MAXIM MAX6680, MAX6681 Technical data

General Description

The MAX6680/MAX6681 are precise, two-channel digi­tal thermometers. Each accurately measures the tem­perature of its own die and one remote PN junction and
reports the temperature on a 2-wire serial interface. The
remote junction can be a diode-connected transistor
like the low-cost NPN type 2N3904 or PNP type
2N3906. The remote junction can also be a common­collector PNP, such as a substrate PNP of a micro­processor.

The MAX6680/MAX6681 include pin-programmable
default temperature thresholds for the OVERT output,
which provides fail-safe clock throttling or system shut­down. In addition, the devices are pin programmable to
select whether the OVERT output responds to either the
local, remote, or both temperatures.

The 2-wire serial interface accepts standard System
Management Bus (SMBus)™ commands such as Write
Byte, Read Byte, Send Byte, and Receive Byte to read
the temperature data and program the alarm thresholds
and conversion rate. The MAX6680/MAX6681 can func­tion autonomously with a programmable conversion
rate, which allows the control of supply current and
temperature update rate to match system needs. For
conversion rates of 4Hz or less, the remote sensor tem­perature can be represented in extended mode as 10
bits + sign with a resolution of 0.125°C. When the con­version rate is 8Hz, output data is 7 bits + sign with a
resolution of 1°C. The MAX6680/MAX6681 also include
an SMBus timeout feature to enhance system reliability.

The MAX6681 is an upgrade to the MAX6654. The
MAX6680/MAX6681 remote accuracy is ±1°C with no
calibration needed. They are available in a 16-pin
QSOP package and operate throughout the -55°C to
+125°C temperature range.

Applications

Features

♦ Two Alarm Outputs: ALERT and OVERT
♦ Pin-Programmable Threshold for OVERT Limit
♦ Programmable Under/Overtemperature ALERT

Limit

♦ Dual Channel: Measures Remote and Local

Temperature

♦ 11-Bit, 0.125°C Resolution for Remote Temperature

Measurements

♦ High Accuracy ±1°C (max) from +60°C to +100°C

(Remote)

♦ No Calibration Required

♦ SMBus/I

2

C™-Compatible Interface

♦ SMBus Timeout Prevents SMBus Lockup

MAX6680/MAX6681

±1°C Fail-Safe Remote/Local Temperature

Sensors with SMBus Interface

________________________________________________________________ Maxim Integrated Products 1

Typical Operating Circuit

Ordering Information

19-2305; Rev 1; 1/05

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

PART TEMP RANGE PIN-PACKAGE

MAX6680MEE -55°C to +125°C 16 QSOP

MAX6681MEE -55°C to +125°C 16 QSOP

SMBus is a trademark of Intel Corp.

I

2

C is a trademark of Philips Corp.

Pin Configurations appear at end of data sheet.

Desktop Computers

Notebook
Computers

Servers

Thin Clients

Workstations

0.1µF

200Ω

3.3V

MICROPROCESSOR

2200pF

V

DXP

DXN

SENS_SEL

INT_SEL

ADD0

ADD1

CC

STBY

MAX6680
MAX6681

SMBDATA

SMBCLK

ALERT

RESET

OVERT

CRIT1CRIT0GND

10kΩ
EACH

DATA

CLOCK

INTERRUPT
TO µP

TO SYSTEM
SHUTDOWN

MAX6680/MAX6681

±1°C Fail-Safe Remote/Local Temperature
Sensors with SMBus Interface

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(Circuit of Typical Operating Circuit, VCC= 3.0V to 5.5V, TA= -25°C to +125°C, unless otherwise specified. Typical values are at V

CC

= 3.3V and TA= +25°C.)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.

VCC...........................................................................-0.3V to +6V

DXP.............................................................-0.3V to (V

CC

+ 0.3V)

DXN ......................................................................-0.3V to +0.8V

SMBCLK, SMBDATA, ALERT, OVERT .....................-0.3V to +6V

RESET, INT_SEL, STBY, ADD0, ADD1.....................-0.3V to +6V

CRIT1, CRIT0, SENS_SEL ........................................-0.3V to +6V

SMBDATA, ALERT, OVERT, Current ..................-1mA to +50mA

DXN Current ......................................................................±1mA

Continuous Power Dissipation (T

A

= +70°C)

16-Pin QSOP (derate 8.3mW/°C above +70°C) ..........664mW

Junction Temperature .....................................................+150°C

Storage Temperature Range ............................-65°C to +150°C

Lead Temperature (soldering, 10s) ................................+300°C

Average Operating Current
(Note 3)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Temperature Resolution,
Legacy Mode

Temperature Resolution,
Extended Mode

TRJ = +60°C to +100°C, VCC = 3.3V -1.0 +1.0

TRJ = +50°C to +120°C, VCC = 3.3V -2.0 +2.0Rem ote Tem p er atur e E r r or ( N ote 1)

= -55°C to +125°C, VCC = 3.3V -3.0 +3.0

T

RJ

TA = +60°C to +100°C, VCC = 3.3V -1.5 +1.5

Local Temperature Error

Line Regulation 3.0V ≤ VCC ≤ 5.5V 0.2 0.6 m°C/V

Supply Voltage Range V

Undervoltage Lockout Threshold UVLO Falling edge of VCC disables ADC 2.60 2.80 2.95 V

Undervoltage Lockout Hysteresis 90

Power-On Reset (POR)
Threshold

POR Threshold Hysteresis 90 mV

Conversion Time

Standby Supply Current SMBus static 3 10 µA

Operating Current During conversion 0.55 1.0 mA

DXP and DXN Leakage Current In standby mode 2 µA

Remote-Diode Source Current I

TA = 0°C to +125°C, VCC = 3.3V -3.0 +3.0

= -55°C to +125°C, VCC = 3.3V (Note 2) -5.0 +5.0

T

A

CC

, falling edge 1.5 2.0 2.5 V

V

CC

Legacy 62.5

Extended 125

0.25 conversions/s 35 70

2 conversions/s 120 180

High level 80 100 120

RJ

Low level 8 10 12

1°C

8 Bits

0.125 °C

11 Bits

3.0 5.5 V

°C

°C

mV

ms

µA

µA

MAX6680/MAX6681

±1°C Fail-Safe Remote/Local Temperature

Sensors with SMBus Interface

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Typical Operating Circuit, VCC= 3.0V to 5.5V, TA= -25°C to +125°C, unless otherwise specified. Typical values are at V

CC

= 3.3V and TA= +25°C.)

