Thermal Sensing Requires
System-Level Design
By Donal McNamara, Applications Engineer, Analog Devices,
Limerick, Ireland
Using digital temperature sensors to monitor
heat-generating components or ambient environments requires an understanding of package
thermal characteristics and conductive heat
transfer applied at the pc-board level.
A
ll electronics generate heat. No matter how
low the power specification is on a device,
it will generate some heat. But heat generation becomes particularly significant when it
reaches high levels, because excessive heat is
not good for electronic circuits. At overly high temperatures, circuit performance deteriorates, systems shut down
and, in extreme cases, the equipment creates a fire hazard.
On the other hand, extreme cold presents challenges, too,
since a critical component or even an entire circuit may
not operate as temperatures approach -40°C.
The temperature profile of every electronic application
depends on the cumulative effect of heat generated by each
device and on the ambient environment. So when faced
with harsh environmental conditions and/or high levels of
power dissipation in circuit, equipment designers often must
measure the ambient and/or component temperatures and
adjust the operating parameters accordingly.
Once the designer has established that temperature sensing is required, the next step is to design an accurate temperature-measuring system. This is not as straightforward
as it may seem. If done without any consideration given to
basic temperature-measurement principles, the design could
suffer from inaccurate temperature readings or the wrong
temperature zone being monitored.
While the focus here will be on applying digital temperature sensors, which constitute the bulk of the silicon
temperature sensor market, the principles also apply to more
mature discrete temperature sensors such as thermistors,
thermocouples and resistive temperature detectors.
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Fig. 1. In conduction, heat is transferred from a hot body to a cold body
via a solid medium.
Material
Thermal conductivity (W/mK)
Diamond
Silver
Copper
Gold
1000 to 2600
406
385
320
Table 1. Material thermal conductivities.
which it would perform best. The aim is to maintain the
equipment temperature within an optimum range by such
methods as controlling current consumption or airflow
through the thermal zone affected.
Accuracy of the temperature measurement is the next
factor a designer must consider. If the operating temperature
range is small, then a digital temperature sensor with high
accuracy, perhaps ±0.5°C, may be necessary. On the other
hand, the designer may have a wide operating temperature
range with which to work. In the latter case, a temperature
sensor with accuracy on the order of ±2°C would suffice and
such a sensor would obviously be less expensive.
Once these considerations have been addressed, the
designer may have a good idea of the performance specifications the temperature sensor must have and be able to select
a sensor from half a dozen options available. The challenges
then become selecting the right sensor package, placing the
sensor in the correct position on the pc board to measure
the appropriate temperature and, finally, designing the
System Planning
As a first step in developing a temperature-measurement
system, the designer must determine what thermal zones
need to be monitored, as well as the temperature ranges in
which the application could safely work and the ranges in
Power Electronics Technology May 2007
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THERMAL SENSING
e-Front runners
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“Micro DC-DC Converter
with Integrated Inductor”
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Fig. 2. Keeping the temperature sensor close to the main heat source
and in contact with the same ground plane enables accurate tracking
of the target heat source.
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Type Number: FB6831J
Vin=2.7V-5.5V, Vout min=0.8V,
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Switching frequency=2.5MHz
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Fig. 3. Captured data from the pc-board layout in Fig. 2 reveals effective
thermal coupling between the temperature sensor and the target heat
source, enabling accurate real-time tracking.
Cellular phone, Digital video camera,
Digital still camera, Portable instruments, etc.
pc-board layout to get the best and most accurate temperature measurement. But to accomplish these goals, designers
must understand the basic theory of how temperature sensors work, or they risk making system-design errors.
Packages
•
SON (10pin)
VIN
2.7V to 5.5V
Heat-Transfer Theory
CIN
4.7uF
2 GND
PVDD 9
3 COP
CE 8
5 IN
RFB1
100k
The naturally occurring direction of heat transfer is normally from a high-temperature object to a lower-temperature
object. Heat transfer from a cold region to a hot region (as
in refrigerators, for example) is only possible with the addition of external energy to the heat-transfer system. Apart
from these conditions, there are three basic modes of heat
transfer: conduction, convection and radiation.
