Voltage Regulation; Voltage Regulators; Electronic On/Off - Motorola MCS 2000 Service Instructions Manual

7-2

Voltage Regulation

(Refer to schematic page 10-33 or 10-34 for reference)

Voltage Regulators

Voltage regulation for the controller is provided by 4 separate devices; U0500 (LM2925) +5V, U0501
(LM2941) +9.3 V, U0502 +8V, and UNSW 5V (a combination of R0004 and VR0001). Additional
regulators are located in the RF section and the control head/handset.
Voltage regulation providing 5V for the digital circuitry is done by U500. Input and output capacitors
(C0500/0501 and C0503/0504) are used to reduce high frequency noise and provide proper
operation during battery transients. This regulator provides a reset output (pin 5) that goes to 0 volts
if the regulator output goes out of regulation. This is used to reset the microprocessor during power
on to prevent improper operation. C0502 sets the duration of this reset signal to a minimum of about
12 ms.
Regulator U0501 is used to generate the 9.3 volts required by some audio circuits, the RF circuitry
and power control circuitry. Input and output capacitors (C0505/0506 and C0508/0509) are used to
reduce high frequency noise. R0503/R0504 sets the output voltage of the regulator. If the voltage at
pin 1 is greater than 1.2 volts the regulator output decreases and if the voltage is less than 1.2 volts
the regulator output increases. This regulator output is electronically enabled by a 0 volt signal on pin
2. Q0500 and associated circuitry (R0500/0501/0502 and C0507) are used to disable the regulator
when the radio is turned off.
Regulated 8V is provided by U0502. It provides a separate supply to the microphone circuitry. The
microphone cannot be supported directly from A+/SWB+ because any fluctuation on A+ would be
impressed on the microphone line and affect transmitted audio. The microphone circuitry can not be
connected to the other regulators because, there is a function which allows the radio to be switched
on by momentarily grounding the microphone line (see Electronic On/Off).
UNSW 5V is only used in a few areas which draw low current and requires 5 V while the radio is off.

Electronic On/Off

Unlike older radios whose on/off feature was controlled only by the mechanical on/off switch, this
radio has circuitry which allow radio software and/or external triggers to turn the radio on or off
without direct user action. Some examples of this usage are 1) automatic turn on when emergency
switch is engaged; 2) automatic turn on when ignition is sensed and off when ignition is off; 3) radio
deaffiliation, with certain features the radio can be programmed to stay on for a brief period after the
user has "turned it off". During this "extra on" period the radio may transmit a signal to a base station
letting the station know that the radio has been turned off.
Q0511 is used to provide SW B+ to the various radio circuits. Q0511 acts as an electronic on/off
switch where pin 4 acts as the control pin. The switch is on when pin 4 is low and off when pin 4 is
high. When the radio is off Q0510 is cutoff and the voltage at Q0511-4 is at A+. This effectively
prevents current flow through Q0511 from pins 2/3 to 5/6/7/8. When the radio is turned on the voltage
at the base of Q0510 is high (at least above 3V) and Q0510 switches on (saturation) and pulls down
the voltage at Q0511-4 to ~0.07V. With FET Q0511 now "enabled" current flows through the device.
This path has a very low impedance (less than 1 ohm) from pins 2/3 to 5/6/7/8. This effectively
provides the same voltage level at SWB+ as at A+.
The electronic on/off circuitry can be enabled by the microprocessor, the emergency switch, the
mechanical On/Off switch on the control head, or the ignition sense circuitry. If any of the 4 paths
leading to the base of Q0510 provides a high voltage through diodes CR0510 or CR0511, the
electronic "ON" is engaged.
December 6, 2004
Controller Section Theory of Operation: Voltage Regulation
68P81083C20-D