Note 1: T

A

= +25°C to +85°C.

Note 2: If both the local and the remote junction are below T

A

= -20°C, then VCC> 3.15V.

Note 3: Conversions done in extended mode. For legacy mode, current is approximately half.
Note 4: Timing specifications guaranteed by design.
Note 5: The serial interface resets when SMBCLK or SMBDATA is low for more than t

TIMEOUT

.

Note 6: A transition must internally provide at least a hold time to bridge the undefined region (300ns max) of SMBCLK’s falling edge.

CRIT0, CRIT1, ADD0, ADD1, RESET, INT_SEL, SENS_SEL

Logic Input Low Voltage V

Logic Input High Voltage V

Input Leakage Current I

(ALERT, OVERT)

SMBus INTERFACE (SMBCLK, SMBDATA, STBY)

SMBus-COMPATIBLE TIMING (Note 5)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

IL

IH

LEAK

Output Low Sink Current VOL = 0.4V 1 mA

Output High Leakage Current VOH = 5.5V 1 µA

Logic Input Low Voltage V

Logic Input High Voltage V

Input Leakage Current I

Output Low Sink Current I

Input Capacitance C

Serial Clock Frequency (Note 5) f

Bus Free Time Between STOP
and START Condition

START Condition Setup Time 4.7 µs

Repeat START Condition Setup
Time

START Condition Hold Time t

STOP Condition Setup Time t

Clock Low Period t

Clock High Period t

Data Setup Time (Note 6) t

Receive SCL/SDA Rise Time t

Receive SCL/SDA Fall Time t

Pulse Width of Spike Suppressed t

SMBus Timeout (Note 5) SMBDATA low period for interface reset 25 37 45 ms

IL

IH

LEAK

OL

IN

SCL

t

BUF

t

SU:STA

HD:STA

SU:STO

LOW

HIGH

HD:DAT

R

F

SP

VCC = 3.0V 2.2

VCC = 5.5V 2.4

VIN = GND or V

VOL = 0.6V 6 mA

90% to 90% 50 ns

10% of SMBDATA to 90% of SMBCLK 4 µs

90% of SMDCLK to 90% of SMBDATA 4 µs

10% to 10% 4.7 µs

90% to 90% 4 µs

CC

2.4 V

-1 +1 µA

5pF

4.7 µs

250 ns

050ns

0.8 V

0.8 V

±2 µA

100 kHz

1µs

300 ns

V

MAX6680/MAX6681

±1°C Fail-Safe Remote/Local Temperature
Sensors with SMBus Interface

4 _______________________________________________________________________________________

Typical Operating Characteristics

(TA = +25°C, unless otherwise noted.)

MAX6680/81 toc01

SUPPLY VOLTAGE (V)

STANDBY SUPPLY CURRENT (µA)

5.04.54.03.5

1

2

3

4

5

6

7

8

9

10

0

3.0 5.5

STANDBY SUPPLY CURRENT

vs. SUPPLY VOLTAGE

MAX6680/81 toc02

CONVERSION RATE (Hz)

OPERATING SUPPLY CURRENT (µA)

8.0000

4.0000

2.0000

1.0000

0.5000

0.2500

0.1250

100

200

300

400

500

600

0

0.0625

AVERAGE OPERATING SUPPLY CURRENT

vs. CONVERSION RATE

8Hz IS 1°C
RESOLUTION

MAX6680/81 toc03

TEMPERATURE (°C)

TEMPERATURE ERROR (°C)

1251007550250-25

-2

-1

0

1

2

3

-3

-50 150

TEMPERATURE ERROR

vs. REMOTE-DIODE TEMPERATURE

MAX6680/81 toc04

TEMPERATURE (°C)

TEMPERATURE ERROR (°C)

100500

-2

-1

0

1

2

3

-3

-50 150

LOCAL TEMPERATURE ERROR

vs. DIE TEMPERATURE

MAX6680/81 toc05

FREQUENCY (Hz)

TEMPERATURE ERROR (°C)

1M10k100

0

0.2

0.4

0.6

0.8

1.0

1.2

-0.2
110M100k1k10 100M

TEMPERATURE ERROR

vs. POWER-SUPPLY NOISE FREQUENCY

VIN = 100mV SQUARE WAVE
APPLIED TO V

CC

WITH NO

0.1µF V

CC

CAPACITOR

LOCAL
DIODE

REMOTE
DIODE

MAX6680/81 toc06

FREQUENCY (Hz)

TEMPERATURE ERROR (°C)

10M1M100k10k1k10010

-1

0

1

2

3

4

5

-2
1 100M

TEMPERATURE ERROR

vs. COMMON-MODE NOISE FREQUENCY

VIN = 100mV

P-P

SQUARE WAVE

AC-COUPLED TO DXN

MAX6680/81 toc07

FREQUENCY (Hz)

TEMPERATURE ERROR (°C)

