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Texas Instruments Thermal Considerations of the PGA411 Application notes
Application Report
SLAA738 – May 2017
Thermal Considerations of the PGA411
Duy Nguyen
ABSTRACT
This application report covers thermal performance of the PGA411-Q1 through the testing of two boards,
the PGA411-Q1 Evaluation Module (EVM) and the industrial EMC Compliant Single-Chip Resolver-toDigital Converter (RDC) Reference Design. Tests included parameter changes of boost voltage, peak-topeak output voltage, and load currents. The tests were evaluated by observing the temperature changes
for each adjusted parameter. Results show that the board layout was a major factor in thermal
performance.
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Contents
Introduction ................................................................................................................... 3
Set Up ......................................................................................................................... 3
Boost Voltage’s Relation to Power ........................................................................................ 5
Peak-to-Peak Voltage Output in Relation to Power .................................................................... 6
Load Current’s Relation to Power ......................................................................................... 8
PCB Thermal Analysis ...................................................................................................... 9
EVM versus Industrial Board Comparison .............................................................................. 12
Conclusion .................................................................................................................. 14
References .................................................................................................................. 14
List of Figures
1
2
3
4
5
6
7
8
9
10
11
12
13
14
......................................................................................... 3
Buck Converter Schematic of Industrial TI Design ..................................................................... 4
Efficiency versus Boosted Voltage Graph ................................................................................ 5
Efficiency versus Pre-gain Graph.......................................................................................... 7
Temperature versus Power Across Device Graph ...................................................................... 8
Model of Package on PCB ................................................................................................ 10
Resistive Network of Thermal Model .................................................................................... 11
Heat Flow Diagram in Regard to Parallel and Perpendicular Breaks ............................................... 12
EVM Infrared Image ....................................................................................................... 12
Industrial Infrared Image .................................................................................................. 12
EVM Layout ................................................................................................................. 13
Industrial Layout ............................................................................................................ 13
Industrial Mid-layer Ground Plane ....................................................................................... 13
Industrial Bottom Layer Ground Plane .................................................................................. 13
Power Flow Overview of System
List of Tables
.............................................................................................
1
Instruments Used For Testing
2
Parameter of Constants for Boosted Voltage Test ...................................................................... 5
5
3
Board Temperature versus Boosted Voltage at Ambient Temperature of 22.4°C .................................. 6
4
Parameter Constants for Peak-to-Peak Output Voltage Test .......................................................... 7
5
Board Temperature in Regard to Pre-gain Setting ...................................................................... 7
6
Parameter Constants for Load Current Sweep Test .................................................................... 8
7
Definition of Symbols ........................................................................................................ 9
8
Thermal Symbol Definitions ............................................................................................... 10
Trademarks
All trademarks are the property of their respective owners.
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Introduction
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Introduction
In environments with high ambient temperatures, managing the junction temperature on integrated circuits
is crucial. Keeping the heat across the device in check will extend the life of the device and ensure that
the device will not fail due to exceeding its maximum operating limit. The PGA411-Q1 is an automotive
qualified device with a junction temperature operation range from -40°C to +150°C and with an ambient
operating temperature of -40°C to +125°C. To ensure the device does not exceed the 150°C junction
rating, knowing which parameter settings affect the heating of the device is important to help minimize the
heat across the device. Testing was done between two boards, the PGA411-Q1 Evaluation Module and
the industrial EMC Compliant Single-Chip Resolver-to-Digital Converter (RDC) Reference Design, to see
how board layout affected the thermal performance. The results show that the industrial design
outperformed the EVM, with one test showing a difference of about 10°C between the two boards for the
same power across the PGA411 device.
2
Set Up
The goal of this application report is to provide insight on parameters which affect the thermal
performance of the PGA411-Q1 and to understand how to minimize the thermal dissipation with proper
board layout practices. The power of an ideal system can be stated as the power in being equal to the
power out. Figure 1 shows how the power is represented in the system of this application report. The
power supply unit provides all of the input power of the system, whereas the consumed power will be from
the PGA411-Q1 device, the load, and the external components from the boost converter. The load is tied
to the PGA411-Q1’s exciter amplifier outputs, OE1 and OE2. Normally the load is the resolver’s stator coil,
though for testing, it will be a variable resistor to test various parameters.
