Understanding Temperature Specifications:
An Introduction
AN4017
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Abstract
The following technical note is intended to give the reader a basic understanding of the temperature specifications found in
Cypress’s product data sheets. In the past, Cypress has simply stated that the device meets a commercial, industrial, or automotive temperature specification and listed the associated ambient temperature range. For example, we specify a commercial
temperature part as 0°C ≤ TA ≤ 70°C. However, there are many factors that affect the thermal operation of a device and specifying the temperature in this manner does not address them. This technical note will explore these issues and provide the reader
with enough background to understand the thermal performance.
Fundamentals
Power is required to operate integrated circuits (ICs). This
power is provided to the IC in the form of voltage and current
via power supply pins. It is this consumption of power that
creates heat and results in junction temperatures different
from the surrounding ambient temperature. As long as power
is being dissipated, the junction temperature (temperature of
the chip) will always be hotter than the ambient temperature.
The degree depends on several factors including, but not limited to:
The ability for a given device to dissipate this internally generated heat is expressed as thermal resistance with units of
°C/watt. Basically, the thermal resistance is derived to show
how much the junction temperature will increase based on
the power being dissipated by the device.
Definitions
We must first define some of the terms required to specify the
operating condition of our devices.
■
Heat from neighboring ICs
■
Airflow
■
IC packaging material
■
IC packaging technique (e.g., flip chip versus wire bond)
■
Number of leads on the IC package
■
Printed circuit board materials
■
Air temperature
TA = Ambient temperature. This is the temperature of the air
in close proximity to the device. It can be measured directly.
The air temperature (TA) dictates the minimum temperature
at which the device will operate. No matter how much heat
sinking or airflow is supplied, the device will not get colder
than the surrounding air. Once the IC begins to dissipate
power, the junction temperature (TJ) will increase above the
air temperature. One can reduce the increase of junction
temperature by adding airflow or heat sinks, but as long as
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power is being dissipated, the junction will rise to a temperature above TA.
TC = Case temperature. This is the temperature of the case
of the semiconductor. It is typically measured at the center of
the top side of the device.
TJ = Junction temperature. This is the temperature of the
active silicon circuit itself. It must be calculated or inferred
from the case and/or ambient temperature.
Power Dissipation (Pd) = This is the power consumed while
the device is in operation. It is this consumption of power that
creates heat. It is typically expressed in watts as a sum of the
static power and dynamic power.
Airflow = The movement of air over and around a device.
Airflow is used to remove heat from the system.
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Thermal Resistance = An empirically derived set of constants that describes the heat flow characteristics of a given
system, expressed in °C/watt. It is a measure of the ability of
a package to transfer the heat generated by the device inside
it to the ambient. Some factors that affect thermal resistance
include: (1) the die size of the IC chip, (2) the mold compound
and (3) lead frame / substrate design. θJA (Junction to Ambient thermal resistance), θJC (Junction to Case thermal resistance), and θCA (Case to Ambient thermal resistance) are the
thermal parameters generally used to characterize a
package.
θJA is the Junction to Ambient thermal resistance. It represents the ability of the package to conduct heat from the IC
chip inside the package to the environment. It is defined as
the difference between the junction temperature and the
ambient temperature when the device is dissipating 1W of
power. θJA (expressed in °C/W) = (TJ – TA)/Pd. For a given
package and lead frame, some factors that affect θJA are:
(1) the die size of the IC chip, (2) the length of the printed circuit board traces attached to the IC package on the system
board, and (3) the amount of airflow across the package.
θJC is the Junction to Case thermal resistance. It is defined
as the temperature difference between the junction and a reference point on the package when the device is dissipating
1 Watt of power. θJC (expressed in °C/W) = (TJ – TC)/Pd. It is
mainly a function of the thermal properties of the materials
constituting the package.
θCA is the Case to Ambient thermal resistance. It is defined
as the temperature difference between a reference point on
the package and the ambient temperature when the device is
dissipating 1 Watt of power. θCA (expressed in °C/W) =
(TC – TA)/Pd. It is mainly dependent on the surface area
available for convection and radiation and the ambient conditions, among other factors. This can be controlled by using
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heat-sinks, providing greater surface area and better conduction path, or by air or liquid cooling.
The Junction to Ambient thermal resistance is the sum of the
thermal resistances of Junction to Case and Case to Ambient. In other words, the relationship between the thermal
parameters can be expressed as: θJA = θJC + θCA
Calculating the Junction Temperature
Given the Junction to Ambient thermal resistance (θJA) and
the ambient temperature, one can arrive at the junction temperature of the chip after calculating the power dissipated by
the device as follows:
TJ = PdθJA + TA where
Pd = Core power + I/O switching power
Core power = VDD(max.) x IDD and
IO Switching power = α x f x CLx V2 x (number of I/Os that
are switching), where:
α is the activity factor, or the ratio of frequency at which
outputs toggle to the clock frequency
= 0.5 for single data rate devices like Std Sync,
NoBL™ SRAMs;
= 1 for double data rate devices like
DDR/QDR™ SRAMs)
f = operating frequency
CL = external load capacitance
V is output voltage swing
=
VDD with unterminated load;
=
VDD/2 for a terminated load, assuming
termination voltage is equal to VDD/2
θJA = Junction to Ambient thermal resistance
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SRAM Example
Temperature Specifications
Let’s look at an example using the 100-lead SRAM TQFP
device (specifically, part number CY7C1381D-100AC). The
thermal resistance is 28.66°C/W for a 4-layer board with 0 ft/s
of airflow.
