Casio SK 8A - 14. POWER SUPPLY CIRCUIT

14. POWER SUPPLY CIRCUIT
The Power Supply Circuit consists of a voltage regulator and an APO (Auto Power Off) circuit.
When power is ON and operation is normal, the APO signal from the CPU maintains HIGH level.
Transistors T4 ~ T9 turn on providing the voltages to the circuits.
The signal APO becomes LOW, when the keyboard is not operated for seven minutes, and
transistors T4 ~ T9 turn off causing the voltages except VA to shut off.
Voltages VA (5V) and VDD1 (ground) are always provided to the CPU even at APO.
Transistors T1, T2 and T3 form a voltage regulator.
Using the LED characteristic of current regulation, transistor T3 stabilizes the battery voltage at
5V and the darlington-connected transistor T1 and T2 prevent over current in the circuits.