Conduction is the most common means of heat transfer
in a solid. On a microscopic scale, conduction occurs as
rapidly moving or vibrating atoms and molecules collide
with neighboring atoms and molecules, transferring some
of their energy (heat) to these neighboring atoms.
Convection is usually the dominant form of heat transfer
in liquids and gases. This is a term used to characterize the
combined effects of conduction and fluid flow. In convection,
M 10
1 PGND
4 CRES
VOUT
1.5V / 500mA
VDD 7
VOUT 6
COUT
4.7uF
RFB0
150k
“Step Forward, Raise Value”
Fuji Electric Device Technology
America, Inc.
Piscataway, NJ 08854, U.S.A.
Phone: 972-733-1700
Fax: 732-457-0042
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Power Electronics Technology May 2007
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Potential application
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Advantages
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THERMAL SENSING
the pc board and mounted components.
The other is the temperature of the ambient air before it is affected by any heatgenerating components on the pc board.
To accomplish either goal, the designer
must be aware of one important fact: The
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pins of the temperature sensor transfer
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60% to 65% of the heat to the thermal
sensor die (for silicon sensors).
The ground pin is connected to the
substrate. Therefore, the ground pin has
the least thermal resistance between the
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temperature sensor and heat source. Yet,
many designers are under the misconFig. 4. This layout uses several techniques for accurately measuring ambient temperature while ception that heat is mainly transferred
guarding against potential inaccuracies caused by heat-producing components.
through the plastic package of a digital
temperature sensor.
heat transfer occurs partly through conduction and partly
Though there are many succinct descriptions available
through transport by movement of hot and cold portions
in numerous textbooks and temperature-sensor datasheets,
of the fluid.
the intricacies of a digital temperature-sensor circuit are
Radiation is the only form of heat transfer that can occur
beyond the scope of this article. However, a designer may
in the absence of any form of medium. As such, it is the only
apply three pc-board tips to ensure the temperature sensor
means of heat transfer through a vacuum. Thermal radiation
tracks and accurately measures the temperature of the correct
is a direct result of the movement of atoms and molecules in
pc-board region that will accurately reflect the temperature
a material. Because the amount of radiated energy increases
of the target heat source or device. The first tip is to use a
with rising temperature, a net transfer of energy from higher
common ground plane between the temperature sensor
temperatures to lower temperatures results.
and heat source. Second, ensure that all ground pins of the
temperature sensor are connected to the heat-source ground
Thermal Conduction in PC Boards
plane. Third, keep the temperature sensor and heat source
If one end of a pc board is at a higher temperature, energy
as close as possible to each other on the pc board.
is transferred through the pc board toward the colder end.
Fig. 2 illustrates how these techniques are applied to a
The higher-speed particles collide with the slower ones,
pc-board layout in which the temperature of a heat-proresulting in a net transfer of energy to the slower ones. This
ducing component is to be monitored. Fig. 3 reveals that
is shown in Fig. 1, and the rate of heat transfer for conducthis arrangement is not only accurate, but also allows for
tion is given by:
temperature changes to be tracked in real time.
(K × A(THOT − TCOLD ))
H=
,
Measuring Ambient Temperature
L
where H equals energy conducted in time (J/sec), K equals
Some customers want to monitor air temperature, but still
thermal conductivity of the copper (385 W/mK) at room
take advantage of the the accuracy, linearity, speedy response
temperature, A equals the area of copper on the pc board
and convenience of an IC temperature sensor. However, in
(m2), T equals temperature (°C) and L equals the distance
doing so, they must prevent the heat dissipated by the main
between hot and cold bodies (m).
heat source on the pc board from affecting the temperature
Heat travels faster from a hot body to a cold body if the
measurement.
area of the medium it is conducting through (copper, for
There are five guidelines that can be applied to prevent
example) is increased. Likewise, if the area of the medium
the heat-dissipating component from affecting the temperais reduced, the heat-transfer rate is reduced. Common sense
ture sensor and to accurately monitor ambient temperature.
