dm00074301

AN4233
Application note
Sound Terminal®: a method for measuring
the total thermal resistance (Rth) in the final application
By Marco Brugora
Introduction
The purpose of this document is to provide a methodology for measuring the total thermal
resistance junction-to-ambient (Rth j-a) of a Sound Terminal® amplifier in a final application.
The methodology allows identifying Rth, considering the effect of the PCB and in particular
the influence of the GND plane, the number of layers of the board, and the copper
connective paths.
January 2013
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1/12
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Contents
AN4233
Contents
1
Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1
Equipment needed and HW and SW configuration . . . . . . . . . . . . . . . . . . 3
1.2
Test procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2
Schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
Test results with the STA333IS and 2- or 4-layer PCB . . . . . . . . . . . . . . 6
3.1
STA333IS: 500 mA output current and 2-layer PCB . . . . . . . . . . . . . . . . . . 6
3.2
STA333IS: 1000 mA output current and 2-layer PCB . . . . . . . . . . . . . . . . . 7
3.3
STA333IS: 500 mA output current and 4-layer PCB . . . . . . . . . . . . . . . . . . 8
3.4
STA333IS: 1000 mA output current and 4-layer PCB . . . . . . . . . . . . . . . . . 9
4
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
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1
Description
Description
As mentioned, this test methodology allows measuring with high accuracy Rth j-a of the
Sound Terminal® amplifier when it is assembled in a PCB.
To perform the test it is not necessary to modify the hardware in the final application
although some registers must be configured in order to have the appropriate setup of the
device.
Setting the registers to have ternary modulation and with no input signal (or amplitude
< –60 dBFs), each output of the power stage will be set to GND. Connecting the output to a
positive source and with a resistor connected in series to limit the current, it is possible to
sink a DC current from the output. A reliable and cheap method to measure the output
current is to convert the current to voltage using a high-precision sense resistor (for instance
0.1 Ω) connected in series to each output. Using a voltmeter it is possible to also measure
the voltage level on the output of each bridge and this information, along with the current
level, allows calculating with high accurancy the RDSon of each power output (only for the
low-side portion of the output bridge).
1.1
Equipment needed and HW and SW configuration
To perform the measurement, the equipment and the settings are summarized as follows:
Register setting: the device must be set to have TERNARY modulation scheme.
Output loads: four dummy resistors must be connected to each bridge output and
connected to a positive supply voltage (Vdd or Vcc).
Input signal: it must be null or < –60 dBFs.
High precision resistor (Rsense): four sense resistors with very low resistive value (for
instance 0.1 Ω) must be connected in series between each output and the load. These
will be used to read the current (Iout = VRsense/Rsense)
Current meter (DC): a current meter must be inserted in series to each supply
sources; specifically Vdd (+3.3 V) and Vcc must be monitored. The series resistance of
this equipment must be very low to avoid adding any unwanted voltage drop.
Voltmeters (DC): a voltmeter will be used to measure both Vdd and Vcc after the
current meter and to measure the voltage across each sense resistor. The accuracy of
this equipment must be precise enough to measure the low voltage levels present
across each sense resistor. This equipment will be used to measure VRsense and Vout.
Thermal camera: a thermal camera must be used to measure the device temperature.
Care must be taken to avoid any possible mistake during the temperature
measurement; it is mandatory to define the right emission coefficient and to avoid
reflection from the device package (special paint could be used or a small portion of
non-reflective tape could be applied on top of the case). It is mandatory to use a
thermal camera with a resolution high enough to discriminate the surface of the device.
It is not recommended to use an IR thermometer because with this tool it is not
possible to measure with sufficient precision the device due to the optical angle and
also the position of the beam.
From the description above it is clear that all the measurements have been performed in the
DC domain. This aspect is very important because whatever parasitic effects due to the
PWM switching signal will be cancelled as well as the losses in the snubber networks,
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Description
AN4233
output dumping network and in the magnetic materials, all of which are usually difficult to
estimate.
1.2
Test procedure
The steps to perform the test are as follows:
1.
