TDA8924 Datasheet, PDF (17/35 Pages) NXP Semiconductors – 2 x 120 W class-D power amplifier

Philips Semiconductors
2 × 120 W class-D power amplifier
Objective specification
TDA8924
To trigger the maximum current protection in the
TDA8924, the required output current must exceed 12 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
2 Ω and 4 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 11.3 A for the TDA8924.
As a rule of thumb the following expressions can be used
to determine the minimum allowed load impedance
without generating audio holes:
Z_L ≥ -V----^P_I--O-⁽--_R-1---_M--^–---_–-t--^m--I-_r-ⁱ-ⁿⁱ-_p--f_p--^o-_l-_es--^c---⁾ – 0.6 for SE application.
Z_L ≥ -2---V---I--^PO---⁽_R--1--_M---^–--_–---t-I-^m-_r--_i-^ip-ⁿ-_p-f--_lo-_e--^s--^c---⁾ – 1.2 for BTL application.
Legend:
Z_L = load impedance
f_osc = oscillator frequency
t_min = minimum pulse width (typical 190 ns)
V_P = single-sided supply voltage (if the supply = ±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.
Output current limiting goes with a signal on the protection
pin (pin PROT). This pin is HIGH under normal operation.
It goes LOW when current protection takes place.
This signal could be used by a signal processor. In order
to filter the protection signal a capacitor can be connected
between pin PROT and V_SS. However, this capacitor
slows down the protective action as well as it filters the
signal. Therefore, the value of the capacitor should be
limited to a maximum value of 47 pF.
For a more detailed description of the implications of
output current limiting see also the application notes (tbf).
16.7 Pumping effects
The TDA8924 class-D amplifier is supplied by a
symmetrical voltage (e.g V_DD = +24 V, V_SS = −24 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. The overvoltage protection is
only meant to prevent the amplifier from supply pumping
effects.
For a more detailed description of this phenomenon see
the application notes (tbf).
16.8 Reference design
The reference design for the single-chip class-D audio
amplifier using the TDA8924 is illustrated in Fig.9. The
Printed-Circuit Board (PCB) layout is shown in Fig.10. The
Bill Of Materials (BOM) is given in Table 1.
2003 Jul 28
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