10M1M100k10k1k

0

1

2

3

-1

100 100M

TEMPERATURE ERROR

vs. DIFFERENTIAL NOISE FREQUENCY

VIN = 10mV

P-P

SQUARE WAVE

APPLIED TO DXP-DXN

MAX6680/81 toc08

DXP-DXN CAPACITANCE (nF)

TEMPERATURE ERROR (°C)

908070605040302010

-4

-3

-2

-1

0

1

-5
0100

TEMPERATURE ERROR

vs. DXP-DXN CAPACITANCE

MAX6680/MAX6681

±1°C Fail-Safe Remote/Local Temperature

Sensors with SMBus Interface

_______________________________________________________________________________________ 5

Pin Description

PIN

MAX6680 MAX6681

12V

2, 5 1, 5

3 3 DXP

44DXN

6 6 ADD1

7 7 RESET

8 8 GND Ground
99OVERT Overtemperature Active-Low Output. Open drain.

10 10 ADD0 SMBus Slave Address Select Pin (see ADD1).
11 11 ALERT SMBus Alert (Interrupt) Active-Low Output. Open drain.

12 12 SMBDATA SMBus Serial-Data Input/Output, Open Drain

13 13 INT_SEL

14 14 SMBCLK SMBus Serial-Clock Input

15 15 STBY

16 16 SENS_SEL

NAME FUNCTION

Supply Voltage Input, 3V to 5.5V. Bypass VCC to GND with a 0.1µF capacitor.

CC

CRIT1,

CRIT0

A 200Ω series resistor is recommended, but not required for additional noise
filtering. See the Typical Operating Circuit.

Hardware-Programmable Default Alarm Threshold for OVERT Limits. Use Table
4 to set default temperatures.

Combined Remote-Diode Current Source and A/D Positive Input for Remote­Diode Channel. DO NOT LEAVE DXP FLOATING; connect DXP to DXN if no
remote diode is used. Place a 2200pF capacitor between DXP and DXN for
noise filtering.

Combined Remote-Diode Current Sink and A/D Negative Input. DXN is
internally biased to one diode drop above ground.

SMBus Address Select Pin (Table 9). ADD0 and ADD1 are sampled upon
power-up. Excess capacitance (>50pF) at the address pins when floating may
cause address-recognition problems.

Reset Input. Drive RESET high to set all registers to their default values (POR
state). Drive RESET low or leave floating for normal operation.

Input. Connect high or leave floating to conform to the standard SMBus ALERT
protocol. See the
comparator mode, where ALERT is asserted whenever any of the temperature
conditions is violated by the remote sensor. In this mode, ALERT can only be
deasserted by the condition returning within the temperature limits by enabling
the mask bit in the Configuration register.

Input. Hardware Standby. Connect to ground to place in device in standby.
Supply current drops below 10µA and all registers’ data are maintained.

Input. Selects which temperature sensor (local, remote, or both) activates
OVERT.

High = Remote, Low = Local, Open = Local and Remote

ALERT

Interrupts section. Connect to GND to invoke

MAX6680/MAX6681

Detailed Description

The MAX6680/MAX6681 are temperature sensors designed
to work in conjunction with a microprocessor or other
intelligence in thermostatic, process-control, or monitoring
applications. Communication with the MAX6680/MAX6681
occurs through the SMBus serial interface and dedicated
alert pin. The overtemperature alarm OVERT is asserted if
the software or hardware programmed temperature thresh­olds are exceeded. OVERT can be connected to a fan,
system shutdown, or other thermal management circuitry.

The MAX6680/MAX6681 convert temperatures at a pro­grammed rate or a single conversion. Legacy mode
conversions have a 1°C resolution. Legacy resolution
represents temperature as 7 bits + sign bit and allows
for faster autonomous conversion rates at 8Hz. The
remote diode temperature can also be represented in
extended-resolution mode. Extended resolution repre-

sents temperature as 10 bits + sign bit and is available
for autonomous conversions that are 4Hz or slower and
single-shot conversions.

The MAX6680/MAX66681 default low-temperature mea­surement limit is 0 °C. The device temperature measure­ment can be placed in extended temperature range by
setting bit 3 of the Configuration register to 1. In extend­ed temperature range, the remote and local temperature
measurement range is extended down to -64°C.

ADC and Multiplexer

The averaging ADC integrates over a 60ms period
(each channel, typically, in the 7-bit + sign “legacy”
mode). Using an averaging ADC attains excellent noise
rejection.

The multiplexer automatically steers bias currents
through the remote and local diodes. The ADC and
associated circuitry measure each diode’s forward volt-

±1°C Fail-Safe Remote/Local Temperature
Sensors with SMBus Interface

6 _______________________________________________________________________________________

Figure 1. MAX6680/MAX6681 Functional Diagram

V

DXP

DXN

CC

RESET

CIRCUITRY

RESET

MUX

REMOTE

LOCAL

ADC

MAX6680
MAX6681

2

CONTROL

LOGIC

SENS_SEL

STBY

INT_SEL

DIODE

ALERT

FAULT

S

Q

R

OVERT

S

Q

R

CRIT0

CRIT1

SMBus

REGISTER BANK

COMMAND BYTE

REMOTE TEMPERATURE

LOCAL TEMPERATURE

ALERT THRESHOLD

ALERT RESPONSE ADDRESS

OVERT THRESHOLD (EXT)
OVERT THRESHOLD (INT)

8

READ

8

WRITE

7

ADDRESS
DECODER

SMBDATA

SMBCLK

ADD0

ADD1

age and computes the temperature based on this volt­age. If the remote channel is not used, connect DXP to
DXN. Do not leave DXP and DXN unconnected.
When a conversion is initiated, both channels are con­verted whether or not they are used. The DXN input is
biased at one VBEabove ground by an internal diode
to set up the ADC inputs for a differential measurement.
Resistance in series with the remote diode causes
about 1/2°C error per ohm.