External
Components
Power In
PSU
Power Out
PGA411-Q1
Load
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Figure 1. Power Flow Overview of System
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Set Up
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Pin
Pout total
Pin
V in u I in
Pout total
Pload
P device
(1)
(2)
Pload
P device
Pexternal _ components
(3)
V rms u I rms
(4)
V in u I in
efficiency
V rms u I rms
Pout avg
Pin
(5)
u 100%
(6)
Equation 1 shows the overall power of the system. Equation 2 calculates the input power by taking the
supply current and multiplying it by the input voltage of the PGA411-Q1. Equation 3 shows that the total
output power can be calculated by summing the power of the load, the power across the device, and the
power dissipated through the boost converter’s external components. The power on the external
components is expected to be significantly low and will be assumed to be zero. Equation 4 shows that the
power across the load can be found by multiplying the RMS voltage across the load by the RMS current
through the load. To find the power across the load, use equations 1, 2, 3, and 4 to isolate for Pload to get
Equation 5. It is important to solve for Pdevice because most of the power across the device will dissipate as
heat and a small amount will dissipate to the external components of the boost converter.
All tests done in this application report were done using the PGA411-Q1 Evaluation Module and the EMC
Compliant Single-Chip Resolver-to-Digital Converter (RDC) Reference Design with the intention of seeing
how both designs relate in terms of thermal dissipation. The power efficiencies are expected to be the
same for both boards because the same device being tested will be used in both designs. However, the
heat at the junction is expected to different.
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Figure 2. Buck Converter Schematic of Industrial TI Design
The industrial board does not use a 5-V input like the EVM board and instead uses the TPS54140A buck
converter which requires a 12-V to 42-V input. To get more accurate data, the R45 resistor from the
industrial board was removed, and a wire was soldered directly to the 5-V rail to supply the 5-V input from
an external power supply. This procedure allows for a more similar set up to the EVM because it bypasses
the buck converter, which otherwise would introduce power loss on the input due to the buck converter’s
power efficiency. Figure 2 shows the R45 resistor circled in red, placed directly after the buck converter to
the 5-V rail.
4
Thermal Considerations of the PGA411
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Boost Voltage’s Relation to Power
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Table 1. Instruments Used For Testing
3
Device
Model
Digital Multimeter (Voltage reading)
HP 34401A
Digital Multimeter (Current reading)
HP3448A
Infrared Thermal Imaging Camera
Flir i50
Power Supply Unit
Agilent E3646A
Resistor box
Clarostate model number 240-C
Boost Voltage’s Relation to Power
Understanding which parameters affect power input and output is important for intentionally lowering heat
seen by the device. The first parameter for consideration will be the boosted voltage of the exciter
amplifier. This test was done with a constant load resistance of 150 Ω. This resistance was chosen to
mimic a real resolver sensor’s resistance and allowed for testing with both 4-Vrms and 7-Vrms modes without
exceeding the current output limits on OE1 and OE2. The offset value was chosen to ensure the 7-Vrms
mode would not clip and kept constant for the 4-Vrms mode, as well as to ensure the same supply current
draw due to the offset parameter. For the test, the boosted voltage was changed from 10 V to 17 V in 1-V
steps for both 7-Vrms and 4-Vrms mode, and the input power and output power were measured for each
step. For the 4-Vrms mode, a typical pre-gain value was used. However, for the 7-Vrms mode, 1.50-V/V pregain was used to avoid clipping on the upper bounds of the output at 10 V of boosted voltage. The offset
value was chosen to avoid clipping on the lower bounds of the output for the 7-Vrms mode. The efficiency
was calculated by using Equation 6 as the ratio of output power to input power.
Table 2. Parameter of Constants for Boosted Voltage Test
Offset
4-Vrms Mode
7-Vrms Mode
1.2 V
1.2 V
Pre-gain
1.55
1.50
Frequency
10 KHz
10 KHz
Resistance
150 Ω
150 Ω
45
Industrial 4Vrms
Industrial 7Vrms
40
EVM 4Vrms
EVM 7Vrms
Efficiency (%)
35
30
25
20
15
10
5
0
10
11
12
13
14
15
Boosted Voltage (V)
16
17
D003
Figure 3. Efficiency versus Boosted Voltage Graph
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Boost Voltage’s Relation to Power
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From 10 V to 17 V, the efficiency decreases over each step for both 4-Vrms and 7-Vrms mode. The reason
for this is because the current, resistance, and peak-to-peak voltage are constant, so from 10 V to 17 V,
the output power does not increase, but the supply current to the device increases for each step up in
voltage, therefore increasing the input power.