Connect the board to the supply generators (Vcc, Vdd; Vdd could be provided by the
APWlink if this board is connected to the DUT)
2.
Connect the outputs to the series made by the sense resistors and limiter resistors to a
positive supply source. The voltage source could be the same used to supply the
device (Vcc), in this manner simplifying the connections and the number of PSUs. The
limiter resistor must be selected to reach the target current to sink from each output.
3.
Set the digital input to have no audio input signal.
4.
Turn on the PSUs.
5.
Set the device register to have a TERNARY modulation. This action could be executed
using an APWlink board and the APWorkbench software.
6.
Measure using the voltmeter each supply voltage and adjust the level if it is too low due
to the additional voltage drop caused by the current meter internal resistance.
7.
Read the current sunk from each supply source.
8.
With a voltmeter, for each output measure the voltage across the sense resistor (this
level allows knowing the output current) and the voltage between the output pin and
GND. This last measurement allows measuring the voltage drop inside the device due
to RDSon. The measurement must be carried out, positioning the voltage probe as
close as possible to the output pin or output ball to avoid any voltage drop due to the
copper track.
9.
With the thermal camera measure the device temperature. It is mandatory to wait to
have a stable measurement and then save the thermal picture.
The above sequence can be repeated, setting some output current levels and modifying the
Vcc supply level.
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2
Schematic diagram
Schematic diagram
Figure 1 shows the schematic diagram which indicates the current meters connected in
series to Vdd and Vcc and the 8 test points dedicated to measure V_Rsense and the Vout
level for each channel.
Rs1 through Rs4 are the sense resistors and R1 through R4 are the resistors used to limit
the current flowing from Vcc to the output.
Figure 1.
Schematic diagram
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Test results with the STA333IS and 2- or 4-layer PCB
3
AN4233
Test results with the STA333IS and 2- or 4-layer PCB
The following examples show the results achieved when testing the STA333IS device
assembled in two different boards, the first one has 2 layers while the second one has 4
layers. The test has also been performed with two different output current levels (500 mA
and 1000 mA) for each board.
3.1
STA333IS: 500 mA output current and 2-layer PCB
Table 1.
STA333IS: 500 mA output current and 2-layer PCB - current measurement
[mV]
[V]
[mA]
Vdd
[V]
I_Vdd
[A]
RDS
[mΩ]
I_Vcc
[mV]
PRDS
[mW]
Vcc
[mΩ]
Out 1A
100
51.53
0.5153
65.88
33.947964
127.84786
8.4
19.6
3.3
31
Out 1B
100
51.53
0.5153
72.89
37.560217
141.45158
Out 2A
100
51.42
0.5142
73.43
37.757706
142.80436
Out 2B
100
51.65
0.5165
67.28
34.75012
130.26137
Rsense V_Rsense I_Channel
Table 2.
VRDS
STA333IS: 500 mA output current and 2-layer PCB - power dissipation
Power_Output [W]
0.14402
W
Power_Vdd [W]
0.1023
W
Power_Vcc [W]
0.16464
W
Figure 2.
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[mA]
AN4233
Test results with the STA333IS and 2- or 4-layer PCB
Table 3.
STA333IS: 500 mA output current and 2-layer PCB - summary table
Total power [W]
Temperature (case) [°C]
Temperature (ambient) [°C]
Rth [k/W]
0.41096
W
44.8
°C
26
°C
45.747
°C/W
3.2
STA333IS: 1000 mA output current and 2-layer PCB
Table 4.
STA333IS: 1000 mA output current and 2-layer PCB - current measurement
Rsense
V_Rsense I_Channel
VRDS
[mV]
[V]
[mA]
Vdd
[V]
I_Vdd
[A]
RDS
[mΩ]
I_Vcc
[mV]
PRDS
[mW]
Vcc
[mΩ]
Out 1A
100
102.8
1.028
147.7
151.8356
143.67704
17.05
19.87
3.3
32
Out 1B
100
103.9
1.039
162.63
168.97257 156.52551
Out 2A
100
103.7
1.037
163.1
169.1347
Out 2B
100
104.24
1.0424
149.63
155.97431 143.54375
Table 5.