A/D Conversion Sequence

A conversion sequence consists of a local temperature
measurement and a remote temperature measurement.
Each time a conversion begins, whether initiated auto­matically in the free-running autoconvert mode
(RUN/STOP = 0) or by writing a One-Shot command,
both channels are converted, and the results of both
measurements are available after the end of conver­sion. A BUSY status bit in the Status register shows that
the device is actually performing a new conversion. The
results of the previous conversion sequence are still
available when the ADC is busy.

Remote-Diode Selection

The MAX6680/MAX6681 can directly measure the die
temperature of CPUs and other ICs that have on-board
temperature-sensing diodes (see the Typical Operating
Circuit) or they can measure the temperature of a dis­crete diode-connected transistor. The type of remote
diode used is set by bit 5 of the Configuration Byte. If
bit 5 is set to zero, the remote sensor is a diode-con­nected transistor, and if bit 5 is set to 1, the remote sen­sor is a substrate or common-collector PNP transistor.
For best accuracy, the discrete transistor should be a
small-signal device with its collector and base connect­ed together. Accuracy has been experimentally verified
for all of the devices listed in Table 1.

The transistor must be a small-signal type with a rela­tively high forward voltage; otherwise, the A/D 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, 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 100Ω. Tight speci­fications for forward-current gain (50 < ß < 150, for
example) indicate that the manufacturer has good
process controls and that the devices have consistent
VBEcharacteristics.

Thermal Mass and Self-Heating

When sensing local temperature, these temperature
sensors are intended to measure the temperature of the
PC board to which they are soldered. The leads pro-

vide 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 measurements impractical. Because the ther­mal mass of the PC board is far greater than that of the
MAX6680/MAX6681, the device follows temperature
changes on the PC board with little or no perceivable
delay.

When measuring the 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
tracks the actual temperature within a conversion cycle.
When measuring temperature with discrete remote sen­sors, smaller packages (e.g., a SOT23) yield 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 measurement accuracy.

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 cur­rent at the ALERT output. For example, with VCC=

5.0V, an 8Hz conversion rate, and ALERT sinking 1mA,
the typical power dissipation is V

CC

✕

550µA + 0.4V

✕

1mA, which equals 2.75mW; θ

J-A

for the 16-pin QSOP
package is about +120°C/W, so assuming no copper
PC board heat sinking, the resulting temperature rise is:

Even under these engineered circumstances, it is diffi­cult to introduce significant self-heating errors.

ADC Noise Filtering

The integrating ADC used has good noise rejection for
low-frequency signals such as 60Hz/120Hz power-sup-

∆TmW CW C=×°=°2 75 120 0 330./.

MAX6680/MAX6681

±1°C Fail-Safe Remote/Local Temperature

Sensors with SMBus Interface

_______________________________________________________________________________________ 7

Table 1. Remote-Sensor Transistor
Manufacturers

Note: Transistors must be diode connected (base shorted to
collector).

MANUFACTURER MODEL NO.

Central Semiconductor (USA) CMPT3904

On Semiconductor (USA) 2N3904, 2N3906

Rohm Semiconductor (USA) SST3904

Samsung (Korea) KST3904-TF

Siemens (Germany) SMBT3904

Zetex (England) FMMT3904CT-ND

MAX6680/MAX6681

ply hum. In noisy environments, high-frequency noise
reduction is needed for high-accuracy remote mea­surements. 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.

PC Board Layout

Follow these guidelines to reduce the measurement
error of the temperature sensors:

1) Place the MAX6680/MAX6681 as close as is practi­cal to the remote 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.

2) Do not route the DXP-DXN lines next to the deflec­tion coils of a CRT. Also, do not route the traces
across fast digital signals, which can easily intro­duce 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 2).

4) Route through as few vias and crossunders as pos­sible 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 negligi­ble 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 2 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 V

CC

for best noise

filtering (see the 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 micro­phones. For example, Belden 8451 works well for dis­tances 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 approxi­mately 1/2°C error.

Low-Power Standby Mode

Standby mode reduces the supply current to less than
10µA by disabling the ADC. Enter hardware standby 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 memory, and the SMB inter­face is alive and listening for SMBus commands, but
the SMBus timeout is disabled. The only difference is
that in software standby mode, the One-Shot command
initiates a conversion. With hardware standby, the One­Shot command is ignored. Activity on the SMBus caus­es the device to draw extra supply current (see the
Typical Operating Characteristics).

Driving the STBY pin low overrides any software con­version command. If a hardware or software standby
command is received while a conversion is in progress,
the conversion cycle is interrupted, and the tempera-

±1°C Fail-Safe Remote/Local Temperature
Sensors with SMBus Interface

8 _______________________________________________________________________________________

Figure 2. Recommended DXP-DXN PC Traces

GND

10mils

10mils

10mils

DXP

MINIMUM

DXN

10mils

GND

ture registers are not updated. The previous data is not
changed and remains available.

SMBus Digital Interface

From a software perspective, the MAX6680/MAX6681
appear as a series of 8-bit registers that contain tem­perature data, alarm threshold values, and control bits.
A standard SMBus-compatible 2-wire serial interface is
used to read temperature data and write control bits
and alarm threshold data. The device responds to the
same SMBus slave address for access to all functions.

The MAX6680/MAX6681 employ four standard SMBus
protocols: Write Byte, Read Byte, Send Byte, and
Receive Byte (Figure 3). The shorter Receive Byte pro­tocol allows quicker transfers, provided that the correct
data register was previously selected by a Read Byte
instruction. Use caution with the shorter protocols in
multimaster systems, since a second master could
overwrite the command byte without informing the first
master.