Table 3. Board Temperature versus Boosted Voltage at Ambient Temperature of 22.4°C
Boosted Voltage (V)
EVM 4-Vrms
Temperature (°C)
Industrial 4-Vrms
Temperature (°C)
EVM 7-Vrms Temperature
(°C)
Industrial 7-Vrms
Temperature (°C)
10
37.6
36.8
37.5
36.5
11
38.7
37.5
39.4
37.8
12
40.2
38.7
40.9
39
13
41.7
39.5
42.5
40.8
14
43.3
40.6
44.7
42
15
44.6
41.9
46.7
43.9
16
46.1
43.2
49.1
45.4
17
47.3
44.3
51.2
47.1
The temperature of the device being tested will also be affected by the output power remaining constant
but the input power increasing. Table 3 shows how both boards and both output modes experience a
temperature increase as the boosted voltage increases. The increase in temperature for the EVM board
for 4-Vrms mode and 7-Vrms mode are 9.7°C and 13.7°C respectively. For the industrial design, at 4-Vrms
mode and 7-Vrms mode, the temperature increases were 7.5°C and 10.6°C respectively. The industrial
board has a lower temperature than the EVM board for all boosted voltages when compared to the same
output mode. The ambient temperature of the room where the testing occurred was about 22.4°C.
3.1
Take Away Due to Boosted Voltage
By understanding that the boosted voltage decreases efficiency and affects the device’s temperature, the
boosted voltage should be kept as low as possible to minimize efficiency loss and unnecessary increases
in temperature. Keeping the boosted voltage at 10 V would be optimal but not always possible due to the
load resistance and load current under operation. In a typical case, the peak-to-peak output voltage is
preferred to be set higher because it results in a higher signal-to-noise ratio. The boosted voltage must be
adjusted higher to meet this requirement. In this case, TI recommends selecting the minimal boosted
voltage required to avoid clipping. Doing so will minimize efficiency losses and unnecessary temperature
increases in the PGA411.
4
Peak-to-Peak Voltage Output in Relation to Power
The peak-to-peak output voltage is a parameter which can be fine-tuned by adjusting the pre-gain setting
and then largely amplified by the 4-Vrms and 7-Vrms stage. Making changes to the pre-gain and final-gain
stage affects the peak-to-peak output voltage and therefore also affects the power efficiency and heating
of the device. Testing was done to see how much of a difference the peak-to-peak output voltage had on
power efficiency for all steps of the pre-gain stage with both 4-Vrms and 7-Vrms mode. Temperature
measurements were taken at every five-step increment.
The boosted voltage parameters selected were picked based on commonly-used values. The load
selected was 150 Ω for both output modes to simulate a real resolver sensor. Offset and frequency were
held constant at 2.0 V and 10 KHz respectively.
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Table 4. Parameter Constants for Peak-to-Peak Output Voltage Test
4-Vrms Mode
7-Vrms Mode
Offset
2.0 V
2.0 V
Frequency
10 KHz
10 KHz
Boosted Voltage
12 V
15 V
Resistance
150 Ω
150 Ω
45
40
Efficiency (%)
35
30
25
20
15
10
EVM 4Vrms
EVM 7Vrms
5
0
1.15
1.25
1.35
1.45
1.55
1.65
Pregain (V/V)
Industrial 4Vrms
Industrial 7Vrms
1.75
1.85 1.9
D004
Figure 4. Efficiency versus Pre-gain Graph
Table 5. Board Temperature in Regard to Pre-gain Setting
Pre-gain
4-Vrms EVM
Temperature (°C)
7-Vrms EVM Temperature
(°C)
4-Vrms Industrial
Temperature (°C)
7-Vrms Industrial
Temperature (°C)
1.15
1.40
39.3
46
37.4
42.2
39.9
46.7
38.1
43
1.65
40.7
47
38.7
43.7
1.90
41
47
39
43.7
From Table 5, it can be observed that for both boards with higher peak-to-peak output voltage,
temperature increases occurred. When the 7-Vrms mode was used to amplify the peak-to-peak output
voltage, both boards stopped increasing in temperature at the 1.65 pre-gain setting and the temperature
remained constant to 1.90 pre-gain. Due to the output power increasing at about the same rate as the
input power, the power seen across the device stays constant, therefore the temperature remains at a
constant. This observation, however, would differ depending on the boosted voltage used due to its effect
on input power.
