Application Notes

AN10327
TDA856x and TDA8571J power amplifiers
Rev. 01.00 — 15 October 2004
Application note
Document information
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Content
Keywords
Automotive, audio, power amplifier, Stereo, Quad, BTL, class AB, bipolar
Abstract
This document contains application information for the power amplifier
TDA856x series and the TDA8571J
AN10327
Philips Semiconductors
TDA856x ,TDA8571J
Revision history
Rev
Date
Description
1.0
20041015
First version
Contact information
For additional information, please visit: http://www.semiconductors.philips.com
For sales office addresses, please send an email to: [email protected]
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1. Introduction
1.1 Amplifier overview
This document describes the application specific subjects of the following audio power
amplifiers : TDA856x family and TDA8571J.
These amplifiers, which are made in a bipolar process, are mainly used in automotive
applications such as car radios, boosters and multimedia applications. The differences
between the types are mainly the number of output channels, different load values and
output power. The following matrix shows an overview of the mentioned amplifiers and
their properties.
Table 1:
Amplifier Overview
* DDD = Dynamic Distortion Detection
Channels
Gain
Load
[dB]
[Ohm]
Output
power
[W]
DDD*
Package
[%]
TDA8560Q
2 x BTL
40
2
2 x SE
2 X 40
10
DBS13P
TDA8562Q
4 x SE
20
4
4 x SE
4 x 12
10
DBS17P
TDA8563Q
2 x BTL
20
2
2 x SE
2 x 40
10
DBS13P
TDA8563AQ
2 x BTL
20
2
2 x SE
2 x 40
2.2
DBS13P
TDA8566Q
2 x BTL
26
2
2 x diff.
2 x 40
7.5
DBS17P
TDA8566TH
2 x BTL
26
2
2 x diff.
2 x 40
10
HSOP20
TDA8567Q
4 x BTL
26
4
4 x SE
4 x 25
10
DBS23P
TDA8568Q
4 x BTL
40
4
4 x SE
4 x 25
10
DBS23P
TDA8569Q
4 x BTL
26
2
4 X SE
4 x 40
10
DBS23P
TDA8571J
4 x BTL
34
4
4 x SE
4 x 26
10
DBS23P
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Application note
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2. Application Information
2.1 Input capacitors
The amplifiers need capacitors on the inputs to get a DC decoupling of the input source
(pre-amplifier stage). The impedance of the input stage together with the input
capacitors, create a low frequency roll-off point. A larger input capacitor means a lower
frequency roll-off point. The values that should be used are mentioned in the datasheet
of the amplifier type.
The following figure shows the influence of the input capacitors on the frequency roll-off
point for the TDA8566TH.
(A) Input capacitor 470nF
(B) Input capacitor 220nF
(C) Input capacitor 100nF
Fig 1. Roll-off frequency at different input capacitor values
The low frequency roll-off point can easily be calculated :
f low _ −3dB =
1
2 ⋅ π ⋅ Z in ⋅ Cin _ tot
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For example the low frequency roll-off point for the TDA8566TH, when using 220nF input
capacitors, equals :
f low _ −3dB =
1
2 ⋅ π ⋅ 120 ⋅ 10 3 ⋅ 110 ⋅ 10 −9
= 12 Hz
In this case the total input capacitance is halved since the input source is “seeing” the
input capacitors in series. This is due to the differential input configuration which is drawn
in the next figure.
Fig 2. Differential input stage TDA8566TH
Furthermore it is recommended to use input capacitors with a low DC leakage (film
capacitors), since any DC leakage at the inputs will result in a DC offset at the outputs.
Electrolytic capacitors usually have a relatively high DC leakage and should therefore not
be used.
2.2 Differential inputs
The TDA8566 is provided with differential input circuits. This has the advantage that
disturbances on the inputs, with relation to ground, are greatly eliminated.
However, if there’s a mismatch of the input capacitors, the common mode rejection ratio
(CMRR) decreases for low frequencies, since the impedance of the input capacitors will
increase then.
