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This circuit generates +3.3V, +5V from the input +15V.
It is comprised of the circuit that steps +15V down to +3.3V by the switching power supply of
IC801, Q801, and L810 and the one that steps +15V down to +5V by the switching power supply
of IC802, Q802, and L811.
It has three protection circuits: IP801 against +3.3V short, IP802 against +5V short, and the
overvoltage protection that detects the voltage buildup of +3.3V/+5V, turns FET of Q9 on, and
then interrupts IP1.
8.3.2. Ringer Generating Circuit
The bell signal forms a sine wave out of the rectangular wave of 20/25Hz transmitted from ASIC
of MPR, amplifies it up to AC75Vrms, and generates it.
+40V is stepped up to +180V and -100V by SW power circuit that oscillates at 200KHz, and the
voltage between these is amplified with a focus on +30V to generate the bell signal of
AC75Vrms.
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