Bose SoundDock 10 Service Manual page 73

SoundDock 10 Theory of Operation
package when the SPS board is operated on a lab bench at higher (>1A/25W) power
levels.
Output Inductor (L502)
The output inductor works in conjunction with the output capacitor (C521) to filter the
output voltage. This inductor works well away from its saturation point and is adequately
sized to keep the output voltage ripple to a minimum.
Voltage Feedback Circuitry
The combination of U200 (1/4 of quad NOR gate), Q504, Q510, Q502 and associated
resistors and capacitors provide the capability for the DSP to control the DC voltage for
the +24V bus over a range of +7V to +24V. The capability to reduce the +24V rail to +7V
is necessary to meet low power requirements in standby mode, and the capability to ramp
the voltage slowly and continuously between these two voltages is necessary for proper
operation of the PWM controller (U505) between transitions.
The basic function of this circuit is to synthesize two variable resistors, one connected
between the node at TP536 and +24V, and one connected between the node at TP530
and +24V, with the value of the resistors controlled by the duty cycle of the BIGMAX_OFF
3.3V CMOS logic signal from the DSP module. One of the synthesized resistors is in
parallel with R521, the value of which determines the setpoint of the +24V rail, and the
second synthesized resistor is in parallel with R517, the value of which determines the
bias level for the LED internal to the optocoupler, U501. Refer to the optocoupler and
PWM controller sections on the operation of the main switching power supply voltage
control loop for an explanation of how R521 and R517 control the voltage and LED
bias....
When Big Max is playing audio or in charging standby, the +24V rail is controlled to +24V.
In this condition, BIGMAX_OFF is logic LOW. This results in +5V at node TP595
(through the inversion of the NOR GATE), resulting in zero emitter current in Q504. The
node at TP546 is thus pulled high by R561, keeping Q510 and Q502 turned off. With the
collectors of these two transitors essentially infinite impedance, the optocoupler
essentially sees a 10.2k resistor for the top side of the setpoint divider and a 10k resistor
to +24V for the LED bias. This is the static condition necessary for controlling the supply
rail to +24V.
When Big Max is in low power standby, the +24V rail is controlled to approximately +6.9V.
In this condition, BIGMAX_OFF is logic HIGH. This results in 0V at node TP595. With
the base of Q504 held at +5V, the voltage between Q504's emitter and TP595 results in
approximately 0.53mA emitter and collector current, which is sufficient to pull the base of
both Q510 and Q502 to low enough voltages with respect to their emitters such that they
are both in saturation. Under this condition, the optocoupler sees 10.2k in parallel with
909 ohms + 1.62k (R560 and R532 in series), or a combined resistance of 2.0k for the
top side of the setpoint divider, and 10k in parallel with 2.2k or a combined resistance of
1.8k for the LED bias. This is the static condition necessary for controlling the supply rail
to +6.9V.
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