Power Supplies; Voltage Regulator Circuit; Power On / Off - Motorola GM-950 Manual

3.2

Power Supplies

The power supply to the control head is taken from the host radio FLT A+ voltage via connector
J0901 pin 2. The voltage FLT A+ is at battery level and is used for the LEDs, the back light, to power
up the radio via On / Off button and to supply the voltage regulator circuit. The regulator circuit
provides the stabilized +5 volt which is used for the microprocessor circuit, the display, the display
driver and the keypad buttons. The voltage +5V USW also provided by the regulator circuit is used to
buffer the internal RAM of the microprocessor (U0901). The regulated +5V taken from the host radio
via connector J0901 pin 10 (line +5V SOURCE) is only used to switch on or off the voltage regulator
in the control head.
3.3

Voltage Regulator Circuit

Voltage regulator U0891 provides 5V for the control head. The supply voltage FLT A+ for the voltage
regulator is fed via parallel resistors R0893/4 and dual diode D0891 to pin 8 of U0891. The +5 volt
output is switched on and off by the host radio's 5 volt source via line +5V SOURCE and control
transistor Q0891. When the host radio is switched off the voltage on line +5V SOURCE is at ground
level and switches off transistor Q0891. Pull up resistor R0892 pulls input SHUTDOWN (pin 3) of the
voltage regulator U0891 to FLT A+ level and switches off the output of U0891 (pin 1). When the host
radio is switched on the voltage on line +5V SOURCE of about +5 volts switches on transistor
Q0891 which in turn pulls input SHUTDOWN (pin 3) to ground and switches on the output of U0891.
Input and output capacitors (C0892 / C0893 and C0895 - C0896) are used to reduce high frequency
noise and provide proper operation during battery transients. Diode D0891 prevents discharge of
C0893 by negative spikes on the FLT A+ voltage. 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
(U0901) and the display driver (U0902) to prevent improper operation.
The voltage +5V USW derived from voltage FLT A+ is stabilized using resistor R0896 and diode
VR0891 This voltage is used to buffer the microprocessor's internal RAM. C0898 allows the battery
voltage to be disconnected for a couple of seconds without losing RAM parameters. Diode D0892
prevents radio circuitry from discharging this capacitor. The +5V at the second anode of D0892
speeds up charging of C0898 when the host radio is turned on by a high level at the ignition input
while the supply voltage is applied to the radio. This prevents the microprocessor from accidently
entering bootstrap mode.
3.4

Power On / Off

The On/Off button when pressed switches the radio's voltage regulators on by pulling ON OFF
CONTROL to high via D0931 and connects the base of Q0932 to FLT A+. This transistor pulls the
line ANALOG 3 low to inform the µP that the On/Off button is pressed. If the radio is switched off, the
µP will switch it on and vice versa. If the On/Off button is pressed and held while the radio is on, the
software detects a low state on line ANALOG 3 and switches the radio off. If the radio is switched on
either manually or automatically its +5V source switches on the control head voltage regulator
U0891 via line +5 SOURCE and transistor Q0891 and the control head microprocessor starts
execution.
Introduction/Theory of Operation
Theory of Operation
4D.1-3