Bose 3-2-1GS Series II Handbuch zur Fehlersuche - Seite 5
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1.1. Switching Power Supplies
The +12V and +5V switching power supplies use ST L4973D3.3 regulator ICs (U17 [sheet 14,
B5] and U2 [A6]). The power supplies are designed as step-down Buck converters. The voltage
fed back to the chip on pin 13 determines the output voltage; the chip's control circuitry will work
to keep this voltage at +3.3V. The +5.1V and +12V supplies use resistor divide-down networks
to obtain the +3.3V feedback voltage.
The reference designators listed below correspond to the +12V switching power supply; the
designs of the +5V regulator is nearly identical.
IC Pin
Components Connected
1
R17, C50
10
C25
11
R33
12
R36 C22 C24
13
R30 R37
18
C51
19
C58
20
Q5
A number of additional series inductors and parallel capacitors exist to provide filtering func-
tions.
The compensation networks used for these switching power supplies have been chosen to
provide stability under all conditions and to provide minimal RF interference to the tuner.
The supplies function in both continuous and discontinuous mode depending on the load.
1.2. Linear Power Supplies
U18 [C3] is the +9V linear regulator. VR1 [A3] and VR2 [A2] are the 3.3V and 1.8V linear regula-
tor, respectively.
1.3. Supply Synchronization
In order to control the noise interference to the AM tuner, a variable frequency to alternate
switching synchronization scheme was implemented. When the console is in the Tuner mode,
the SPDIF receiver (U8001 [sheet 12, B4]) delivers a 2.8224MHz clock signal (SPDIFIN_BCK)
from the 11.2896MHz crystal oscillator. SPDIFIN_BCK drives the binary count (U3 [sheet 14,
C4]) and generates a required synchronizing pulse (F_SYNCH) and feed to pin 20 of U17 [C5]
and pin 20 of U2 [B6]. Q4 [D3] and Q5 [D3] are for buffering and level shifting of the synchroniz-
ing pulse. The synchronization pulse alternation is controlled by tuner board based on the AM
station to be tuned.
THEORY OF OPERATION
Function
Sets free running switching frequency
(when not controlled externally)
Bootstrap to drive internal D-MOS
Inhibits supply
Lead-lag filter for compensation loop
Voltage feedback for control
+5.1V for external reference
Sets supply soft-start time constant
Supply frequency switching synchronization
(see section 2.4 below)
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