Comparing the EVM board and the industrial board, it can be observed the industrial board has lower
temperatures for all peak-to-peak output voltage settings when comparing the same output modes. The
difference for the 4-Vrms mode is about 2°C and for the 7-Vrms mode the difference is from 3.8°C to 3.3°C.
When adjusting the peak-to-peak output voltage with the 4-Vrms mode and 7-Vrms mode, the temperatures
for both boards were higher for the 7-Vrms mode. This increase is due to the power across the device being
greater at a higher peak-to-peak output voltage that the 7-Vrms mode provided.
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Peak-to-Peak Voltage Output in Relation to Power
4.1
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Take Away From Peak-to-Peak Output Voltage
With a higher peak-to-peak output voltage, the power efficiency increases as expected, but a more
important observation is that with a lower peak-to-peak output voltage, there is lower power across the
device and thus lower temperature on the device when compared to higher peak-to-peak output voltages.
For a more sensitive application where device temperature is more of a concern due to a high ambient
temperature, using lower peak-to-peak output voltage with the 4-Vrms output mode may be considered. In
applications which temperature is less of a concern, using a higher peak-to-peak output voltage may be
considered to get the most out of the power efficiency with relatively low increases to temperature. The
peak-to-peak output voltage should be kept at a level that allows for the sine and cosine differential inputs
to stay within the range of 600 mVpp to 1.5 Vpp.
5
Load Current’s Relation to Power
To observe thermal dissipation of both boards, doing a current sweep by adjusting the load resistance was
done. The load current is a parameter which is related to the load impedance and is dependent on the
resolver sensor. This data provides understanding on how both boards behave in terms of thermal
dissipation. The test will cover peak load current values from 25 mApeak to 145 mApeak, where the 145mApeak upper limit was chosen due to the OE1 and OE2 maximum recommended operating conditions.
The temperatures, power in and power out, were measured for each current load change. The power
across the device was calculated and graphed against the temperature across the device seen in
Figure 5.
Table 6 shows the parameters set for the initial test. The offset value was chosen to be kept at default to
allow for more footroom and avoid clipping at the bottom of the output waveform. The gain and boosted
voltage setting was selected on which output mode was considered. With the 7-Vrms mode, a boosted
voltage of 15 V and gain of 1.90 was chosen as an expected typical value of use. The same reasoning
was used for the 4-Vrms mode with a gain of 1.55 and boosted voltage of 12 V.
Table 6. Parameter Constants for Load Current Sweep Test
4-Vrms Mode
7-Vrms Mode
Offset
2.0 V
2.0 V
Gain
1.55
1.90
Frequency
10 KHz
10 KHz
Boosted Voltage
12 V
15 V
65
Temperature (qC)
60
55
50
45
40
35
30
350
Industrial 7Vrms
EVM 7Vrms
Industrial 4Vrms
EVM 4Vrms
550
750
950
Power Across Device (mW)
1150
D005
Figure 5. Temperature versus Power Across Device Graph
8
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5.1
Take Away From Load Current
The data in Figure 5 shows the temperature of the device is linear with respect to the power across the
device. Regarding the EVM and industrial board, the temperature on the device is higher for the EVM
board than it is for the industrial board. The 7-Vrms and 4-Vrms modes both show the same results with very
similar slopes for both boards. The main difference between these two boards is the layout and therefore
the temperature across the device is influenced by the board layout.
6
PCB Thermal Analysis
From the previous tests done, it was observed that the industrial board was more efficient at dispersing
the heat across the device than the EVM board. The reason behind the temperature differences in both
boards is due to the printed-circuit board (PCB) layout.
