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MAX1618MUB-MAX1618MUB+
Remote Temperature Sensor with SMBus Serial Interface
________________General DescriptionThe MAX1618 precise digital thermometer reports the
temperature of a remote sensor. The remote sensor is a
diode-connected transistor—typically a low-cost, easily
mounted 2N3904 NPN type—that replaces conventional
thermistors or thermocouples. Remote accuracy is ±3°C
for multiple transistor manufacturers, with no calibration
needed. The MAX1618 can also measure the die temper-
ature of other ICs, such as microprocessors, that contain
an on-chip, diode-connected transistor.
The 2-wire serial interface accepts standard System
Management Bus (SMBus™) Write Byte, Read Byte, Send
Byte, and Receive Byte commands to program the alarm
thresholds and to read temperature data. The data format
is 7 bits plus sign, with each bit corresponding to 1°C, in
two’s complement format. Measurements can be done
automatically and autonomously, with the 16Hz conversion
rate or programmed to operate in a single-shot mode.
The thermostat mode configures the ALERToutput as an
interrupt or as a temperature reset that remains active only
while the temperature is above the maximum temperature
limit or below the minimum temperature limit. The ALERT
output polarity in thermostat mode can be configured for
active high or active low. Fan control is implemented using
this ALERToutput.
The MAX1618 is available in a small (1.1mm high) 10-pin
µMAX package.
________________________ApplicationsDesktop and NotebookCentral Office
ComputersTelecom Equipment
Smart Battery PacksTest and Measurement
LAN ServersMultichip Modules
Industrial Controls
____________________________FeaturesSingle Channel: Measures Remote CPU
TemperatureNo Calibration RequiredSMBus 2-Wire Serial InterfaceProgrammable Under/Overtemperature AlarmsOvertemperature Output for Fan Control
(Thermostat Mode)Supports SMBus Alert Response Accuracy
±3°C (+60°C to +100°C)
±5°C (-55°C to +120°C)3µA (typ) Standby Supply Current900µA (max) Supply Current in Autoconvert Mode+3V to +5.5V Supply RangeSmall 10-Pin µMAX Package
MAX1618
Remote Temperature Sensor
with SMBus Serial Interface
___________________Pin Configurationypical Operating Circuit19-1495; Rev 1; 10/99
SMBus is a trademark of Intel Corp.
MAX1618
Remote Temperature Sensor
with SMBus Serial Interface
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VCC= +3.3V, configuration byte register = X8h, TA
= 0°C to +85°C, unless otherwise noted.) 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.
VCCto GND..............................................................-0.3V to +6V
DXP, ADD_ to GND....................................-0.3V to (VCC+ 0.3V)
DXN to GND..........................................................-0.3V to +0.8V
SMBCLK, SMBDATA, ALERT, STBYto GND...........-0.3V to +6V
SMBDATA Current.................................................-1mA to 50mA
DXN Current.......................................................................±1mA
ESD Protection (all pins, Human Body Model)...............±2000V
Continuous Power Dissipation (TA= +70°C)
µMAX (derate 5.6mW/°C above +70°C)....................444mW
Operating Temperature Range (extended)......-55°C to +125°C
Junction Temperature.....................................................+150°C
Storage Temperature Range............................-65°C to +150°C
Lead Temperature (soldering, 10sec)............................+300°C
MAX1618
Remote Temperature Sensor
with SMBus Serial Interface
ELECTRICAL CHARACTERISTICS (continued)(VCC= +3.3V, configuration byte register = X8h, TA
= 0°C to +85°C, unless otherwise noted.)
ELECTRICAL CHARACTERISTICS (VCC= +3.3V, configuration byte register = X8h, TA
= -55°C to +125°C, unless otherwise noted.) (Note 5)
Typical Operating Characteristics
(TA = +25°C, unless otherwise noted.)
MAX1618
Remote Temperature Sensor
with SMBus Serial Interface
ELECTRICAL CHARACTERISTICS (continued)
(VCC= +3.3V, configuration byte register = X8h, TA= -55°C to +125°C, unless otherwise noted.) (Note 5)
Note 1:Guaranteed, but not 100% tested.
Note 2:A remote diode is any diode-connected transistor from Table 7. TRis the junction temperature of the remote diode. See
Remote Diode Selectionfor remote-diode forward voltage requirements. Temperature specification guaranteed for a diode
with ideality factor (MTR= 1.013). Additional error = (1.013/m - 1) ·273 + Temp where M = Ideality of remote diode used.
Note 3:The SMBus logic block is a static design that works with clock frequencies down to DC. While slow operation is possible, it
violates the 10kHz minimum clock frequency and SMBus specifications and may monopolize the bus.
Note 4:Note that a transition must internally provide at least a hold time to bridge the undefined region (300ns max) of SMBCLK’s
falling edge.
Note 5:Specifications from -55°C to +125°C are guaranteed by design, not production tested.