157.28062
STA333IS: 1000 mA output current and 2-layer PCB - power dissipation
Power_Output [W]
0.64592
W
Power_Vdd [W]
0.1056
W
Power_Vcc [W]
0.33878
W
Figure 3.
[mA]
STA333IS: 1000 mA output current and 2-layer PCB 2 - thermal measurement
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Test results with the STA333IS and 2- or 4-layer PCB
Table 6.
AN4233
STA333IS: 1000 mA output current and 2-layer PCB - summary table
Total power [W]
Temperature (case) [°C]
Temperature (ambient) [°C]
Rth [k/W]
1.0903
W
72.3
°C
24
°C
44.300
°C/W
3.3
STA333IS: 500 mA output current and 4-layer PCB
Table 7.
STA333IS: 500 mA output current and 4-layer PCB - current measurement
Rsense
V_Rsense I_Channel
VRDS
[mV]
[V]
[mA]
Vdd
[V]
I_Vdd
[A]
RDS
[mΩ]
I_Vcc
[mV]
PRDS
[mW]
Vcc
[mΩ]
Out 1A
100
51.1
0.511
65.15
33.29165
127.49511
8.36
19.21
3.3
32
Out 1B
100
50.46
0.5046
69.55
35.09493
137.83195
Out 2A
100
50.94
0.5094
69.9
35.60706
137.22026
Out 2B
100
51.1
0.511
64.8
33.1128
126.81018
Table 8.
STA333IS: 500 mA output current and 4-layer PCB - power dissipation
Power_Output [W]
0.13711
W
Power_Vdd [W]
0.1056
W
Power_Vcc [W]
0.1606
W
Figure 4.
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AN4233
Table 9.
Test results with the STA333IS and 2- or 4-layer PCB
STA333IS: 500 mA output current and 4-layer PCB - summary table
Total power [W]
0.4033
W
Temperature (case) [°C]
42
°C
Temperature (ambient) [°C]
26
°C
39.672
°C/W
Rth [k/W]
3.4
STA333IS: 1000 mA output current and 4-layer PCB
Table 10.
STA333IS: 1000 mA output current and 4-layer PCB - current measurement
Rsense
V_Rsense I_Channel
VRDS
[mV]
[V]
[mA]
Vdd
[V]
I_Vdd
[A]
RDS
[mΩ]
I_Vcc
[mV]
PRDS
[mW]
Vcc
[mΩ]
Out 1A
100
101.8
1.018
137
139.466
134.5776
16.58
19.5
3.3
32
Out 1B
100
100.5
1.005
146.5
147.2325 145.77114
Out 2A
100
101.4
1.014
147.5
149.565
145.46351
Out 2B
100
101.9
1.019
137
139.603
134.44553
Table 11.
STA333IS: 1000 mA output current and 4-layer PCB - power dissipation
Power_Output [W]
0.57587
W
Power_Vdd [W]
0.1056
W
Power_Vcc [W]
0.32331
W
Figure 5.
[mA]
STA333IS: 1000 mA output current and 4-layer PCB - thermal measurement
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Conclusion
Table 12.
AN4233
STA333IS: 1000 mA output current and 4-layer PCB - summary table
Total power [W]
Temperature (case) [°C]
Temperature (ambient) [°C]
Rth [k/W]
4
1.00478
W
60.9
°C
26
°C
34.734
°C/W
Conclusion
This methodology is very simple to implement because it measures only DC current and
voltage levels. The result achieved is very reliable.
The flexibility and the easy configurability of the external components allow implementing
tests with any output current level, estimating the thermal behavior with the operating
condition using the same PCB used in the final application.
Another positive aspect is that this methodology provides an estimation of the combination
of RDSon + output PCB tracks. This data is useful for evaluating the proper connection of
the output pads or balls.
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5
Revision history
Revision history
Table 13.
Document revision history
Date
Revision
16-Jan-2013
1
Changes
Initial release.
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AN4233
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