TDA8922 datasheet(18/36 Pages) PHILIPS

18 / 36 page

2003 Mar 20

Philips Semiconductors

Objective speciﬁcation

× 25 W class-D power ampliﬁer

TDA8922

To trigger the maximum current protection in the

TDA8922, the required output current must exceed 4 A.

This situation occurs in case of:

• Short-circuits from any output terminal to the supply

lines (VDD or VSS)

• Short-circuit across the load or speaker impedances or

a load impedance below the specified values of

4 and 8

Ω.

Even if load impedances are connected to the amplifier

outputs which have an impedance rating of 4

Ω, this

impedance can be lower due to the frequency

characteristic of the loudspeaker; practical loudspeaker

impedances can be modelled as an RLC network which

will have a specific frequency characteristic: the

impedance at the output of the amplifier will vary with the

input frequency. A high supply voltage in combination with

a low impedance will result in large current requirements.

Another factor which must be taken into account is the

ripple current which will also flow through the output power

switches. This ripple current depends on the inductor

values which are used, supply voltage, oscillator

frequency, duty factor and minimum pulse width. The

maximum available output current to drive the load

impedance can be calculated by subtracting the ripple

current from the maximum repetitive peak current in the

output pin, which is 4 A for the TDA8922.

As a rule of thumb the following expressions can be used

to determine the minimum allowed load impedance

without generating audio holes:

for SE application.

for BTL application.

Where:

ZL = load impedance

fosc = oscillator frequency

tmin = minimum pulse width (typical 190 ns)

VP = single-sided supply voltage

(so, if the supply is

±30 V symmetrical, then VP =30V)

IORM = maximum repetitive peak current in output pin;

see also Chapter 9

Iripple = ripple current.

See the application notes (tbf) for a more detailed

description of the implications of output current limiting.

16.7

Pumping effects

The TDA8922 class-D amplifier is supplied by a

symmetrical voltage (e.g VDD = +25 V and VSS = −25 V).

When the amplifier is used in a SE configuration, a

so-called ‘pumping effect’ can occur. During one switching

interval, energy is taken from one supply (e.g. VDD), while

a part of that energy is delivered back to the other supply

line (e.g. VSS) and visa versa. When the voltage supply

source cannot sink energy, the voltage across the output

capacitors of that voltage supply source will increase:

the supply voltage is pumped to higher levels. The voltage

increase caused by the pumping effect depends on:

• Speaker impedance

• Supply voltage

• Audio signal frequency

• Capacitor value present on supply lines

• Source and sink currents of other channels.

The pumping effect should not cause a malfunction of

either the audio amplifier and/or the voltage supply source.

For instance, this malfunction can be caused by triggering

of the undervoltage or overvoltage protection or unbalance

protection of the amplifier.

See the application notes (tbf) for a more detailed

description of the implications of output current limiting.

16.8

Reference design

The reference design for a single-chip class-D audio

amplifier using the TDA8922TH is illustrated in Fig.10.

The Printed-Circuit Board (PCB) layout is shown in Fig.11.

The Bill Of Materials (BOM) is given in Table 1.

16.9

PCB information for HSOP24 package

The size of the PCB is 74.3

× 59.10 mm, dual sided 35 µm

copper with 121 metallized through holes.

The standard configuration has a symmetrical supply

(typical

±20 V) with stereo SE outputs (typical 2 × 8 Ω).

The PCB is also suitable for a mono BTL configuration

× 8 Ω) with symmetrical and asymmetrical supply.

It is possible to use several different output filter inductors

such as 16RHBP or EP13 types to evaluate the

performance against the price or size.

16.10 Classiﬁcation

The application shows optimized signal and EMI

performance.

min

osc

–

()

ORM

ripple

–

-----------------------------------------------------

0.6

–

≥

min

osc

–

()

ORM

ripple

–

---------------------------------------------------------

1.2

–

≥

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