MAX1618
Remote Temperature Sensor
with SMBus Serial Interface1001000
TEMPERATURE ERROR vs.
COMMON-MODE NOISE FREQUENCY
MAX1618 toc04
COMMON-MODE NOISE FREQUENCY (MHz)
TEMPERATURE ERROR (
STANDBY SUPPLY CURRENT
vs. SUPPLY VOLTAGEMAX1618 toc07
SUPPLY VOLTAGE (V)
STANDBY SUPPLY CURRENT (
TEMPERATURE ERROR
vs. DXP-DXN CAPACITANCE
MAX1618 toc05
DXP-DXN CAPACITANCE (nF)
TEMPERATURE ERROR (101001000
STANDBY SUPPLY CURRENT
vs. CLOCK FREQUENCYMAX1618 toc06
CLOCK FREQUENCY (kHz)
STANDBY SUPPLY CURRENT (
RESPONSE TO THERMAL SHOCK
MAX1618 toc08
TIME (sec)
TEMPERATURE (
°C)
____________________________Typical Operating Characteristics (continued)(TA = +25°C, unless otherwise noted.)
MAX1618
Remote Temperature Sensor
with SMBus Serial Interface
Pin Description
Detailed DescriptionThe MAX1618 is a temperature sensor designed to
work in conjunction with an external microcontroller
(µC) or other intelligence in thermostatic, process-con-
trol, or monitoring applications. The µC is typically a
power-management or keyboard controller, generating
SMBus serial commands by “bit-banging” general-pur-
pose input-output (GPIO) pins or through a dedicated
SMBus interface block.
Essentially an 8-bit serial analog-to-digital converter
(ADC) with a sophisticated front end, the MAX1618
contains a switched-current source, a multiplexer, an
ADC, an SMBus interface, and the associated control
logic (Figure 1). Temperature data from the ADC is
loaded into a data register, where it is automatically
compared with data previously stored in over/under-
temperature alarm threshold registers. The alarm
threshold registers can be set for hysteretic fan control.
ADC and Multiplexer The averaging ADC integrates over a 30ms period (typ)
with excellent noise rejection. The ADC converts at a
rate of 16Hz. The multiplexer automatically steers bias
currents through the remote diode, measures the for-
ward voltage, and computes the temperature.
The DXN input is biased at 0.65V above ground by an
internal diode to set up the analog-to-digital (A/D)
inputs for a differential measurement. The worst-case
DXP-DXN differential input voltage range is 0.25V to
0.95V.
Excess resistance in series with the remote diode
causes about +1/2°C error/Ω. A 200µV offset voltage at
DXP-DXN causes about +1°C error.
A/D Conversion SequenceIf a Start command is written (or generated automatical-
ly in the free-running autoconvert mode), the result of
the measurement is available after the end of conver-
sion. A BUSY status bit in the status byte shows that the
device is performing a new conversion. The result of the
previous conversion is always available even when the
ADC is busy.
MAX1618
Remote Temperature Sensor
with SMBus Serial InterfaceFigure 1. Functional Diagram
Low-Power Standby ModeStandby mode disables the ADC and reduces the sup-
ply-current drain to 3µA (typ). Enter standby mode by
forcing the STBYpin low or via 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. Activity on the
SMBus draws extra supply current (see Typical
Operating Characteristics). In software standby mode,
the MAX1618 can be forced to perform A/D conversions
through the one-shot command, despite the RUN/STOP
bit being high.
Enter hardware standby mode by forcing the STBYpin
low. In a notebook computer, this line may be connect-
ed to the system SUSTAT# suspend-state signal.
The STBYpin 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 62ms conversion period
is always about 450µA. Between conversions, the
instantaneous supply current is about 25µA due to the
current consumed by the conversion rate timer. In
standby mode, supply current drops to about 3µA. With
very low supply voltages (under the power-on reset
threshold), the supply current is higher due to the
address input bias currents. It can be as high as 160µA,
depending on ADD0 and ADD1 settings.
SMBus Digital InterfaceFrom a software perspective, the MAX1618 appears as a
set of byte-wide registers that contains temperature data,
alarm threshold values, or control bits. Use a standard
SMBus 2-wire serial interface to read temperature data
and write control bits and alarm threshold data.
The MAX1618 employs four standard SMBus protocols:
Write Byte, Read Byte, Send Byte, and Receive Byte
(Figure 2). The two shorter protocols (Receive and Send)
allow quicker transfers, provided that the correct data
register was previously selected by a Write or 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.
The temperature data format is 7 bits plus sign in two’s
complement form for each channel, with each data bit
representing +1°C (Table 1), transmitted MSB first.
Measurements are offset by +1/2°C to minimize internal
rounding errors; for example, +99.6°C is reported as
+100°C.