The thermal parameters exhibit worst-case values when
there is no airflow. Also, with higher temperatures the thermal
performance becomes even more critical. Because of this, as
we move from commercial temperature ranges to industrial or
automotive temperature ranges, temperature specifications
become much higher.
Assuming the device is running at 100 MHz with a 40-pF
capacitive load and all I/Os switching, the power dissipated is
calculated as follows:
Pd = Core power + I/O switching power
Core power = VDD(max.) x IDD = 3.6 x 175 x 10–3 = 0.63W
In order to ensure good thermal management, it is essential
that the junction temperature remains well below the maximum rated value. This is because an increase in junction
temperature (TJ) can adversely affect the long-term operating
life of a device.
I/O Switching power = α x f x CLx V2 x (number of I/Os that
are switching) = 0.5 x 100 x 106 x 40 x 10–12 x (3.6)2 x 36
= 0.93W
Therefore total power dissipated Pd = 1.56W
The junction temperature increase is then calculated using
the thermal resistance value:
TJ = TA + (Thermal resistance × Power)
= TA + (θJA × Pd)
= TA + (28.66°C/W × 1.56W)
= TA + 44.71°C
The manner in which an IC package is mounted and positioned in its surrounding environment has significant effects
on operating junction temperatures. These conditions are
controlled by the system designer and are worthy of serious
consideration in the PC board layout and system ventilation
and airflow features.
Forced air cooling will significantly reduce thermal resistance.
Airflow parallel to the long dimension of the package is generally a little more effective than airflow perpendicular to the
long dimension of the package.
(Note: θJA used is a referenced value and will vary by
device).
If the application is rated for commercial temperature range,
we can have an ambient temperature from 0°C to 70°C.
Assume a typical environment within the system is 30°C—the
resulting junction temperature is:
TJ = 30°C + 44.71°C
= 74.71°C
If the same system had airflow, the junction temperature
would be lower.
In a worst case scenario, we can have TA = 70°C:
TJ = 70°C + 44.71°C
= 114.71°C
However, note that a typical application will have boards with
more layers and better heat sinking characteristics. We see
that the temperature at the junction will be much hotter than
the temperature of the air around it, and that airflow and
board construction have a large impact on the junction temperature.
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System Considerations
External heat sinks applied to an IC package can improve
thermal resistance by increasing heat flow to the surrounding
environment. Heat sink performance will vary by size, material, design and system airflow. In general, they can provide a
substantial improvement.
Package mounting can affect thermal resistance. For example, surface mount packages dissipate significant amounts of
heat through the leads that attach to the traces.
The metal (copper traces) on PC boards conduct heat away
from the package and dissipate heat to the ambient; thus the
larger the trace area the lower the thermal resistance.
Also, as package sizes shrink and more devices are mounted
on the board the thermal characteristics become a major concern.
Measurement of TJ
Measurement of junction temperature to confirm whether it is
well below the specification is a possible but difficult procedure.
A practical and easier method is to measure the case temperature. This can be done through a direct measurement—such
as a thermocouple or Resistance Temperature Detector
(RTD) placed in contact with the device under test—or with a
noninvasive method, such as an infrared heat detector. Once
TC is known, TJ can then be calculated using the junction-tocase thermal resistance and the power as mentioned in the
previous example.
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In some cases involving Cypress’s SRAM memory devices,
the case temperature will be within a few degrees of the junction temperature so the calculation will not be necessary.
Therefore it is advisable to measure the case temperature,
and ensure it is below the maximum specified junction temperature. If it is higher than the maximum specified junction
temperature, the junction will be even hotter and the application will be running outside of specification.
Summary
The thermal characteristics of a device have been and will
continue to be a major concern for board designers. It is crucial that the thermal parameters, especially junction temperature, are well below the specified limit. This is because an
increase in junction temperature can adversely affect the long
term operating life of a device. Since it is impractical to measure junction temperature directly, it is better to measure the
case temperature of the device and then calculate the junction temperature. Some factors affecting TJ are controlled by
the IC manufacturer and others are controlled by the system
designer. Also, temperature specifications as well as thermal
management become a major concern to board designers as
package sizes shrink and board density increases.
NoBL is a trademark of Cypress Semiconductor Corporation. QDR RAMs and Quad Data Rate RAMs comprise a new family of
products developed by Cypress Semiconductor, Renesas, IDT, NEC, and Samsung. All product and company names mentioned
in this document are the trademarks of their respective holders.
Cypress Semiconductor
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San Jose, CA 95134-1709
Phone: 408-943-2600
Fax: 408-943-4730
http://www.cypress.com
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