When the conversion rate is 8Hz, temperature data can
be read from the Read Internal Temperature (00h) and
Read External Temperature (01h) registers. The tem-

perature data format in these registers is 7 bits + sign
in two’s-complement form for each channel, with the
LSB representing 1°C (Table 2). The MSB is transmitted
first. Extended range extends the temperature data
range of the local and remote sensor to -64°C. Extended
range is activated by setting bit 3 of the Configuration
register to 1.

When the conversion rate is 4Hz or less, temperature
data can be read from the Read Internal Temperature
(00h) and Read External Temperature (01h) registers,
the same as for faster conversion rates. An additional 3
bits can be read from the Read External Extended
Temperature (10h), which extends the remote tempera­ture data to 10 bits + sign and the resolution to 0.125°C
per LSB (Table 3).

When a conversion is complete, the Main register and
the Extended register are updated almost simultane­ously. Ensure that no conversions are completed
between reading the Main and Extended registers so
that when data that is read by both registers contain
the result of the same conversion.

MAX6680/MAX6681

±1°C Fail-Safe Remote/Local Temperature

Sensors with SMBus Interface

_______________________________________________________________________________________ 9

ACK

7 bits

ADDRESS ACKWR

8 bits

DATA ACK

1

P

8 bits

S COMMAND

Write Byte Format

Read Byte Format

Send Byte Format Receive Byte Format

Slave Address: equiva­lent to chip-select line of
a 3-wire interface

Command Byte: selects which
register you are writing to

Data Byte: data goes into the register
set by the command byte (to set
thresholds, configuration masks, and
sampling rate)

ACK

7 bits

ADDRESS ACKWR S ACK

8 bits

DATA

7 bits

ADDRESS RD

8 bits

/// PCOMMAND

Slave Address: equiva­lent to chip-select line

Command Byte: selects
which register you are
reading from

Slave Address: repeated
due to change in data­flow direction

Data Byte: reads from
the register set by the
command byte

ACK

7 bits

ADDRESS WR

8 bits

COMMAND ACK P ACK

7 bits

ADDRESS RD

8 bits

DATA /// PS

Command Byte: sends com­mand with no data, usually
used for one-shot command

Data Byte: reads data from
the register commanded
by the last Read Byte or
Write Byte transmission;
also used for SMBus Alert
Response return address

S = Start condition Shaded = Slave transmission
P = Stop condition /// = Not acknowledged

Figure 3. SMBus Protocols

MAX6680/MAX6681

To ensure valid extended data, read extended resolu­tion temperature data using one of the following
approaches:

1) Put the MAX6680/MAX6681 into standby mode by
setting bit 6 of the Configuration register to 1. Initiate
a one-shot conversion using Send Byte command
0Fh. When this conversion is complete, read the
contents of the temperature data registers.

2) If the MAX6680/MAX6681 are in run mode, read the
Status register. If a conversion is in progress, the
BUSY bit is set to 1. Wait for the conversion to com­plete as indicated by the BUSY bit being set to zero,
then read the temperature data registers.

Diode Fault Alarm

There is a continuity fault detector at DXP that detects
an open circuit between DXP and DXN, or a DXP short
to VCC, GND, or DXN. If an open or short circuit exists,
the External Temperature register is loaded with 1000

0000. Additionally, if the fault is an open circuit, bit 2

(OPEN) of the Status byte is set to 1 and the ALERT
condition is activated at the end of the conversion.
Immediately after power-on reset, the Status register
indicates that no fault is present until the end of the first
conversion.

Alarm Threshold Registers

Four registers store ALERT threshold values—one high­temperature (T

HIGH

) and one low-temperature (T

LOW

)
register each for the local and remote channels. If
either measured temperature equals or exceeds the
corresponding ALERT threshold value, the ALERT out­put is asserted.

The POR state of both ALERT T

HIGH

registers is 0111

1111 or +127°C and the POR state of T

LOW

registers is

1100 1001 or -55°C.

Two additional registers, RWOE and RWOI, store
remote and local alarm threshold data information cor­responding to the OVERT output (see the

OVERT

Overtemperature Alarm section).

ALERT

The ALERT output operates in two modes—the typical
interrupt mode and comparator mode. The INT_SEL
input determines the mode. When INT_SEL is connect­ed to VCChigh, using a weak pullup resistor, or left
floating, the ALERT functions in the interrupt mode.

ALERT

Interrupt Mode

An ALERT interrupt occurs when the internal or external
temperature reading exceeds a high or low tempera­ture limit (user programmed) or when the remote diode
is disconnected (for continuity fault detection). The

ALERT interrupt output signal is latched and can be
cleared only by either reading the Status register or by
successfully responding to an Alert Response address.
In both cases, the alert is cleared even if the fault con­dition still exists, but is reasserted at the end of the next
conversion. The interrupt does not halt automatic con­versions. The interrupt output pin is open drain so that
multiple devices can share a common interrupt line.
The interrupt rate never exceeds the conversion rate.

Comparator Mode

Connecting INT_SEL to ground operates the ALERT
output in comparator mode. In the comparator mode,
whenever the temperature of the remote or local temp
sensor goes outside the limits set by T

HIGH

or T

LOW

,

the ALERT output becomes inactive after the tempera-

±1°C Fail-Safe Remote/Local Temperature
Sensors with SMBus Interface

10 ______________________________________________________________________________________

Table 2. Data Format (Two’s Complement)

FRACTIONAL

TEMPERATURE

CONTENTS OF

EXTENDED REGISTER

0.000 000X XXXX

0.125 001X XXXX

0.250 010X XXXX

0.375 011X XXXX

0.500 100X XXXX

0.625 101X XXXX

0.750 110X XXXX

0.875 111X XXXX

Table 3. Extended Resolution Register

Note: Extended mode applies only for conversion rates of 4Hz
and slower.

