Bose SoundDock 10 Service Manual page 74

SoundDock 10 Theory of Operation
When Big Max transitions from playing audio (or charging standby) to low power standby,
the DSP module begins to pulse width modulate BIGMAX_OFF with a duty cycle that
ramps continuously from 0% to 100%. The pulse width modulated signal is low-pass
filtered by R559 and C549 to provide a DC voltage at TP577 that is proportional to the
duty cycle of BIGMAX_OFF. Q504 can be described as a common-base current source,
with the collector current set by the voltage across R558 (between the fixed emitter
voltage of Q504 and TP577). As the duty cycle of BIGMAX_OFF increases from 0%, the
collector current in Q504 increases, which biases Q510 into linear constant-current mode.
In this condition, the combination of Q504, Q510, R561, and R560 act as a current source
controlled by the duty cycle of BIGMAX_OFF. This current is injected into the feedback
node of the opto-coupler.
Optocoupler (U501)
The optocoupler closes the asymmetric half bridge control loop by sending the output
voltage from the secondary side to the PWM controller on the primary side. The
optocoupler modulates the LED output according to the comparison of the voltage on pin
4 to an internal 2.5V regulator. The higher pin 4 with respect to 2.5V, the more we turn on
the PNP transistor on the primary side (pins 6 and 7). This in turn lowers the current
sense comparator threshold on the PWM controller.
The optocoupler is equipped with a CMP pin which is proportional to the output of the
comparison of pin 4 to the 2.5V regulator. This configuration can be thought of as an
inverting opamp. R518 and C517 create a low pass filter.
PWM Controller (U505)
The PWM controller in short outputs a PWM signal proportional to the ratio of the output
to input voltage ratio. This signal is then used to turn on and off the MOSFETS through
IC's that will be talked about shortly.
The PWM controller has a built in comparator that turns on the output until the Isense pin
voltage exceeds that of pin 1 (COMP). The PWM controller therefore uses current mode
feedback.
Inverter (U506)
The inverter is necessary to generate a pulse stream opposite of the output of the PWM
controller. This is necessary because the gate driver requires separate input signals to
drive each of the MOSFET's. In theory one could put the inverter at the output of the gate
driver, however the special voltage levels required to drive the upper MOSFET (Q500)
render this impractical. This inverter needs to be able to run at 8.5V because this voltage
supply is readily available from the PWM controller. If we wanted to use a standard 5V
inverter part we would have to generate a separate 5V supply on the primary. The RC
combination (R563 and C569) provide additional delay that results in increased deadtime
between lower FET (Q501) turn off and upper FET (Q500) turn on.
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