Radio Power Distribution - Motorola CDM1550 LS+ Detailed Service Manual

2-2 Theory of Operation
2.2.1 Radio Power Distribution
The de power distribution throughout the radio board is shown in Figure 2-2. Voltage regulation for
the controller is provided by four separate devices:
U0651 (MC78M05) +5 volts
U0641 (LM2941) +9.3 volts
U0611 (LM2941) +12 volts
VSTBY 5V (a combination of R0621 and VR0621)
U3211 Additional 5 volt regulator located in the RF section.
The de voltage applied to connector J0601 supplies power directly to the following:
Electronic on/off control
RF power amplifier
12 volt regulator
9.3 volt regulator
Audio PA
5.6 volt stabilization circuit
9.3 volt regulator (U0641) supplies power to the 5 volt regulator (U0651) and 6 volt voltage
divider Q0681
Regulator U0641 generates the 9.3 volts required by some audio circuits, the RF and power control
circuits. Input and output capacitors C0641 and C0644
I
C0645 are used to reduce high frequency
noise. Resistors R0642
I
R0643 set the output voltage of the regulator. If the voltage at pin 1 is
greater than 1.3 volts the regulator output decreases and if the voltage is less than 1.3 volts the
regulator output increases. This regulator output is enabled by a 0 volt signal on pin 2. Transistors
00661, Q0641, and R0641 are used to disable the regulator when the radio is turned off.
Voltage regulator U0651 provides 5 volts operating voltage for the digital circuits. Operating voltage
is from the regulated 9.3volts supply. Input and output capacitors (C0651 I C0652 and C0654 I
C0655) reduce high frequency noise and provide proper operation during battery transients. Voltage
sense device U0652 or alternatively U0653 provides a reset output that goes to 0 volts if the
regulator output goes below 4.5 volts. This resets the controller to prevent improper operation.
Diode D0651 prevents discharge of C0652 by negative spikes on the 9.3 volt supply.
Transistor Q0681 and resistors R0681
I
R0682 divide the regulated 9.3 volts down to about 6 volts.
This voltage supplies the 5 volt regulator, located on the RF section. By reducing the supply voltage
of the regulator, the power dissipation is divided between the RF section and the controller section.
The VSTBY signal, derived directly from the supply voltage by components R0621 and VR0621,
buffers the internal RAM. Capacitor C0622 allows the battery voltage to be disconnected for several
seconds without losing RAM information. Dual diode D0621 prevents radio circuits from discharging
this capacitor. When the supply voltage is applied to the radio, C0622 is charged via R0621 and
D0621. When the radio is switched on, the IJP enters the wrong mode if the voltage across C0622 is
still too low. The regulated 5 volts charges C0622 via diode D0621.