2003 Mar 20
18
Philips Semiconductors
Objective specification
2
×
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 (V
DD
or V
SS
)
•
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:
Z
L
= load impedance
f
osc
= oscillator frequency
t
min
= minimum pulse width (typical 190 ns)
V
P
= single-sided supply voltage
(so, if the supply is
±
30 V symmetrical, then V
P
= 30 V)
I
ORM
= maximum repetitive peak current in output pin;
see also Chapter 9
I
ripple
= 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 V
DD
= +25 V and V
SS
=
−
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. V
DD
), while
a part of that energy is delivered back to the other supply
line (e.g. V
SS
) 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
µ
m
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
(1
×
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.
Z
L
V
P
1
t
min
f
osc
×
–
(
)
×
I
ORM
I
ripple
–
-----------------------------------------------------
0.6
–
≥
Z
L
2V
P
1
t
min
f
osc
×
–
(
)
×
I
ORM
I
ripple
–
---------------------------------------------------------
1.2
–
≥
