AN10327_技术文档

AN10327

Rev. 01.00 — 15 October 2004

TDA856x and TDA8571J power amplifiers

Application note

Document information

Info

Content

Keywords

Abstract

Automotive, audio, power amplifier, Stereo, Quad, BTL, class AB, bipolar

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: sales.addresses@www.semiconductors.philips.com

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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

Inputs

Output

power

[W]

DDD*

Package

[dB]

40

20

26

40

26

34

[Ohm]

[%]

10

2.2

7.5

10

TDA8560Q

TDA8562Q

TDA8563Q

TDA8563AQ

TDA8566Q

TDA8566TH

TDA8567Q

TDA8568Q

TDA8569Q

TDA8571J

2 x BTL

4 x SE

2

4

2

4

2

4

2 x SE

4 x SE

2 x SE

2 x diff.

4 x SE

4 X SE

4 x SE

2 X 40

4 x 12

2 x 40

4 x 25

4 x 40

4 x 26

DBS13P

DBS17P

DBS13P

DBS17P

HSOP20

DBS23P

2 x BTL

4 x BTL

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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 :

1

f_{low_−3dB}

=

2⋅π ⋅ Z_in⋅C_{in_tot}

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For example the low frequency roll-off point for the TDA8566TH, when using 220nF input

capacitors, equals :

1

f_{low_−3dB}

=

=12Hz

2⋅π ⋅120⋅10³⋅110⋅10⁻⁹

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

Cvp

V1

14.4V

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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Philips Semiconductors

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Vp

+

current mirror

Vp

-

Q1

PNP

Q2

PNP

Q3

NPN Upper power

C Vp

output

Q5

Q4

NPN parasitic

NPN

Q6

NPN Lower power

Diode parasitic

D1

0

Output

power

stage

one

Gnd

channel

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

+

current mirror

Vp

-

Q1

Q2

PNP

Q3

NPN Upper power

C Vp

output

Q5

NPN parasitic

Q4

NPN

Q6

NPN Lower power

Diode parasitic

D1

0

D2

DIODE SCHOTTKY

BYV10-40

BAT140A

Output

power

stage

one

Gnd

channel

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 :

2⋅ P

R_load

2

π

out

P

= P

− P = V_p⋅

⋅

− P

out

diss

sup ply

out

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For a stereo amplifier this means :

2⋅ P

R_load

4

π

out

P

= V_p⋅

⋅

− 2⋅ P

out

diss

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.

2⋅ P

R_load

2

π

out

P

=V_p⋅

⋅

− P

out

diss _ music

This means that when :

V_p=14.4V

P

= 2x5W

out

R_load= 4Ω

The dissipated power for music signals equals :

2

π

2⋅2⋅5

4

P

= V_p⋅

⋅

−10 =10.5W

diss _ music

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 :

T

P

R_th

=

In fact, T is the temperature difference across the thermal resistance while P is the

dissipated power of the amplifier, so :

∆T

R_th

=

P

diss

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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 :

T_vj−T_amb

R_{th( j−c)}+ R_th(c−h)+ R_th(h−a)

=

P

diss

When the value of the heatsink is determined for music signals, the equation leads to :

T_vj−T_amb

R_th(h−a)

=

− R_{th( j−c)}− R_th(c−h)

P

diss _ music

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 :

T_vj=150°C

the absolute maximum junction temperature at which the

amplifier does not breakdown (value mentioned in datasheet)

T_amb= 70°C

the ambient temperature in which the amplifier is used, ie. In the

dashboard of a car

P

=10.5W the dissipated power for music signals

diss _ music

R_{th( j−c)}=1.3K /W

the thermal resistance of the amplifier according to the datasheet

R_th(c−h)= 0.1K /W the thermal resistance of thermal paste

The thermal resistance of the required heatsink equals :

150 − 70

10.5

R_th(h−a)

=

−1.3− 0.1 = 6.2K /W

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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

Introduction .........................................................3

Amplifier overview...............................................3

2.

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

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.

Disclaimers ........................................................17

Contents.............................................................18

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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


型号：	AN10327
厂家：	NXP
描述：	TDA856x and TDA8571J power amplifiers TDA856x和TDA8571J功放
文件：	总19页 (文件大小：754K)
中文：	中文翻译
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