This extended range has opened up an entire set of new applications,
such as remote sensing, airflow sensing, actuating fans,
and buzzers. Modern IC temperature sensors also offer direct
compatibility with many popular computer buses, including the
single-wire pulse-width-modulation (PWM) bus, the two-wire
I2C bus, the three- and four-wire serial periheral interface (SPI)
bus, and the two-wire SMBus, to interface with microcontrollers
and other digital systems.
Texas Instruments’ TMP421 ±1°C remote and local temperature
sensor monitor in an SOT23-8 case exemplifies the trend
toward remote temperature measurements. It features a built-in
local temperature sensor consisting of low-cost npn and pnp
diode-connected transistors or diodes that are an integral part of
FPGAs, microprocessors, and microcontrollers.
Many high-end CPUs and DSPs have specific interfaces to IC
temperatures sensors to keep tabs on a processor’s temperature—a
critical task at gigahertz clock rates. The flexibility of an IC sensor
lets users program threshold levels, hysteresis, and shutdowns. It
also allows operation with digital signals coming from an ADC.
“As CPU designs approach 45-nm line widths, accurately measuring
a processor’s temperature becomes more challenging,” says
Tadija Janjic, business unit manager for TI. “A designer must know
what he or she is measuring and pay attention to the sensor’s package.
The bigger the package, the larger the thermal constraints.”
National Semiconductor uses its TruTherm technology in
temperature sensors for monitoring ICs made on 90-nm processes
and below. The company measured the temperature of 11 different
65-nm processors from 20°C to 100°C. Temperature errors
(measurement variations) were less than 0.5°C with its TruTherm
sensors versus a 3°C variation using other IC temperature sensors consisting of traditional
remote diodes (Fig. 5).
IC temperature sensors and
their small profiles are being
used in dual-inline memory
modules (DIMMs), too. TI’s
TMP102 digital temperature
sensor offers an SMBus/
two-wire serial interface in an
SOT563 package. According
to the company, its thin profile
suits it for mounting under a
DIMM without compromising
DIMM dimensions.
The application-specific
STTS424/E02 digital sensor
chips from STMicroelectronics
conform to the
JEDEC JC42.4 specification.
The STTS424 is a
standalone model, while the
STTS424E02 integrates
the sensor with 2 kbits of
serial presence-detection
EEPROM. Both interface
with the I2C bus and
SMBus.
Designers must account
for two key considerations when using an
IC temperature sensor. First, they must
determine what quantity needs to be measured
and where the object to be measured
is located in the circuit. For example, the
application may require the IC sensor to be
mounted either close to or far away from
the object. This is important for communicating
with other sensors in remote locations
for functions like fan-speed control
and overall system temperature control.
The second consideration is measurement
accuracy. Although many types of
temperature sensors are available to satisfy
just about every accuracy requirement, the
correct temperature-sensor selection isn’t
so straightforward. If you choose a sensor
without first understanding its basic operating
principles, the measurement could suffer
from inaccurate readings, or the wrong
temperature zone could be monitored.
For an IC temperature sensor, the sensor’s
temperature is essentially that of the
transistor’s junction diode. So when measuring
the temperature of, say, a CPU, a
thermal-diode monitor that’s integrated
either on the sensor chip itself or on the
CPU should be used for accurate measurements.
This monitor can also be implemented
as a discrete solution on a PCB.
GREATER ACCURACY
Semiconductor IC temperature sensors
are moving toward greater accuracy of
0.5°C and better, in concert with today’s
shrinking IC line widths and increasing
chip densities. “There’s a lot of interest in
very high-accuracy, low-cost IC sensors
and in new applications like HVAC,” says
Analog Devices’ Pratt. “This is the case in
the food transport business of perishable
goods where thermocouples are now more
commonly used.”
“IC temperature sensors are enabling
more sophisticated sensing in terms of
accuracy, ease of use, and a direct digital
output for domestic and industrial HVAC
applications, as well as use in white goods
appliances. We might see a trend toward
having an IC with an embedded temperature
sensor on it, but then we’re giving up
the redundancy gained from a discrete
approach,” says Linear’s Williams. “The
raw measurement capability of an IC temperature
sensor has not changed much.
What has changed is the packaging.”