TEMP (°C)

127.00 0111 1111 0111 1111

25 0001 1001 0001 1001

1 0000 0001 0000 0001

0.00 0000 0000 0000 0000

-1 0000 0000 1111 1111

-25 0000 0000 1110 0111

-64 0000 0000 1000 0000

Diode Fault

(Short or

Open)

LEGACY MODE

DIGITAL OUTPUT

1000 0000 1000 0000

EXTENDED

DIGITAL OUTPUT

RANGE

ture returns within the limits. An open diode also sets
this output.

Alert Response Address

The SMBus Alert Response interrupt pointer provides
quick fault identification for simple slave devices that
lack the complex, expensive logic needed to be a bus
master. Upon receiving an ALERT interrupt signal, the
host master can broadcast a Receive Byte transmission
to the Alert Response slave address (see the Slave

Addresses section). Then, any slave device that gener­ated an interrupt, attempts to identify itself by putting its
own address on the bus (Table 4).

The Alert Response can activate several different slave
devices simultaneously, similar to the I2C General Call.
If more than one slave attempts to respond, bus arbitra­tion rules apply, and the device with the lower address
code wins. The losing device does not generate an
acknowledge and continues to hold the ALERT line low
until cleared. (The conditions for clearing an alert vary

MAX6680/MAX6681

±1°C Fail-Safe Remote/Local Temperature

Sensors with SMBus Interface

______________________________________________________________________________________ 11

Figure 4. SMBus Write Timing Diagram

SMBCLK

AB CDEFG HIJ

K

SMBDATA

t

SU:STA

t

HD:STA

t

LOWtHIGH

t

SU:DAT

t

HD:DAT

t

SU:STO

t

BUF

L

M

A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
E = SLAVE PULLS SMBDATA LINE LOW

F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO MASTER
H = LSB OF DATA CLOCKED INTO MASTER
I = MASTER PULLS DATA LINE LOW

J = ACKNOWLEDGE CLOCKED INTO SLAVE
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION
M = NEW START CONDITION

Figure 5. SMBus Read Timing Diagram

AB CDEFG HIJ

t

LOWtHIGH

SMBCLK

SMBDATA

t

t

HD:STA

SU:STA

A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
E = SLAVE PULLS SMBDATA LINE LOW

t

SU:DAT

F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO SLAVE
H = LSB OF DATA CLOCKED INTO SLAVE
I = MASTER PULLS DATA LINE LOW

t

HD:DAT

K

t

SU:STO

J = ACKNOWLEDGE CLOCKED INTO SLAVE
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION
M = NEW START CONDITION

M

L

t

BUF

MAX6680/MAX6681

depending on the type of slave device.) Successful
completion of the Alert Response protocol clears the
interrupt latch, provided the condition that caused the
alert no longer exists. If the condition still exists, the
device reasserts the ALERT interrupt at the end of the
next conversion.

OVERT

Overtemperature Alarm

Two registers, RWOE and RWOI, store remote and local
alarm threshold data corresponding to the OVERT out­put. The values stored in these registers are high-tem­perature thresholds. If any one of the measured
temperatures equals or exceeds the corresponding
alarm threshold value, an OVERT output is asserted.

The overtemperature thresholds are both hardware and
software programmable. The overtemperature thresh­olds can be hardware programmed by pin strapping
CRIT0 and CRIT1. Use Table 4 to set the desired
remote and local threshold temperatures. Upon POR or
driving the RESET pin high, the Overtemperature regis­ter takes on the hardware-programmed values.
Afterward, any write to the Overtemperature registers
overwrites the hardware-programmable values.

OVERT always operates in comparator mode and is
asserted when the temperature rises to a value pro­grammed in the appropriate threshold register. It is
deasserted when the temperature drops below this
threshold minus the programmed value in the Hysteresis
(HYST) register. An OVERT output can be used to acti­vate a cooling fan, send a warning, initiate clock throt­tling, or trigger a system shutdown to prevent component
damage. The HYST byte sets the amount of hysteresis
to deassert the OVERT output. The data format for the
HYST byte is 7 bits + sign with 1°C resolution. Bit 7 of
the HYST register should always be zero.

Command Byte Functions

The 8-bit Command Byte register (Table 5) is the mas­ter index that points to the various other registers within
the MAX6680/MAX6681. This register’s POR state is
0000 0000, so a Receive Byte transmission (a protocol
that lacks the command byte) occurring immediately
after POR returns the current local temperature data.

One Shot

The One-Shot command immediately forces a new con­version cycle to begin. If the One-Shot command is
received when the MAX6680/MAX6681 is in software
standby mode (RUN/STOP bit = 1), a new conversion is
begun, after which the device returns to standby mode.
If a conversion is in progress when a One-Shot com­mand is received, the command is ignored. If a One-Shot
command is received in autoconvert mode (RUN/STOP

bit = 0) between conversions, a new conversion
begins, the conversion rate timer is reset, and the next
automatic conversion takes place after a full delay
elapses.

Configuration Byte Functions

The Configuration Byte register, Table 6, is a read-write
register with several functions. Bit 7 is used to mask
(disable) ALERT interrupts. Bit 6 puts the device into
software standby mode (STOP) or autonomous (RUN)
mode. Bit 5 selects the type of external junction (set to
0 for a substrate PNP on an IC or set to 1 for a discrete
diode-connected transistor) for optimized measure­ments. Bit 4 selects the extended temperature mea­surement for the remote sensor. If high, the temperature
data is available as 10 bits + sign with a 0.125°C reso­lution, otherwise, 7 bits + sign with 1°C resolution. Bit 4
extends the temperature range of the remote and local
temperature sensor to -64°C. Bit 2 disables the SMBus
timeout, as well as the Alert Response. Bit 1 provides a
software reset from the SMBus. Bit 0 is reserved and
returns a zero when read.

