AN4017 Understanding Temperature Specifications An Introduction.pdf

AN4017
Understanding Temperature Specifications: An Introduction
Associated Part Family: Sync SRAMs
Associated Project: No
Software Version: NA
Related Application Notes: AN42468
To get the latest version of this application note, or the associated project file, please
visit http://www.cypress.com/go/AN4017.
AN4017 gives a basic understanding of the temperature specifications found in Cypress's product datasheets. There
are many factors that affect the thermal operation of a device. This application note also gives you an understanding
of the thermal parameters and temperature specifications of the device.
1
Introduction
Power is required to operate integrated circuits (ICs). This power is provided to the IC in the form of voltage and
current through power supply pins. The consumption of power creates heat and results in junction temperatures
different from the surrounding ambient temperature. There are several factors that affect the junction temperature:
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Heat from neighboring ICs
Airflow
IC packaging material
IC packaging technique (example flip chip versus wire bond)
Number of leads on the IC package
Printed circuit board (PCB) materials
Ambient temperature
The air temperature (TA) dictates the minimum temperature at which the device operates. 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) increases above the ambient temperature. You can reduce the junction
temperature by adding airflow or heat sinks, but as long as the power is dissipated, the junction rises to a
temperature above TA.
Thermal resistance is the ability for a given device to dissipate the internally generated heat, expressed in units of
°C/W. Basically, the thermal resistance is derived to show how much the TJ increases based on the power dissipated
by the device.
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Understanding Temperature Specifications: An Introduction
2
Definitions
The following are some important definitions that pertain to the operating condition of the devices.
TA = Ambient temperature. This is the temperature of the environment, still air.
TC = Case temperature. This is the temperature of the case of the semiconductor device.
TJ = Operating Junction temperature. This is the temperature of the device circuit itself under given operating
conditions. TJ must be calculated or inferred from the case and/or ambient temperature.
TJmax = Maximum Junction temperature. This is the maximum temperature that the device tolerates to guarantee
reliable operation. The system designer needs to ensure that TJ < TJmax to guarantee reliability.
Table 1. Maximum Junction Temperature
SRAM Type
TJmax
Sync SRAMs
125 °C
nvSRAM
150 °C
Async SRAM
150 °C
Dual port RAMs and FIFOs
125 °C
F-RAM
125 °C
Max junction temperature is listed in Table 1 for various Cypress memory devices.
Power Dissipation (Pd) = This is the power consumed while the device is in operation and this power consumption
creates heat. It is typically expressed in Watts.
Airflow = The movement of air over and around a device that is used to remove heat from the system.
Thermal Resistance = An empirically derived set of constants that describe the heat flow characteristics of a given
system, expressed in °C/W. Thermal resistance is a measure of the ability of a package to transfer the heat
generated by the device inside a package 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), θCA (case-to-ambient thermal resistance), and θJB (junction to
board) are the thermal parameters generally used to characterize a package.
Figure 1. Types of Thermal Resistance
θJA is the junction-to-ambient thermal resistance. θJA represents the ability of the package to conduct heat from the IC
chip inside the package to the environment. θJA is defined as the difference between the junction temperature and the
ambient temperature when the device is dissipating 1 W 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. θJA value is available in the datasheet of the device.
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Understanding Temperature Specifications: An Introduction
θJC is the junction-to-case thermal resistance. θJC is defined as the temperature difference between the junction and a
reference point on the package when the device is dissipating 1 W 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. θJC value is available in the
datasheet of the device.
θJB is the junction-to-board thermal resistance. θJB is defined as the temperature difference between the junction and
the board when the device is dissipating 1 W of power. θJB (expressed in °C/W) = (TJ – TB)/Pd. TB is the temperature
of the PCB board taken at a predefined location near the die. θJB can be provided upon request.
θCA is the case-to-ambient thermal resistance. θCA is defined as the temperature difference between a reference point
on the package and the ambient temperature when the device is dissipating 1 W of power. θCA (expressed in °C/W) =
(TC – TA)/Pd. θCA 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 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-toambient. In other words, the relationship between the thermal parameters can be expressed as: θJA = θJC + θCA
3
Calculating the Junction Temperature
When the junction-to-ambient thermal resistance (θJA) and the ambient temperature are given, you can calculate the
junction temperature of the chip after calculating the power dissipated by the device, as follows:
TJ = Pd θJA + TA
Where,
θJA = Junction-to-ambient thermal resistance
TA = Ambient temperature
Pd = Core power + I/O switching power + ODT Power
Core power = VDD(max.) x IDD and
2
I/O switching power = α × f × CL × V 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 (such as DDR/QDR™ SRAMs)
f = operating frequency
CL = external load capacitance
V is output voltage swing,
For example,
= Vddq for unterminated load
= Vddq/2 for a terminated load with pull-up termination
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Understanding Temperature Specifications: An Introduction
ODT Power is the power dissipated in the input on die termination resistors. For Non-ODT parts this power is zero.
See Figure 2 for a description of ODT Power consumption.
If the driver source impedance is R, the input on die termination resistors are 2R as shown below.
