Bose AV28, Manual

Page: 7 / 73

7
Theory of Operation
Voltage Type PCB Location
(schematic page)
Input Outputs
+33 Linear Power Pack 120VAC +12V, +5.1V, +3.3V
+12 Switching Tuner (3) V_RAW DVD drive, VFD, +10V
+10 Linear Tuner (3) +12 Tuner, Transceiver, Audio path, +8V
+8 Linear Main (9) +10 Video circuits
+5.1 Switching Main (9) V_RAW DVD drive, Audio path, +3.3
+3.3 Switching Main (9) V_RAW Flash, DRAM, U1 I/O & core, misc. logic
+2.5 Linear Main (9) +3.3 U1 PLL circuits
2.0 Power Supply Electronics
2.1 Architecture
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.
Power Supply Block Diagram
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.
+33V DCS
9X
Linear
+12V
SMPS
+5.1V
SMPS
+3.3V
SMPS +2.5V
Linear
+10V
Linear
+8V
Linear
AC line
voltage
from wall