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The next figure shows the CMRR of the different input capacitor configurations :
(A) 100nF input capacitors, unmatched (<3%)
(B) 220nF input capacitors, unmatched (<2%)
(C) 220nF input capacitors, matched (<0.2%)
(D) ideal input capacitors
Fig 3. CMRR with different input capacitors
It may be clear that using matched input capacitors give the best CMRR results (line C).
So, in order to take optimum advantage of the differential inputs, the input capacitors
should be equal (matched) and have a low tolerance. Also, when a very high CMRR is
required it is therefore best to use input capacitors with a high capacitance.
When only a pre-amplifier without differential outputs is available, the TDA8566 can also
be driven single ended. In this way one of the inputs should be tied to signal ground via
the capacitor, while the other input is driven.
Since this is a compromise, one must consider that the CMRR ratio will get worse.
2.3 Loss of ground
The definition of a loss of ground with a power amplifier can be described as following :
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The ground of the power supply (car-battery) is connected to the output of
the amplifier, instead of to the amplifier-ground, after which the amplifier is
switched on.
In a practical situation a loss of ground condition could occur during assembly in the
factory, the car manufacturer (OEM) or in the case of an aftersales customer.
The following picture shows a loss of ground condition
Vp
Input
+
Output
+
Vp
14.4V
Cvp
V1
Power Amplifier
Ground
loss of ground
0
Fig 4. Loss of ground
According to figure 4, during a LOG, the peak current which charges the buffer capacitor
Cvp, will flow from Cvp into the amplifier ground pin and can destroy the amplifier.
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Vp
+
Vp
current mirror
Q1
PNP
Q2
PNP
Q3
NPN Upper power
C Vp
output
Q4
NPN
Q5
NPN parasitic
Q6
Diode parasitic
D1
0
NPN Lower power
Output
power
stage
one
channel
Gnd
Fig 5. Amplifier simplified internal schematic
According to the internal schematic of the amplifier, the failure mechanism is described
step by step.
During a Loss of ground, when the amplifier is turned on :
1. The buffer capacitor Cvp is charged and the current flows from Cvp to the amplifier
ground pin via the parasitic diode D1 to ground
2. Since D1 is conducting, the voltage on the collector of the lower power Q6 equals –
0.7V (under substrate level)
3. This causes a turn-on of a parasitic NPN Q5
4. The current mirror is ‘activated’ and pulls a current
5. Then the upper power Q3 will be turned on and a very large current will flow, since
the full Vp is across it
6. This will destroy the upper power transistor
In order to withstand the LOG it has to be prevented that the upper power is conducting.
The root cause is the conduction of the parasitic diode D1, which causes a substrate
level of –0.7V.
To prevent the conduction of D1 it is adviced to use a schottky diode between each of
the outputs and ground, according to figure 6. (So for a 4 channel BTL amplifier 8
schottky diodes are to be used)
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Since the schottky diode has a lower treshold (0.1 .. 0.3V) it will prevent a current flow
through D1 and so the turn-on of the upper power.
During turn-on of the amplifier, the capacitor will be charged via the schottky diode
instead of via D1.
For the schottky D2 it is recommended to use a Philips type BYV10-40 or a double SMD
type BAT140A.
Vp
+
Vp
current mirror
Q1
PNP
Q2
PNP
Q3
NPN Upper power
C Vp
output
Q4
NPN
Q5
NPN parasitic
Diode parasitic
D1
Q6
0
D2
NPN Lower power
DIODE SCHOTTKY
Output
power
stage
one
channel
Gnd
BYV10-40
BAT140A
Fig 6. Schottky diode
2.4 Critical conditions
2.4.1 Stability
When using capacitors from the outputs to ground (EMC) one must consider that the
TDA856x / TDA8571 is stable for capacitances smaller than 2.2nF and larger than
100nF. So, when capacitors are used outside of this range, boucherot filters at the
outputs could be necessary.
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2.4.2 Ground loops
Ground loops are unwanted signal paths that can occur during measurements of the
power amplifier, which can result in a higher THD performance of the amplifier. A many
seen fault is after connecting two ground connectors of an oscilloscope probe : one at
the signal ground of the input of the amplifier and one on the ground of the power supply.
