Bose 3-2-1 Manuel de dépannage - Page 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

R17, C50

C25

R33

R36 C22 C24

R30 R37

C51

C58

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)