5
Power Supply VRAW
Transformer
From
To 3.3 V
Regulator
VRAW
Vraw provided power to the Audio Power Amplifiers, U150, U150, U350, and the +5V regulator. Its potential is
monitored by the DSP, U7000, via the circuit associated with VRAWSEN. When active the potential can vary
from 12 vdc to 14.0 vdc. The circuit controlling the FET switch, Q10, is configured to force commutate (turn off)
the switch whenever the potential of the filter capacitor, either C9 or C10, exceeds the target potential of approxi-
mately 13 volts. The nature of the circuit causes Q10 to turn off during high peak currents. These high peak
currents cause a voltage drop on C9’s (C10’s) internal equivalent series resistance (ESR) which, in turn, causes
a drop in the sensed output potential as soon as the switch, Q10 is turned off. In order to prevent the circuit to
immediately turn the switch back on, a fixed voltage hysteresis is applied by the control circuit to lower the turn-
on threshold of the circuit by 200 to 300 millivolts.
R10 and ZR10 form a 6.0V reference potential for the error amplifier formed by Q11, Q12. R11, R12, and R13
scale this reference voltage to an appropriate potential to permit start-up of the circuit. R11 doubles as the feed-
back resistor that sets the hysteresis mentioned above. R18 and R19 are essentially in parallel when Q10 is on,
so R18, R19, and R14 set a relatively constant turn-off potential. When Q10 is off, R18 depresses the turn-on
threshold of the circuit proportional to the open-circuit potential of the transformer secondary. This is in addition
to the hysteresis provided by R11 and serves to prevent Q10 from turning on during the highest potentials of the
input mains, as this is when the peak charging currents would be the largest.
Q24 and R16 serve as the enable input and current sink for the error amplifier. Maintaining a constant potential
across R16, causes a constant potential to be applied Gate to Source of Q1 when Q10 is on. C15 controls the
switching speed of Q10, which improves audible and electrical noise characteristics of circuit. Q13 and the
additional components associated with Q13 provide additional shaping of the switching characteristics of Q10
when little power is consumed from Vraw.