Status Byte Functions

The status byte (Table 7) indicates which (if any) tem­perature thresholds have been exceeded. This byte
also indicates whether the ADC is converting and if
there is an open-circuit fault detected with the external
sense junction. After POR, the normal state of the regis­ters’ bits is zero, assuming no alert or overtemperature
conditions are present. When operating the
MAX6680/MAX6681 in ALERT interrupt mode, bits 2
through 6 of the Status register are cleared by any suc­cessful read of the Status register, unless the fault per­sists. The ALERT output follows the status flag bit. Both
are cleared when successfully read, but if the condition

±1°C Fail-Safe Remote/Local Temperature
Sensors with SMBus Interface

12 ______________________________________________________________________________________

Table 4. OVERT Temperature Threshold
Programming

CRIT1 CRIT0

GND GND +85 +70

GND Open +90 +75

GND V

Open GND +100 +85

Open Open +105 +90

Open V

V

CC

V

CC

V

CC

CC

CC

GND +115 +100

Open +120 +105

V

CC

OVERT THRESHOLD (°C)

REMOTE LOCAL

+95 +80

+110 +95

+125 +110

still exists, they are reasserted at the end of the next
conversion. If the MAX6680/MAX6681 are operating in
the comparator mode, bits 2–6 of the Status register
are cleared only after the local and/or remote tempera­tures return within the set limits.

The bits indicating OVTI and OVTE are cleared only
when the condition no longer exists. Reading the status
byte does not clear the OVERT output or fault bits. One
way to eliminate the fault condition is for the measured
temperature to drop below the temperature threshold
minus the hysteresis value. Another way to eliminate
the fault condition is by writing new values for the
RWOI, RWOE, or HYST registers so that a fault condi­tion is no longer present.

The MAX6680/MAX6681 incorporate collision avoid­ance so that completely asynchronous operation is
allowed between SMBus operations and temperature
conversions.

When autoconverting, if the T

HIGH

and T

LOW

limits are
close together, it is possible for both high-temp and
low-temp status bits to be set, depending on the
amount of time between status read operations. In
these circumstances, it is best not to rely on the status
bits to indicate reversals in long-term temperature
changes. Instead use a current temperature reading to
establish the trend direction.

Hardware/Software Reset

The MAX6680/MAX6681 reset at power-on if pin 7 is
taken high, or by software reset through bit 1 of the
Configuration register. When reset occurs, all registers
go to default values, and the SMBus address pins are
sampled.

Conversion Rate Byte

The Conversion Rate register (Table 8) programs the
time interval between conversions in free-running

MAX6680/MAX6681

±1°C Fail-Safe Remote/Local Temperature

Sensors with SMBus Interface

______________________________________________________________________________________ 13

Table 5. Command-Byte Register Bit Assignments

REGISTER ADDRESS POR STATE FUNCTION

RLTS 00h

RRTE 01h

RSL 02h 0000 0000 Read Status Register

RCL/WCL 03h/09h 0010 0000 Read/Write Configuration Byte

RCRA/WCRA 04h/0A 0000 0010 Read/Write Conversion Rate Byte

RIH/WIH 05h/0Bh

RIL/WIL 06h/0Ch

REH/WEH 07h/0Dh

REL/WEL 08h/0Eh

OSHT 0Fh 0000 One Shot

REET 10h 0000 0000 Read External Extended Temperature

RWOH 11h 0000 0000 Read/Write External Offset High Byte

RWOL 12h 0000 0000 Read/Write External Offset Low Byte
RWOE 19h See Table 4 Read/Write External OVERT Limit

RWOI 20h See Table 4 Read/Write Internal OVERT Limit

HYST 21h

RDID FEh 0100 1101 Read Manufacturer ID

RDRV Ff 0000 0001 Read Device Revision

0000

at 0°C

0000
(0°C)

0111 1111

(+127°C)

1100 1001

( -55°C)

0100 0110

(+127°C)

1100 1001

(-55°C)

0000 0110

(+6°C)

Read Internal Temperature

Read External Temperature

Read/Write Internal ALERT High Limit

Read/Write Internal ALERT Low Limit

Read/Write External ALERT High Limit

Read/Write External ALERT Low Limit

OVERT Hysteresis

MAX6680/MAX6681

autonomous mode (RUN/STOP = 0). This variable rate
control can be used to reduce the supply current in
portable-equipment applications. The conversion rate
byte’s POR state is 02h (0.25Hz). The MAX6680/
MAX6681 use only the 3LSBs of this register. The
5MSBs are “don’t care” and should be set to zero when
possible. The conversion rate tolerance is ±25% at any
rate setting.

Valid A/D conversion results for both channels are avail­able one total conversion time (125ms nominal, 156ms
maximum) after initiating a conversion, whether conver­sion is initiated through the RUN/STOP bit, hardware
STBY pin, One-Shot command, or initial power-up.

Slave Addresses

The MAX6680/MAX6681 device address can be initially
set to nine different values by pin strapping ADD0 and
ADD1 so that more than one MAX6680/MAX6681 can

±1°C Fail-Safe Remote/Local Temperature
Sensors with SMBus Interface

14 ______________________________________________________________________________________

Table 6. Configuration-Byte Bit Assignment

BIT NAME

POR

STATE

FUNCTION

7 (MSB) BUSY 0 When 1, the A/D is busy converting.

6 LHIGH 0

When 1, internal high-temperature alarm has tripped; cleared by POR or readout
of the Status register, if the fault condition no longer exists.

