Philips 37PF9631D/37 Service Manual page 108

EN 108 9. EJ2.0U LA
9.2.1 Class-D Audio Stage
The class-D audio stage can be found in diagram B08A. See
figure "Class-D stage diagram" for details.
Low pass filter
18k
3D56
22k
180 Phase shift and delay
When switching the set on, the circuit will start to oscillate due
to a transient. At t = 0 the charge on capacitors 2D47, 2D48 and
2D67 will be 0 V. After start-up they will be charged to +VCC
because V will be driven to +VCC level at start-up. In this
out
situation the opamp/comparator (item 7D10) will receive a
higher voltage on his inverted input, the opamp will collapse
and V will be -VCC. Capacitors 2D47, 2D48 and 2D67 will
out
now discharge to -VCC. This process repeats and causes an
oscillating effect. The ideal situation would be that this oscillator
would run at an indefinite high frequency. However, due to
component limitations and signal delay, the frequency will be
limited.
A simple second order output filter (built with items 5D12 and
2D53) creates a low-pass filter that passes only the average of
the output wave, which is an ampified version of the input
signal. See figure "Output filter" for details.
5D12/5D13
+
33uH
Figure 9-5 Output filter
A synchronization circuitry between the two amplifiers is
realized around items 2D65, 2D66 and 3D91 to avoid
interference between the two audio channels. See figure
"Synchronization circuitry" for details.
Circuit Descriptions, Abbreviation List, and IC Data Sheets
3D34
3k3
7D14
7D10
2D35
1n
3D81
680
3D71
3D67
10k
3k9
2D48 3D73
2D47
1n8
560
1n8
2D67
100n
Figure 9-4 Class-D stage diagram
-
2D53/2D57
470n
G_15940_063.eps
260506
Dead zone
3D35
1k2
3D39
22k
7D15
7D18
3D39
22k
7D16
2D30 3D39
22n 22k
3D44
330
7D16
7D18
3D81
22k
7D17
3D76
22k
3D64
1k2
Low pass filter 20 – 30Khz
Feedback gain is fixed here
Power switch
2D52
100n
2D28
100n
G_15940_065.eps
260506
SW SW1
2p2 2p2
CP
2p2
33k
2p2
10k
CP1
G_15940_064.eps
260506
Figure 9-6 Synchronization circuitry