ADN011 Flexible Integrated Temp Sensors Lower System Costs

Flexible Integrated Temp Sensors
Lower System Costs
By Bonnie C. Baker, Microchip Technology Inc.
Temperature is one of the most commonly sensed entities
in electronic circuits today. This is largely due to the
multitude of applications where knowing and using the
actual or relative temperature is critical. The most common
temperature sensors that are used to perform this task are:
Thermocouples, Resistive Temperature Detectors (RTDs),
Thermistors and Silicon Temperature Sensors, which are
primarily classified according to their output-signaling
method. Microchip offers logic output, voltage output and
serial or digital output silicon temperature sensors.
Each of these technologies cater to specific temperature
ranges and environmental conditions. This ADN focuses
on two of Microchip’s serial output silicon sensors.
Thermocouples are capable of sensing elevated
temperatures (–400°C to +1,260°C). This sensor is nonlinear and requires a look-up table in the controller.
The RTD is able to sense temperatures with extreme
repeatability and low drift error (–200°C to +850 °C). For
precision, this sensor also requires a look-up table in the
controller due to non-linearities.
Thermistors (–100°C to +150 °C) are normally used for
overtemperature shutdown purposes. Again, this sensor is
non-linear and requires a look-up table in the controller.
Although the Thermocouple, RTD and Thermistor are
capable of being placed in harsh, high-temperature
environments, the silicon temperature sensor becomes
the preferred choice in applications where –55°C to
+125°C temperatures are sensed. They are easily
installed on printed circuit boards, easy to interface
with and don’t require expensive external circuitry.
Applications such as personal computers, mobile
phones, automotive, medical equipment and gaming
consoles have improved performance when the
temperature is monitored and fed back into the
system. These types of temperature sensors produce
an output that represents the ambient temperature (the
air surrounding the device). The style of outputs from
these sensors include analog voltage, logic threshold
or digital “words” (I2C™, SMBus or SPI™). Today, the
most popular types of thermal sensors are voltage,
logic and serial output.
The flexibility of the IC types of sensors provides
programmability of thresholds, hysteresis, shutdown
and digital code from an A/D conversion. For
instance, the 9-bit TCN75 from Microchip is a seriallyprogrammable temperature sensor that notifies
the host controller when the ambient temperature
exceeds a user-programmed set point. Hysteresis
is also programmable. The INT/CMPTR output
is programmable as either a simple comparator
(for thermostat operation) or as a temperatureevent interrupt. Communication with the TCN75 is
accomplished via a two-wire bus that is compatible
with industry standard protocols. This permits reading
the current temperature, programming the set point
and hysteresis, as well as configuring the device.
Figure 1.
The TCN75 can be used in a multi-zone application where communication from chip to chip is accomplished through a two-wire,
bidirectional interface.
© 2004 Microchip Technology, Inc.
DS21901A - Page 1
Figure 1 presents an example of a multi-zone
temperature-sensing application. In this example,
the TCN75 is being used in an application where
temperature is sensed in many locations across a
printed circuit board or from board to board. Typically, IC
semiconductor sensors are mounted on boards with heat
sinks. The IC sensor is used to measure temperature
in the vicinity and to indicate, for example, when
overtemperature conditions exist. The thermal response
time of most silicon temperature sensors is given in
minutes. Therefore, they are not an appropriate match
for transient sensing requirements.
Referring to the block diagram of the TCN75 in Figure
2, this device powers-up in Comparator mode, with a
default set point of 80°C and 5°C hysteresis. These
defaults allow independent operation as a stand-alone
thermostat. A shutdown command may be sent via the
2-wire bus to activate the low-power Standby mode.
Address selection inputs allow up to eight TCN75s to
share the same 2-wire bus for multi-zone monitoring. All
registers can be read by the PICmicro® MCU, with the
INT/CMPTR output’s polarity being user-programmable.
Both polled and interrupt-driven systems are easily
accommodated. With temperature accuracy of ±3°C
(max) from 25°C to 100°C, this device is available in
either an 8-pin SOIC or the smaller MSOP package.
If accuracy is your goal, the TC77 serial output
temperature sensor from Microchip has temperature
accuracy of ±1.0°C (max.) over the temperature range of
Figure 2.
+25°C to +65°C. Outside of this temperature range, the
TC77 is specified to ±2°C (max.) accuracy from 0°C to
+85°C, and ±3°C (max.) accuracy from –55°C to +125°C.
The temperature performance of the TC77 is illustrated
in Figure 3.
The TC77 is a serially-accessible digital temperature
sensor particularly suited for high-accuracy and small
form-factor applications. Temperature data is converted
from the internal thermal-sensing element and made
available at any time as a 13-bit two’s compliment digital
word. This allows the microcontroller in the circuit to
track and monitor a temperature profile. Communication
with the TC77 is accomplished via a SPI and Microwire
compatible interface. It has a 12-bit plus sign temperature
resolution of 0.0625°C per Least Significant bit (LSb).
The TC77’s configuration register can be used to activate
the low-power Shutdown mode (current consumption
= 0.1 µA, typ.). Small size, low cost and ease-of-use
make the TC77 an ideal choice for implementing thermal
management in a variety of systems.
If you are more interested in designing your system
than finessing the temperature-monitoring circuit, the
“no worries” IC semiconductor temperature sensor
is the right product for your application. This type of
device produces a direct-digital, linear output versus
temperature and does not require external components
in your implementation. This integrated solution offers a
low-cost solution that is easy to design into your circuit.
The TCN75 calibration, data and configuration registers
are accessible through a two-wire, serial interface.
Additionally, there is an Alert (or interrupt) pin that can be
used for thermostat applications.
Figure 3.
Typically, the TC77 accuracy is better than 1°C over a
25°C to 65°C temperature range. This device is available
in a 5-pin SOT-23 package. If further accuracy is required
in the higher or lower temperature ranges (–55°C to 25°C
or 65°C to 125°C), calibration, in conjunction with a lookup table, can be used.
For more information, please visit
The Microchip name and logo, the Microchip logo and PICmicro are registered trademarks of Microchip Technology Inc. in the U.S.A. and other countries.
SPI is a trademark of Motorola Inc. I2C is a trademark of the Phillips Corporation.
All other trademarks mentioned herein are the property of their respective owners. All information is subject to change.
© 2004 Microchip Technology Inc. 8/2004
DS21901A - Page 2
© 2004 Microchip Technology, Inc.