Figure 2. ODT Power Consumption
Depending on whether the source is driving a “1” or “0” either Path-1 or Path-2 is active. In either case the Power
dissipated in the ODT resistors is
ODT Power = (5/16) × (Vddq)2 × (1/R) × (number of inputs with ODT resistors), where:
= Vddq is I/O voltage
= 2R is the termination resistor, used for pull-up and pull-down termination
See AN42468 for derivation of the ODT Power Equations.
4
SRAM Example
Let us look at an example using the 100-lead SRAM TQFP device (specifically, part number CY7C1381D). The
thermal resistance is 28.66 °C/W for a 4-layer board with 0 ft/s of airflow. 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 + ODT power
Core power = VDD(max.) × IDD = 3.6 × 175 × 10–3 = 0.63 W
2
6
–12
2
I/O switching power = α × f × CL× V x (number of I/Os that are switching) = 0.5 × 100 × 10 x 40 × 10 × (3.6) × 36
= 0.93 W
ODT power = 0, as there are no input ODT resistors.
Therefore total power dissipated, Pd = 1.56 W
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 × 1.56 W)
= TA + 44.71 °C
Note: θJA used is a referenced value and will vary by device.
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Understanding Temperature Specifications: An Introduction
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 worse-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 higher than the temperature of the air around it, and that airflow
and board construction have a large impact on the junction temperature.
For more information, see the online tool for calculating the junction temperature for Synchronous SRAM products.
5
Temperature Specifications
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.
To ensure good thermal management, it is essential that the junction temperature remains well below the maximum
rated value TJmax. This is because an increase in junction temperature (TJ) can adversely affect the long-term
operating life of a device.
6
System Considerations
The major part of the heat travels through the PCB only. There are essentially four paths for heat to transfer out of the
chip into the PCB:
1.
The small amount of heat transfer from case to ambient through the air around the device.
2.
Heat transfer into the PCB through the top layer.
3.
Heat transfer to the internal dielectric material and copper layers though via array.
4.
Finally, the heat that travels through via array below the chip and into the PCB’s bottom-most copper layer.
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 significantly reduces 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.
External heat sink 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.
The dielectric material used in the PCB with higher thermal conductivity can also help to get the lower thermal
resistances. For example, the thermal resistance from junction to board (θJB) is about 10% lower in the case of
[1]
RT/duroid 6035HTC (Rogers) than FR-4 .
1
Based on Cypress’ 72M QDR-II+ and JEDEC standard PCB board thermal resistance measurement simulation results.
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Understanding Temperature Specifications: An Introduction
The other most effective conduction path is vias. The more the number of vias under the chip, the lower the thermal
resistances; as a result, the junction temperature of the chip will be reduced. Optimized conduction paths from vias
to copper layers and dielectric material through accurate design of the via array provides the most efficient paths in
the design for removing heat from the chip.
Also, as package sizes shrink and more devices are mounted on the board the thermal characteristics become a
major concern.
7
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. The measurement 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-to-case thermal resistance and the power as mentioned in the previous example.
See JEDEC Specification JESD51 - Methodology for the Thermal Measurement of Component Packages (Single
Semiconductor Device) for details.
In general, 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 rated
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.
8
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. As 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.
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Document No. 001-15491 Rev.*I
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Understanding Temperature Specifications: An Introduction
Document History
Document Title: AN4017 - Understanding Temperature Specifications: An Introduction
Document Number: 001-15491
Revision
ECN
Orig. of
Change
Submission
Date
Description of Change
**
1051123
SFV
05/15/2007
New Application Note.
*A
1736124
VIDB
11/29/2007
No technical updates.
Updated to new template (To enable google search functionality).
*B
3111380
VIDB
12/15/2010
No technical updates.
*C
3187210
NJY
03/03/2011
Updated Abstract.
*D
3339187
NJY
08/10/2011
Changed the “Technical note” to “Application note” in the Abstract section.
Included link to online tool for calculating Junction temperature in page 2.
*E
3557468
AVIA
03/21/2012
Updated Definitions:
Added Table 1.
Defined Tjmax, and explained the condition Tj < Tjmax.
Added Figure 1.
Updated Calculating the Junction Temperature:
Updated the equations for power dissipation with ODT Power included.
Added Figure 2.
Added a reference to application note AN42468.
Updated Measurement of TJ:
Added reference to JEDEC JESD-51 for calculating thermal parameters.
Fixed all the grammatical errors across the document.
Converted application note from FrameMaker to Word template.
*F
3606064
PRIT
05/02/2012
Updated Definitions:
Updated Table 1:
Changed value of Tjmax of NVSRAM from 125 °C to 150 °C.
*G
4235003
PRIT
01/02/2014
Updated to new template.
Completing Sunset Review.
*H
4537562
PRIT
10/14/2014
*I
4831299
PRIT
07/14/2015
Updated System Considerations.
Updated Definitions:
Updated Table 1:
Added F-RAM and its corresponding maximum junction temperature.
Updated to new template.
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Document No. 001-15491 Rev.*I
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Understanding Temperature Specifications: An Introduction
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