Power Control Circuit - Numark D2 DIRECTOR Service Manual

SHEET 2 – POWER
This sheet shows the system power supply and the power control circuit.
The system is powered from a 12V DC power source; this is input on connector J8. D1 is a schottky
diode that ensures the system is not damaged should the user accidentally connect a power supply
with incorrect polarity. L1/C3/C20 are and EMC filter to reduce radiated and conducted emissions.
U5 and associated components form a switching regulator to provide the system +5V power supply.
This in turn is regulated by linear regulator U8 to derive the system +3.3V supply.
Power detect circuit: Comparator U10B and associated components provide a power detect circuit
to send the CPU and early warning that the external power supply has been unplugged or switched
off. Normally the output of the comparator will be low. If the input voltage falls below 10.8V, then
the output will go high.
The software uses this POWERFAIL signal to mute the audio output and save system parameters to
flash, so that the system can be restored after a power failure.

Power control circuit

U6 is a small PIC microcontroller than runs software to control the system power supply. U6 is
powered from a permanent +5V supply +5VPERM, provided by U4.
The PIC controls the ON/OFF pin of 5V regulator U5. Since the 3.3V supply is derived from the
+5V supply, both supplies are enabled together.
POWER_SW is the signal from the front panel power switch. It is high when the switch is pressed.
The power switch is monitored by both the PIC and main CPU. The signal is buffered by U10-D
prior to being sent to the CPU. This ensures that the signal POWER is never asserted high while the
system power supply is switched off (which could damage the CPU).
!OFF is a control signal from the CPU, to request that the power is turned off. It is normally high,
and goes low when the CPU wants the power to be turned off.
The power control works in the following way: When the 12V external power is first applied, the
PIC will keep the system power supply switched off and will wait for the power switch to be
pressed. Only then will the the PIC will turn on the system power supply, allowing the main CPU to
start up.
Once the system is running, the main CPU will monitor the power switch. If the power switch is
pressed, the main CPU will close down the system, mute the audio output and save key parameters
to flash. Having done this, the main CPU will set the !OFF signal low. This indicates to the PIC that
the CPU wishes to turn off the power supply and the PIC will respond by switching off the power.
The PIC also monitors the power switch while the system is running, to detect if the power button is
pressed and held down for >7 seconds. If this happens the PIC will turn off the power. This
provides a way to turn off the power in the event that the main CPU software has crashed.