5 LLOW 0

When 1, internal low-temperature alarm has tripped; cleared by POR or readout of
the Status register, if the fault condition no longer exists.

4 RHIGH 0

When 1, external high-temperature alarm has tripped; cleared by POR or readout
of the Status register, if the fault condition no longer exists.

3 RLOW 0

When 1, external low-temperature alarm has tripped; cleared by POR or readout of
the Status register if the fault condition no longer exists.

2 OPEN 0

When 1 indicates an external diode open; cleared by POR or readout of the Status
register, if the fault condition no longer exists.

1 OVI 0 When 1, internal temperature exceeds the RWOI limit.

0 OVE 0 When 1, external temperature exceeds the RWOE limit.

Table 7. Status Register Bit Assignments

BIT NAME POR STATE FUNCTION

7 (MSB) ALERT MASK 0

6 RUN/STOP 0

5 SPNP 1

4 Extended Resolution 0

3 Extended Range 0 Extended temperature range. 0 = normal, 1 = extended to -64°C.

2 SMBus Timeout 0

1 Software Reset 0 Software reset from SMBus from customer.

0 RFU 0 Reserved

Mask ALERT active state when 1. When 1, ALERT does not respond to
any fault related to the four limit registers.

Standby mode control bit; if 1, immediately stops converting and enters
standby mode. If zero, it converts in either one-shot or timer mode.

When 1, the remote sensor is a common-collector substrate PNP. When
zero, the remote sensor is a diode-connected transistor.

When zero, remote- and local-sensors’ temperature data are 7 bits +
sign with 1°C resolution. When 1, the remote-sensor temperature data is
10 bits + sign with 0.125°C resolution.

When set to 1, it disables the SMBus timeout, as well as the alert
response.

reside on the same bus without address conflicts
(Table 9).

The address pin states are checked at POR and RESET
only, and the address data stays latched to reduce qui­escent supply current due to the bias current needed
for high-Z state detection. The MAX6680/MAX6681 also
respond to the SMBus Alert Response slave address
(see the Alert Response Address section).

POR and UVLO

The MAX6680/MAX6681 have a volatile memory. To
prevent unreliable power-supply conditions from cor­rupting the data in memory and causing erratic behav­ior, a POR voltage detector monitors VCCand clears the
memory if VCCfalls below 1.91V (typ, see Electrical
Characteristics). When power is first applied and V

CC

rises above 2.0V (typ), the logic blocks begin operating,
although reads and writes at VCClevels below 3.0V are
not recommended. A second VCCcomparator, the ADC
UVLO comparator, prevents the ADC from converting
until there is sufficient headroom (VCC= 2.8V typ).

Power-Up Defaults

• Interrupt latch is cleared.

• Address select pin is sampled.

• ADC begins autoconverting at a 1Hz rate (legacy
resolution).

• Command register is set to 00h to facilitate quick
internal Receive Byte queries.

•T

HIGH

and T

LOW

registers are set to max and min

limits, respectively.

• Hysteresis is set to 6°C.

• Transistor type is set to a substrate or common-col­lector PNP.

Temperature Offset

The MAX6680/MAX6681 are designed to provide ±1°C
accuracy for common microprocessors and discrete
transistors. To accommodate processes that differ sig­nificantly in their ideality factor, the user can
increase/decrease the Remote Temperature Sensor
Data register with an offset by writing to the External
Offset High and Low Byte registers (11h and 12h,
respectively). The offset temperature data is represent­ed as a 10 bits + sign with a 0.125LSB resolution.

MAX6680/MAX6681

±1°C Fail-Safe Remote/Local Temperature

Sensors with SMBus Interface

______________________________________________________________________________________ 15

Table 8. Conversion-Rate Control Byte

Note: If extended resolution is selected using bit 4 of the
Configuration register, the extended conversion is limited to a
maximum of 4Hz.

ADD0 ADD1 ADDRESS

GND GND 0011 000
GND HIGH-Z 0011 001
GND V

CC

0011 010
HIGH-Z GND 0101 001
HIGH-Z HIGH-Z 0101 010
HIGH-Z V

CC

0101 011

V

CC

GND 1001 100

V

CC

HIGH-Z 1001 101

V

CC

V

CC

1001 110

Table 9. POR Slave Address Decoding
(ADD0 and ADD1)

Chip Information

TRANSISTOR COUNT: 17,150

PROCESS: BiCMOS

DATA CONVERSION RATE (Hz)

00h 0.0625

01h 0.125

02h 0.25

03h 0.5

04h 1

05h 2

06h 4

07h 8

MAX6680/MAX6681

±1°C Fail-Safe Remote/Local Temperature
Sensors with SMBus Interface

16 ______________________________________________________________________________________

Pin Configurations

TOP VIEW

V

CRIT1

DXP

DXN

CRIT0

ADD1

RESET

GND

1

CC

2

3

MAX6680

4

5

6

7

8

16

15

14

13

12

11

10

9

SENS_SEL

STBY

SMBCLK

INT_SEL

SMBDATA

ALERT

ADDO

OVERT

CRIT1

V

DXP

DXN

CRIT0

ADD1

RESET

GND

1

2

CC

3

MAX6681

4

5

6

7

8

16

15

14

13

12

11

10

9

SENS_SEL

STBY

SMBCLK

INT_SEL

SMBDATA

ALERT

ADDO

OVERT

QSOP

QSOP

MAX6680/MAX6681

±1°C Fail-Safe Remote/Local Temperature

Sensors with SMBus Interface

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 17

© 2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.

Package Information

(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)

QSOP.EPS

PACKAGE OUTLINE, QSOP .150", .025" LEAD PITCH

21-0055

1

E

1