to

8in (typ) 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 deflec-

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

across a fast memory bus, which can easily intro-

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

Otherwise, most noise sources are fairly benign.

3) Route the DXP and DXN traces in parallel and in

close proximity to each other, away from any high-

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.

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

DXP-DXN traces (Figure 2). With guard traces in

place, routing near high-voltage traces is no longer

an issue.

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 DXP and the DXN paths have matching

thermocouples. In general, PC-board-induced ther-

mocouples are not a serious problem. 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. 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 10mil

widths and spacings recommended in Figure 2

aren’t absolutely necessary (as they offer only a

MINIMUM

10mils

GND

DXN

DXP

GND

Figure 2. Recommended DXP/DXN PC Traces

MAX6690

2°C Accurate Remote/Local Temperature

Sensor with SMBus Serial Interface

_______________________________________________________________________________________ 9

minor improvement in leakage and noise), but try to

use them where practical.

8) Keep in mind that copper can’t be used as an EMI

shield, and only ferrous materials such as steel

work well. Placing a copper ground plane between

the DXP-DXN traces and traces carrying high-fre-

quency noise signals does not help reduce EMI.

PC Board Layout Checklist

• Place the MAX6690 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

bypass

capacitors close to the MAX6690.

• Add a 200Ω resistor in series with V

for best

noise filtering (see

Typical Operating Circuit

Twisted-Pair and Shielded Cables

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

ularly noisy environments, a twisted pair is recommend-

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

becomes a problem, as tested in a noisy electronics lab-

oratory. 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. Connect the

twisted pair to DXP and DXN and the shield to GND, and

leave the shield’s remote end unterminated.

Excess capacitance at DXN and DXP limits practical

remote-sensor distances (see

Typical Operating

Characteristics

). 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;

1Ω series resistance introduces about +1/2°C error.

Setting bit 4 of the configuration register to 1 invokes

the parasitic resistance cancellation mode. This rejects

external resistance in excess of 100Ω while maintaining

conversion accuracy.

Low-Power Standby Mode

Standby mode disables the ADC and reduces the sup-

ply-current drain to less than 10µA. Enter standby

mode by forcing the STBY/pin low or through the

RUN/STOP bit in the configuration byte register.

Hardware and software standby modes behave almost

identically; all data is retained in memory, and the SMB

interface is alive and listening for reads and writes. The

only difference is that in hardware standby mode, the

one-shot command does not initiate a conversion.

Standby mode is not a shutdown mode. With activity on

the SMBus, extra supply current is drawn (see

Typical

Operating Characteristics

). In software standby mode,

the MAX6690 can be forced to perform A/D conver-

sions through the one-shot command, despite the

RUN/STOP bit being high.

Activate hardware standby mode by forcing the STBY

pin low. In a notebook computer, this line may be con-

nected to the system SUSTAT# suspend-state signal.

The STBY pin low state overrides any software conver-

sion command. If a hardware or software standby com-

mand is received while a conversion is in progress, the

conversion cycle is truncated, and the data from that

conversion is not latched into either temperature read-

ing register. The previous data is not changed and

remains available.

Supply-current drain during the 125ms conversion peri-

od is always about 550µA. Slowing down the conver-

sion rate reduces the average supply current (see

Typical Operating Characteristics

). In between conver-

sions, the supply current is about 25µA due to the cur-

rent consumed by the conversion rate timer. In standby

mode, supply current drops to about 3µA. At very low

supply voltages (under the power-on-reset threshold),

the supply current is higher due to the address pin bias

currents. It can be as high as 100µA, depending on

ADD0 and ADD1 settings.

SMBus Digital Interface

From a software perspective, the MAX6690 appears as

a set of byte-wide registers that contain temperature

data, alarm threshold values, or control bits. A standard

SMBus 2-wire serial interface is used to read tempera-

ture data and write control bits and alarm threshold

data. The device responds to the same SMBus slave

address for access to all functions.

The MAX6690 employs four standard SMBus protocols:

Write Byte, Read Byte, Send Byte, and Receive Byte

(Figures 3, 4, 5). The shorter Receive Byte protocol

allows quicker transfers, provided that the correct data

tion. Use caution with the shorter protocols in multimas-

ter systems, since a second master could overwrite the

command byte without informing the first master.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P16

MAX6690MEE+

Mfr. #:

Buy MAX6690MEE+

Manufacturer:

Maxim Integrated

Description:

Board Mount Temperature Sensors Remote/Local Temperature Sensor

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MAX6690MEE+

MAX6690MEE+

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