Alarm Threshold Registers Two registers, a high-temperature (THIGH) and a low-
temperature (TLOW) register, store alarm threshold
data. If a measured temperature equals or exceeds the
corresponding alarm threshold value, an ALERTinter-
rupt is asserted.
The power-on reset (POR) state of the THIGHregister is
full scale (0111 1111 or +127°C). The POR state of the
TLOWregister is 1100 1001 or -55°C.
Thermostat ModeThermostat mode changes the function of the ALERT
output from a latched interrupt-type output to a self-
clearing thermostat for fan control. This output simply
responds to the current temperature (Figure 3). If the
current temperature is above THIGH, ALERTactivates
MAX1618
Remote Temperature Sensor
with SMBus Serial Interface
Table 1. Data Format (Two’s Complement)
and does not go inactive until the temperature drops
below TLOW.
Enable thermostat mode through the configuration reg-
ister (Table 4), with one bit to enable the feature and
another bit to set the output polarity (active high or
active low). The ALERTthermostat comparison is made
after each conversion, or at the end of any SMBus
transaction. For example, if the limit is changed while
the device is in standby mode, the ALERToutput
responds correctly according to the last valid A/D
result. Upon entering thermostat mode, the ALERT out-
put is reset so that if the temperature is in the hysteresis
band ALERTinitially goes inactive. The power-on reset
(POR) state disables thermostat mode.
Diode Fault AlarmA continuity fault detector at DXP detects whether the
remote diode has an open-circuit condition, short-cir-
cuit to GND, or short-circuit DXP-to-DXN condition. At
the beginning of each conversion, the diode fault is
checked, and the status byte is updated. This fault
detector is a simple voltage detector; if DXP rises
MAX1618
Remote Temperature Sensor
with SMBus Serial InterfaceFigure 3. Fan Control Application
Figure 2. SMBus Protocols
above VCC- 1V (typ) or below VDXN+ 50mv (typ), a
fault is detected and ALERTis asserted. ADC reads
+127°C. Also, if the ADC has an extremely low differen-
tial input voltage, the diode is assumed to be shorted
and a fault is detected. Note that the diode fault is not
checked until a conversion is initiated, so immediately
after power-on reset, the status byte indicates no fault
is present even if the diode path is broken.ALLEERRTT
InterruptsNormally, the ALERTinterrupt output signal is latched
and can be cleared only by reading the Alert Response
address. Interrupts are generated in response to THIGH
and TLOWcomparisons and when the remote diode is
faulted. The interrupt does not halt automatic conver-
sions; new temperature data continues to be available
over the SMBus interface after ALERTis asserted. The
interrupt output pin is open-drain so the devices can
share a common interrupt line.
The interface responds to the SMBus Alert Response
address, an interrupt pointer return-address feature
(see Alert Response Addresssection). Before taking
corrective action, always check to ensure that an inter-
rupt is valid by reading the current temperature.
The alert activates only once per crossing of a given
temperature threshold to prevent any re-entrant inter-
rupts. To enable a new interrupt, rewrite the value of the
violated temperature threshold.
Alert Response AddressThe 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 ALERTinterrupt signal, the
host master can broadcast a Receive Byte transmission
to the Alert Response slave address (0001100). Any
slave device that generated an interrupt then attempts
to identify itself by putting its own address on the bus
(Table 2).
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 arbitra-
tion rules apply, and the device with the lower address
code wins. The losing device does not generate an
acknowledgement and continues to hold the ALERT
line low until serviced (implies that the host interrupt
input is level sensitive). Successful reading of the alert
response address clears the interrupt latch.
Command Byte FunctionsThe 8-bit command byte register (Table 3) is the master
index that points to the other registers within the
MAX1618. The register’s POR state is 0000 0001, so a
Receive Byte transmission (a protocol that lacks the
command byte) that occurs immediately after POR
returns the current remote temperature data.
The one-shot command immediately forces a new con-
version cycle to begin. A new conversion begins in
software standby mode (RUN/STOP bit = high). The
device returns to standby mode after the conversion. 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 = low) 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 FunctionsThe configuration byte register (Table 4) is used to
mask (disable) interrupts, to put the device in software
standby or thermostat mode, change the polarity of the
alert output (thermostat mode only), and to change the
diode bias current. The lower three bits are internally
driven low (000), making them “don’t care” bits. Write
zeros to these bits. The serial interface can read back
this register’s contents.
Status Byte FunctionsThe status byte register (Table 5) indicates which (if
any) temperature thresholds have been exceeded. This
byte also indicates whether the ADC is converting and
whether there is a fault in the remote diode DXP-DXN
path. After POR, the normal state of all the flag bits is
zero, assuming none of the alarm conditions is present.
The status byte is cleared by any successful read of
the status byte. Note that the ALERTinterrupt latch is
not automatically cleared when the status flag bit is
cleared.
MAX1618
Remote Temperature Sensor
with SMBus Serial Interface
Table 2. Read Format for Alert Response
Address (0001 100)