The same condition holds when connecting an audio analyser (Audio Precision). In this
case when the ground connector (cable shield) is connected to the amplifier input signal
ground and when the output is measured, while its ground connector (cable shield) is
connected to the power supply ground.
The following drawing shows such a ground loop condition
Fig 7. Ground loop
In practice one should always try various ground connections when measuring THD.
However, in many cases it is adviced to use only one ground connection from the
measuring device to the power amplifier board.
To check if a ground loop is present, measure the distortion residue on an oscilloscope
together with the output signals of the amplifier. The distortion residue is usually a
monitor output on an audio analyser, eg. Audio Precision System Two, which shows the
difference between the shape of the original waveform that is put on the input of the
power amplifier and the waveform that is present on the output. (be aware of that the
Audio Precision System Two does not scale this distortion residue !)
The distortion residue shows a groundloop; the waveform shows the rectified
frequency of the signal that is put on the amplifier inputs. The following picture shows an
example of a ground loop.
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(A) Amplifier output signal
(B) Distortion residue
Fig 8. Oscilloscope picture of the distortion residue
2.5 PCB Layout recommendations
The following recommendations can be given when designing a PCB
•
•
•
•
Don’t situate input tracks nearby output tracks to prevent interference
Use a HF decoupling capacitor of about 100nF .. 220nF nearby the device,
between each Vp and power ground pin
When for the HF decoupling capacitors SMD components are used, be aware of
differences in behaviour w.r.t. the capacitor material. Good results are found
with NPO capacitors which have a low ESR (electrical series resistance), next
are X7R capacitors and last are Y5V capacitors which have a considerable ESR
In order to minimize the losses in the tracks for Vp and power ground during
high output power, use 75um or thicker copper layer and use a track-width of at
least 5mm
When using a ground plane, prevent ground loops which have a negative effect on the
THD performance. Use only one connection from the ground plane to ground, eg at the
buffer capacitor of Vp. The following drawing shows an example of a proper grounding
and a poor grounding.
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a. Good
b. Not good
Fig 9. Ground loop
•
•
In spite of the fact that amplifiers with differential (balanced) inputs perform a lot
better on ground noise than an amplifier with unbalanced inputs, it is
recommended to separate the small signal ground connection from the power
ground connection that leads to the power supply (car battery), to prevent
possible interference of any disturbances that come from the power supply
The ground references of the amplifier should all have the same potential. This
is to prevent dc shifts between the different grounds. In practice this can be
done by choosing a star ground connection between power ground and signal
ground (DC voltage shifts could otherwise occur through the large currents that
flow through the power ground tracks) The next drawing shows an example
between a proper lay-out and a poor one
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c. Good
d. Not good
Fig 10. PCB Layout
As can be seen in picture 10c is that the signal ground potential of the ground pin (7,11)
is equal to the potential of the signal groundconnector of the input signal.
In picture 10d the potential between pin 7, 11 and the signal ground connector is
unequal, depending on the current-flow through the track, x+y.
Suppose that, at a certain output power, the current through the x+y ground track equals
3A , while the resistance of the track x+y equals 100mOhm, then the voltage across x+y
equals 0.3V and will increase with increasing output power
2.6 Heatsink calculation
2.6.1 Power dissipation
As an example, the heatsink for a TDA8566Q is calculated.
When designing a heatsink, the amount of dissipated power must be calculated first.
For one channel of a conventional class B, BTL amplifier, the dissipated power equals :
Pdiss = Psup ply − Pout = V p ⋅
2
π
⋅
2 ⋅ Pout
− Pout
Rload
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For a stereo amplifier this means :
Pdiss = V p ⋅
4
π
⋅
2 ⋅ Pout
− 2 ⋅ Pout
Rload
According a rule of thumb the following can be assumed : the power dissipation of a
music signal is about half of the worst case dissipation of a sine wave signal.