deduces that the more distance between the two bodies, the
First, use a hash ground plane that will reduce the ground
longer it takes for the cold body to heat up. As an excellent
plane area to increase thermal resistance. Second, keep the
conductor of heat, copper is used in many pc-board designs
temperature sensor as far away from heat sources as posto dissipate heat from a heat source. Silver and diamond are
sible. Third, use a separate ground plane for the temperature
the only other materials to have better thermal conductivity
sensor and keep the number of connections to the main
(Table 1).
ground plane as low as possible. Fourth, use narrow ground
connections to increase thermal resistance. Fifth, use a solid
PC-Board, Component Temperature Sensing
ground plane under the main heat source and expose the
A designer uses a digital temperature sensor to measure
green solder mask. Following this last guideline gives the
two primary thermal zones. One zone is the temperature of
minimum thermal resistance for the main heat source to dis����������������
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Power Electronics Technology May 2007
THERMAL SENSING
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Fig. 6. The value of JA indicates the ease of heat flow for digital temperature sensors through the package.
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sensor implemented in the pc board in Fig. 4 is not affected
by the heat from the main heat source and is accurately
measuring the ambient air temperature.
Fig. 5. Using the pc-board layout tips for ambient temperature monitoring allows the digital temperature sensor to make accurate measurements, free of errors introduced by other heat-dissipating circuitry.
Package Thermal Characteristics
sipate heat, diverting the heat flow away from the sensor.
The pc-board layouts shown in Figs. 2 and 4 are used to
implement the same circuit topology but are physically different, because the layout in Fig. 4 is optimized for measuring
ambient air temperature, according to the previous guidelines. The graph in Fig. 5 clearly shows that the temperature
Apart from size and pin count, there are several other
package considerations such as package thermal resistance,
power dissipated in the device, soldering temperatures and
response to thermal shock. Two package performance metrics
usually indicated in datasheets are junction-to-air thermal resistance (JA) and junction-to-case thermal resistance (JC).
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Fig. 7. For nondigital temperature sensors such as the AD590, a currentoutput temperature sensor, the value of JC indicates the ease of heat
flow.
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The ease of heat flow between the die surface and air is
indicated by JA. It reflects how heat flows from junctionto-ambient temperature via all package paths. The primary
temperature-transfer path is from the package leads to the
board, so the thermal resistance of the leads is more important to digital temperature sensors than package resistance.
The value of JA is especially relevant for packages used
without external heatsinks (Fig. 6). The lower the value
— measured in units of degrees Celsius per watt — the more
effective the package at heat transfer. For example, for an
8-lead MSOP, JA equals 205.9°C/W. However, for an 8-lead
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Fig. 8. The value of JA for a package has a direct impact on thermalperformance margins, as shown in this comparison between the SOIC
( JA = 157°C/W) and MSOP ((JA = 205.9°C/W) package types.
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Fig. 9. This response of the ADT7301 to a thermal shock indicates a
thermal time constant of 2 seconds for the device.
SOIC, JA equals 157°C/W. Therefore, the latter package is
more effective at heat transfer between the internal die and
the ambient environment.
The ease of heat flow between the chip surface and package surface is indicated by JC. It therefore reflects how heat
flows into the external heatsink (Fig. 7). The lower the value,
the more easily the heat flows into a heatsink. This parameter is also directly determined by the package design. For
example, for an 8-lead MSOP, JC = 43.74°C/W, while JC =
56°C/W for an 8-lead SOIC.