From a thermal standpoint, Ohm’s law is similar to Equation 7. This means temperature, power, and
thermal resistance can be related to electrical voltage, current, and resistance respectively. The thermal
resistance from the junction to the ambient air can be found by readjusting Equation 7, where Tj -Ta is the
“voltage drop” or the temperature across the thermal resistance. This is represented in Equation 8. The
temperature at the junction can be found by solving for Tj in Equation 8 and is shown in Equation 9.
T PuT
(7)
Tj Ta
Tja
Pd
(8)
Tj Pd u Tja Ta
(9)
Table 7. Definition of Symbols
Symbol
Definition
T
Temperature (°C)
P
Power (W)
θ
Thermal resistance (°C/W)
θja
Thermal resistance between ambient air and junction (°C/W)
Tj
Junction Temperature (°C)
Ta
Ambient Temperature (°C)
Pd
Power across device (W)
From Equation 9, it is now possible to quantitatively analyze why a temperature difference between the
EVM board and the industrial board occurred. The temperatures measured during the tests were
measured from an IR camera at junction Tj. For all tests done, the ambient temperature, Ta, stayed
relatively constant. It was observed that the temperature was different for about the same power across
the device, therefore the variable, which was different between the two boards, was the thermal resistance
between the junction and the ambient air, θja. Therefore, the thermal resistance between the junction and
the ambient air for the industrial board must be lower than the EVM board to produce a lower junction
temperature. From collected data where the power across both board’s device was about 1 W, the
ambient temperature at around 22.4°C, and the board temperatures at 61.2°C and 52°C for the EVM and
industrial, respectively, the thermal resistance from junction to ambient air can be calculated. For the EVM
board, θja is found to be 38.8°C/W while the industrial board has a value of 29.6°C/W. From this, it can be
concluded that to achieve better thermal performance, the thermal resistance between the junction and
the ambient air should be kept as low as possible. The θja found for both cases was from one set of data
points and is only an estimation. To get a more accurate number, multiple samples on multiple devices
should be collected.
The θja calculation differs from the one found in the PGA411-Q1 Resolver Sensor Interface Data Sheet at
25.5°C/W. This number was found by using the Joint Electron Device Engineering Council (JEDEC)
standards and therefore allows designers to compare packages from different companies to each other
assuming both companies followed the JEDEC standard. This standard uses a different layout design and
therefore the θja parameter will differ from the EVM and industrial board.
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PCB Thermal Analysis
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Case Top Temperature (Tt)
Case Bottom Temperature (Tc)
Junction Temperature (Tt)
Copper Layer
FR-4
Copper Layer
FR-4
Copper Layer
FR-4
Copper Layer
Thermal Vias
Figure 6. Model of Package on PCB
It is necessary to understand what contributes to the thermal resistance between the junction and the
ambient air to minimize it. Figure 6 models an integrated circuit sitting on top of a PCB. The model shows
what is in contact with the junction and the ambient air, which is important for modeling the thermal
resistance.
Figure 7 shows how the thermal model of the junction to ambient air is usually done with a resistive
network and follows the path from the junction to ambient air. The reason for two resistor paths seen in
Figure 7 is due to the junction being able to pass heat to the top of the case to the ambient air as well as
passing heat through the bottom of the case, through the PCB, to the ambient air.
Table 8. Thermal Symbol Definitions
10
Symbol
Definition
Tj
Junction temperature (°C)
Tc
Bottom of case temperature (°C)
Tt
Top of case temperature (°C)
Ta
Ambient air temperature (°C)
θjc
Junction to bottom of case thermal resistance (°C/W)
θjt
Junction to top of case thermal resistance (°C/W)
θca
Bottom of case to ambient air thermal resistance (°C/W)
θta
Top of case to ambient air thermal resistance (°C/W)
θja
Junction to ambient air thermal resistance (°C/W)
Thermal Considerations of the PGA411
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PCB Thermal Analysis
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Figure 7. Resistive Network of Thermal Model
The resistive network on the left of Figure 7 can be simplified to the single resistor seen on the right. The
result is θja, which is what is needed to be kept low. However, to do so requires manipulating the
resistance network seen on the left of Figure 7. From the four thermal resistances in the network, three
cannot be altered. The θjc, θjt, and θta are all determined by the package of the integrated circuit. The θca,
therefore, is the only thermal resistance which can be manipulated and by observation, with θca in series
with θjc and parallel with θjt and θta, to get a lower θja, then θca should be kept as small as possible. θca is the
thermal resistance from the bottom of the case through the PCB layers to the ambient air. The PCB is
made up of multiple layers. There are usually four layers for the PGA411-Q1 device and several layers of
dielectric. The bottom of the case is also usually in contact with thermal vias. Therefore, to minimize θca, it
is necessary to design the PCB with proper thermal practices.
