Bose AV28 Manual page 7

2.0 Power Supply Electronics
2.1 Architecture
Voltage Type
+33 Linear
+12 Switching
+10 Linear
+8 Linear
+5.1 Switching
+3.3 Switching
+2.5 Linear
V_RAW is provided to the console by an external power pack and is approximately 33VDC. The
power pack is a linear transformer. In addition to the usual large capacitor and full-bridge diode
rectifier, a PTC thermistor (effectively a reset-able fuse) is contained in the power pack to protect
the unit under fault conditions. V_RAW enters the console on the tuner PCB, passes through an
NTC thermistor to eliminate power supply startup surges, and is fed to the +12V supply and the
flat-flex cable, which feeds it to the +5.1V and +3.3V supplies on the Main PCB.
AC line
voltage
from wall
2.2 Switching Power Supplies
See SD254135 sheet 9 and SD256131 sheet 3 for the power supply circuits.
The switching power supplies are ST L4973D3.3 regulator ICs (U802 and U803 on the Main PCB,
and U301 on the Tuner PCB). 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 discussed in this section correspond to the +12V regulator on the tuner
board; the designs of the +5.1V and +3.3V regulators are nearly identical. A number of additional
components exist to provide filtering functions.
Theory of Operation
PCB Location Input
(schematic page)
Power Pack 120VAC
Tuner (3) V_RAW
Tuner (3) +12
Main (9) +10
Main (9) V_RAW
Main (9) V_RAW
Main (9) +3.3
DCS +33V
9X
Linear
Power Supply Block Diagram
+12V, +5.1V, +3.3V
DVD drive, VFD, +10V
Tuner, Transceiver, Audio path, +8V
Video circuits
DVD drive, Audio path, +3.3
Flash, DRAM, U1 I/O & core, misc. logic
U1 PLL circuits
+12V +10V
SMPS Linear
+5.1V
SMPS
+3.3V +2.5V
SMPS Linear
7
Outputs
+8V
Linear