An equation is available in the absolute maximum ratings section of most datasheets for temperature-sensing
components. It is defined as a maximum power rating. It is a
general equation for any temperature-sensing device, but not
a major factor in temperature sensing. Temperature sensors
are designed to draw as little power as possible because selfheating can result in a rise in temperature readings. Thus,
the power-dissipation number is intended to warn a designer
Power Electronics Technology May 2007
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THERMAL SENSING
Average ramp-up rate
Preheat
• Temperature minimum (TSMIN)
• Temperature maximum (TSMAX)
• Time (TSMIN to TSMAX)
Time Maintained Above
• Temperature
• Time
• Peak temperature
• Time within 5°C of actual peak temperature
• Ramp-down rate
• Time from 25°C to peak temperature
Tin-lead assembly
(3°C/sec maximum)
Lead-free assembly
(3°C/sec maximum)
100°C
150°C
60 sec to 120 sec
100°C
150°C
60 sec to 120 sec
183°C
60 sec to 150 sec
220°C
10 sec to 30 sec
6°C/sec maximum
6-minute maximum
217°C
60 sec to 150 sec
260°C
20 sec to 40 sec
6°C/sec maximum
6-minute maximum
Table 2. Data for soldering tin-lead and lead-free packages.
that as heat rises, the component’s ability to dissipate power
is diminished, and this affects temperature-measurement
accuracy as well as potentially damaging the device. This ability is determined by the relationship defined by the equation
for maximum power:
(TAMBMAX − TJ )
PMAX =
,
θJA
where PMAX equals the maximum power dissipated in the
device, TAMBMAX equals the maximum ambient temperature
specified in the datasheet, TJ equals the junction temperature,
and JA equals the junction-to-ambient thermal resistance in
degrees Celsius per watt.
Again, the basic guideline for applying the equation for
temperature sensors is to warn the user that extreme heat
exposure has a destructive effect on the temperature sensor.
IC plastic packages are designed to withstand heat between
100°C and 150°C for a limited period of time. Prolonged
exposure will shorten the lifetime of the device. Fig. 8 shows
how this equation determines the relationship between PMAX
and TAMBMAX, and how a MSOP package has lower thermal
margins for heat and temperature than an SOIC due to the
latter’s lower JA value. The graph also reveals that package
components are in danger of being destroyed once the temperature rises above 100°C.
Fig. 10. The LFCSP package provides a metal stub connected to
the base of the packaged die, resulting in a lower thermal time
constant for temperature sensors.
value of the applied temperature step. In Fig. 9, the ADT7301
experiences a thermal shock from 25°C to 125°C; it typically
takes 2 seconds for the ADT7301 to reach 88.2°C, so this is
the value of the thermal time constant.
The same thermal time constant is used for the ADT7301
for both the SOT-23 and MSOP packages. Evaluation data
has shown that package type only has a small effect on the
thermal time-constant value. This indicates that most of the
heat flows through the package leads. Therefore, the values
of JA and JC have little effect on the thermal response of surface-mount digital temperature sensors. In general, effective
ground-pin contact to the ground plane of the heat source
is far more important than the package type. Most modern
temperature sensors draw very little current, on the order of
microamps. As a result, power dissipation and, consequently,
self-heating are not significant design factors.
In the case of current-output temperature sensors (for
example, the AD590, AD592 and TMP17), package types
TO-52, T0-92 CQFP and SOIC rely on a low JC + JA for
fast thermal response. Note that there are no ground pins on
these parts.
The LFCSP (Fig. 10) has a metal stub at the base that is
directly connected to the ground of the die. Connecting this
stub to the pc-board ground plane gives the LFCSP a lower
thermal resistance than most packages and, consequently,
the device has a lower thermal time constant.
PETech
Package Soldering and Thermal Shock
Analog Devices now produces two types of package leads,
tin-lead leads and lead-free leads (as of 2006, all new parts
released from Analog Devices contain lead-free materials).
There are different time and temperature parameters when
soldering both types of leads. The most important difference
between the two lead types is the peak soldering temperature.
The peak soldering temperature specifications for tin-lead
and lead-free assembly are highlighted in Table 2.
Reducing the thermal resistance between the die and heat
source reduces the thermal time constant and improves the
thermal response of the die. One thermal time constant is
the time it takes for a temperature step applied to the sensing
region of the sensor to produce a reading of 63.2% of the
final temperature value, which should be equal to the final
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Power Electronics Technology May 2007