The following list includes examples of proper thermal PCB practices:
• Use as many thermal vias on the thermal pad of the PCB as allowable.
• Use the second layer for the ground plane to be closer to the source of heat.
• Make the mid-layer ground plane stretch as wide as possible (edge-to-edge of PCB).
• Allow for more area to stretch across the ground plane with a larger PCB; although this may not
always be possible due to cost or size constraints.
• Use the bottom layer for ground and make it as wide as possible.
• Avoid discontinuity in the thermal path.
• Because heat flows laterally outwards from the center of the heat source to the edges of the ground
planes, make necessary breaks due to traces or other reasons in the thermal path run in the same
direction as the heat flow rather than perpendicular to it. An example can be seen in Figure 8.
• Use thicker copper layers to help better dissipate heat.
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EVM versus Industrial Board Comparison
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Trace
Via
Thermal Path
Ground Plane
No Breaks
Perpendicular
Parallel
Heat
Source
Heat
Source
Heat
Source
Figure 8. Heat Flow Diagram in Regard to Parallel and Perpendicular Breaks
7
EVM versus Industrial Board Comparison
Figure 9 and Figure 10 are images taken from an IR camera with a load of about 1000 mW across the
PGA411 at an ambient temperature of 22.4°C. The EVM image shows the thermal energy concentrated at
the center, evenly dispersed outwards, while the industrial image has more heat flowing away from the
center and is much less uniform. With proper thermal layout practices, the industrial board is capable of
reducing the temperature rise of the device by 11.2°C. In terms of thermal performance, this shows the
industrial board is about 29% better at dissipating heat compared to the EVM board.
Figure 9. EVM Infrared Image
12
Figure 10. Industrial Infrared Image
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Figure 11 shows exactly how the EVM and industrial boards are laid out. Looking at the EVM board, the
PGA411 sits directly on top of the thermal pad, which is only a small piece of exposed copper meant to
just fit the device on. The industrial board sits on the thermal pad but also has thermal vias in the middle
of the thermal pad. The thermal vias connect to the digital ground plane in the middle of the board and
also to the bottom ground plane. Figure 13 and Figure 14 show the digital ground plane and bottom
ground plane.
Figure 11. EVM Layout
Figure 12. Industrial Layout
Figure 13 and Figure 14 show the amount of area both planes use. The digital ground plane in Figure 13
goes from edge to edge of the board to help disperse the heat across the entire board. The bottom layer
also uses a lot of the area on the board and is exposed to the ambient air which helps disperse the heat
into the air, making it important for this area to be large.
Figure 13. Industrial Mid-layer Ground Plane
Figure 14. Industrial Bottom Layer Ground Plane
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Conclusion
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Conclusion
Thermal performance is an important consideration for any design and is essential for heat-sensitive
applications with high ambient temperatures. From a power efficiency standpoint and in regard to the
boosted voltage parameter, it was found that using the lowest boosted voltage setting would minimize
power efficiency loss and unnecessary increases on device temperature. When considering pre-gain,
using higher pre-gain values allowed for better power efficiency at the cost of minor temperature
increases, where for thermal sensitive applications, the 4-Vrms mode should be considered to minimize
heat across the device. In regard to thermal performance, the industrial design proved to outperform the
EVM due to the use of multiple thermal vias and the use of large ground planes on both the middle and
bottom layer of the PCB.
9
References
•
•
14
Texas Instruments, AN-2020 Thermal Design By Insight, Not Hindsight Application Report
Texas Instruments, AN-1520 A Guide to Board Layout for Best Thermal Resistance for Exposed
Packages Application Report
Thermal Considerations of the PGA411
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IMPORTANT NOTICE FOR TI DESIGN INFORMATION AND RESOURCES
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