Pdiss _ music = V p ⋅
2
π
⋅
2 ⋅ Pout
− Pout
Rload
This means that when :
V p = 14.4V
Pout = 2 x5W
Rload = 4Ω
The dissipated power for music signals equals :
Pdiss _ music = V p ⋅
2
π
⋅
2⋅2⋅5
− 10 = 10.5W
4
2.6.2 Thermal resistance
The equation for the thermal resistance [Rth] equals Ohms law, when temperature [T] is
substituted for voltage and power [P] is substituted for current :
Rth =
T
P
In fact, T is the temperature difference across the thermal resistance while P is the
dissipated power of the amplifier, so :
Rth =
∆T
Pdiss
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When looking at the next drawing it may be clear that the total thermal resistance is the
sum of the thermal resistances from the junction (outputs) of the amplifier to the ambient,
while the temperature difference is the difference between the junction temperature of
the amplifier and the ambient temperature.
Fig 11. Thermal resistance
So this means that the equation can be extended to :
Rth ( j −c ) + Rth (c −h) + Rth ( h−a ) =
Tvj − Tamb
Pdiss
When the value of the heatsink is determined for music signals, the equation leads to :
Rth( h−a ) =
Tvj − Tamb
Pdiss _ music
− Rth ( j −c ) − Rth (c −h )
The thermal resistance from the junction to case (package) is usually drawn like three
(stereo amplifier) or five (quad amplifier) thermal resistances, but can be translated
(according to Ohms law) to one thermal resistance, according to the next figure.
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Fig 12. Thermal resistance of package
Finally, when :
Tvj = 150°C
the absolute maximum junction temperature at which the
amplifier does not breakdown (value mentioned in datasheet)
Tamb = 70°C
the ambient temperature in which the amplifier is used, ie. In the
dashboard of a car
Pdiss _ music = 10.5W
the dissipated power for music signals
Rth ( j −c ) = 1.3K / W
the thermal resistance of the amplifier according to the datasheet
Rth (c −h) = 0.1K / W
the thermal resistance of thermal paste
The thermal resistance of the required heatsink equals :
Rth ( h−a ) =
150 − 70
− 1.3 − 0.1 = 6.2 K / W
10.5
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3. Disclaimers
Life support — These products are not designed for use in life support
appliances, devices, or systems where malfunction of these products can
reasonably be expected to result in personal injury. Philips Semiconductors
customers using or selling these products for use in such applications do so
at their own risk and agree to fully indemnify Philips Semiconductors for any
damages resulting from such application.
Right to make changes — Philips Semiconductors reserves the right to
make changes in the products - including circuits, standard cells, and/or
software - described or contained herein in order to improve design and/or
performance. When the product is in full production (status ‘Production’),
relevant changes will be communicated via a Customer Product/Process
Change Notification (CPCN). Philips Semiconductors assumes no
responsibility or liability for the use of any of these products, conveys no
licence or title under any patent, copyright, or mask work right to these
products, and makes no representations or warranties that these products
are free from patent, copyright, or mask work right infringement, unless
otherwise specified.
Application information — Applications that are described herein for any of
these products are for illustrative purposes only. Philips Semiconductors
make no representation or warranty that such applications will be suitable for
the specified use without further testing or modification.
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4. Contents
1.
1.1
2.
2.1
2.2
2.3
2.4
2.4.1
2.4.2
2.5
2.6
2.6.1
2.6.2
3.
4.
Introduction .........................................................3
Amplifier overview...............................................3
Application Information ......................................4
Input capacitors...................................................4
Differential inputs ................................................5
Loss of ground ....................................................6
Critical conditions................................................9
Stability ...............................................................9
Ground loops ....................................................10
PCB Layout recommendations .........................11
Heatsink calculation ..........................................13
Power dissipation..............................................13
Thermal resistance ...........................................14
Disclaimers ........................................................17
Contents.............................................................18
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© Koninklijke Philips Electronics N.V. 2004
All rights are reserved. Reproduction in whole or in part is prohibited without the prior
written consent of the copyright owner. The information presented in this document does
not form part of any quotation or contract, is believed to be accurate and reliable and may
be changed without notice. No liability will be accepted by the publisher for any
consequence of its use. Publication thereof does not convey nor imply any license under
patent- or other industrial or intellectual property rights.
Date of release:15 October 2004
Document order number: <12NC>
Published in The Netherlands
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