Power Control Circuitry; Figure 3-20. Rpcic Block Diagram - Motorola ASTRO Digital Spectra Plus Detailed Service Manual

Theory of Operation: Power Amplifiers

3.7.1.1.3 Power Control Circuitry

Command Board Circuitry
Inside U500, the Regulator Power Control IC (see
four inverting inputs, and one non-inverting input (at pin 44) which is the reference input for the entire
power control loop of the power amplifier. The 3.2-V reference voltage at U500-44 is produced by
dividing SW +5-V with the voltage-divider circuit, R514 and R515.
29
N.C.
30
N.C.
31
REGULATOR
GROUND
32 13.8V
A+
33
TX P.A. ENABLE
34
35
LOCK
36
CURRENT SENSE
CURRENT SENSE +
37
+
FROM R9875
CURRENT SENSE
38
FROM R9875
39
KEYED 9.4V INPUT
POWER CONTROL
40
The power control loop is controlled by the microprocessor U204 on the VOCON board. Through the
SLIC IC U206, the microprocessor enables the RPCIC by pulling TX PA ENABLE (U500 pin 33) low
while the radio synthesizer is locked (U500 pin 35). U520 writes data to a digital-to-analog converter,
U502, to change and control the power-set voltage from pin 10 of U502 to pin 6 of U500. The voltage
on this line, 1.5 to 5 V, will be inversely proportional to the power out of the PA, with 5 V producing
the lowest power output. This voltage may be set with RSS (Radio Service Software) or CPS
(Customer Programming Software). On U500, the voltage at pin 6 is buffered internally and exits on
pin 7. Through R507, it is connected to pin 2 of U500. Note that pin 2 of U500 is the summing point
of three voltages: forward detect voltage, power set voltage, and temp-sense.
6881076C25-E
Q538
RPCIC ENABLE
28 27 26 25 24
+
9.6 VOLT
REGULATOR
PA ENABLE
U500
WIDE-BAND
CONTROL AMP
ONE-
ENABLE
Q
SHOT
+
AMP
CURRENT
LIMIT SET
BUFFER
+
PACKAGE FLAG
GROUND
GROUND
41 42 43 44
REGULATOR/POWER CONTROL IC U500

Figure 3-20. RPCIC Block Diagram

Figure 3-20) is an operational amplifier that has
23 22 21 20 19
PACKAGE
GROUND
+
5V REGULATOR
+
THERMISTER
BUFFER
DIRECTIONAL
COUPLER BUFFER
1 2 3 4 5
5V
18
17
9.6V SENSE INPUT
16
5V DRIVE TO Q502
15
5V CURRENT SENSE
14
5V FEEDBACK
TEMPERATURE SENSE
13
INPUT
TEMPERATURE SENSE
OUTPUT TO 500-2
12
11
+
POWER SET FROM
10 U502 1.5V 5 VOLTS
FORWARD DET.
9
VOLTAGE
+
8 FORWARD BUFFERED
OUT
POWER SET OUT TO
PIN 2 VIA R507
7
POWER SET
BUFFER
6
TO PIN 10
U502 DAIC
September 5, 2008
3-57
RESISTIVE
SUMMING
NETWORK
R508
68K
R516
100K
R509
68K
R507
47K
MAEPF-22034-O