Philips DVDR985 Technical Training Manual page 87

Power Switching
The 3.3Vdc and the 5Vdc supplies are
switched by the System Control
Microcomputer. Refer to Figure 44. The 5Vdc
Standby and the unswitched 3.9Vdc are sup-
plied to two FETs, 7501 and 7520. 7515, 7511
and 7512 control 7501 and 7520. The -5Vdc
Standby, the 33Vdc control voltage, and the
12Vdc regulated supplies are continuously pre-
sent on 7515 and 7511. The STBY_ctrl control
voltage is High when the unit is Off. The
STBY_ctrl is named the ION switching voltage
on the Analog Board. While the STBY_ctrl is
high 7512 is biased on, keeping the Gate of
the FET, 7511 Low. This keeps the 12Vdc sup-
ply Off.
When the System Control Microcomputer turns
the unit On, STBY_ctrl goes Low, turning Off
7512. The 33Vdc ctrl supply supplies power to
the FET, turning it On. The 12Vdc supply is
present. The 12Vdc supply goes to the gates
of 7501 and 7520, turning them On. The sup-
plies to the gates are regulated by shunt regu-
lators. The values of the monitoring resistors
establish the output levels of the FETs.
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