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9.
FM242 AA
•
At less costs (no expensive coils and capacitors).
Low Pass Filter (LPF)
For L and R separately, a Low Pass Filter (IC7238A and B) is
processing L_LOW and R_LOW.
The output signal of this filter is then fed to the audio amplifier
(identical for right channel).
High Pass Filter (HPF)
For L and R separately, a High Pass Filter (IC7260A and B) is
processing L_HIGH and R_HIGH.
The output signal of this filter is then fed to the audio amplifier
(identical for right channel).
9.5.4
Amplifier (Diagrams A3 to A6)
Each speaker has its' own 15 W class-D amplifier. These
amplifiers combine a good performance with a high efficiency,
resulting in a big reduction in heat generation.
Principle
Audio-power-amplifier systems have traditionally used linear
amplifiers, which are well known for being inefficient. In fact, a
linear Class AB amplifier is designed to act as a variable
resistor network between the power supply and the load. The
transistors operate in their linear region, and the voltage that is
dropped across the transistors (in their role as variable
resistors) is lost as heat, particularly in the output transistors.
Class D amplifiers were developed as a way to increase the
efficiency of audio-power-amplifier systems.
Figure 9-8 Principle Class-D Amplifier
The Class D amplifier works by varying the duty cycle of a
Pulse Width Modulated (PWM) signal.
By comparing the input voltage to a triangle wave, the amplifier
increases duty cycle to increase output voltage, and decreases
duty cycle to decrease output voltage.
The output transistors (item 7365 on diagram A3) of a Class D
amplifier switch from 'full off' to 'full on' (saturated) and then
back again, spending very little time in the linear region in
between. Therefore, very little power is lost to heat. If the
transistors have a low 'on' resistance (R
dropped across them, further reducing losses.
A Low Pass Filter at the output passes only the average of the
output wave, which is an amplified version of the input signal.
In order to keep the distortion low, negative feedback is applied
(via R3308). A second feedback loop (via R3310) is tapped
after the output filter, in order to decrease the distortion at high
frequencies.
The advantage of Class D is increased efficiency (= less heat
dissipation). Class D amplifiers can drive the same output
power as a Class AB amplifier using less supply current.
The disadvantage is the large output filter that drives up cost
and size. The main reason for this filter is that the switching
waveform results in maximum current flow. This causes more
loss in the load, which causes lower efficiency. An LC filter with
a cut-off frequency less than the Class D switching frequency
(350 kHz), allows the switching current to flow through the filter
Circuit Descriptions and List of Abbreviations
+V
-V
CL16532099_002.eps
200801
), little voltage is
DS(ON)
instead of the load. The filter is less lossy than the speaker,
which causes less power dissipated at high output power and
increases efficiency in most cases.
9.5.5
Mute (Diagram A2 to A6)
A mute switch (item 7302) is provided at the PWM inputs (item
7315, LM311). This switch is controlled by the
AUDIO_ENABLE line, which checks the availability of the
+9V_STBY voltage.
9.5.6
Protections
Short-circuit Protection (Diagram A3)
A protection is made against a too high temperature of
transistor 7355 in case of a short-circuit of output FET 7365-1.
Transistor 7340 is sensing the current through transistor 7355
via R3355, and activates the DC-protection line (see below) in
case the current becomes too high. This is the same for all four
amplifier parts.
DC-protection (Diagram A7)
OUT_LH
OUT_LL
VCC_10_POS
3770
3771
A
3780
3781
2760
OUT_RH
OUT_RL
VCC_10_NEG
Figure 9-9 DC Protection
Because of the symmetrical supply, a DC-blocking capacitor,
between the amplifier and the speaker, is not necessary.
However, it is still necessary to protect the speaker for DC
voltages.
The following protections are therefore implemented:
•
Via R3765 and R3775, each stabilised supply voltage line
(via items 7735 and 7745) is checked on deviations.
•
Via R3770/3771/3780/3781, each amplifier output is
checked for DC-voltage.
Via R3765/3775, a virtual earth is imposed on point A. When
one of the supply voltages deviates, a DC voltage will occur on
this point. If point A is positive, T7751 will conduct. If it is
negative, T7761 will conduct.
Both cases will make T7735 conduct, so that the DC-PROT
signal will be made high. This ensures that the power supply is
rapidly trimmed back.
Capacitor C2760 will ensure that only DC-signals at point A will
activate the protection.
9.6
LED/Switch Panel (Diagram LD)
This panel contains:
•
The red and green status LEDs,
•
The RC input receiver,
•
The light sensor, and
•
The 'on/off' switch.
All signals on this panel come directly from the SCAVIO panel:
•
The LED and sensor signals (RED_LED, GREEN_LED
and LIGHT_SEN_IN) are routed to the OTC. When a F21R
Receiver box is connected, the sensor signal is routed to
the OTC of this box (via UART), where it will control the
2
HOP via I
C.
+9V_STBY
5753
2753
3752
3751
3775
7751
7761
3750
3765
3760
CL16532099_001.eps
7735
3754
DC_PROT
7755
200801
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