Motorola SSETM 5000 - Manual (Page 37)

6881094C12-A November 11, 2004
Theory of Operation: Transceiver Board 2-15
2.4.3.8.1 Power and Control
Since U104 is powered from switched B+, it makes its own regulated 4.5 Vdc to power the internal
logic. The supply input is V5EXT at pin 17, and the output is V45 at pin 16. ANO at pin 32 is the
control signal to the RX/TX antenna switch control circuit.
2.4.3.8.2 Automatic Level Control (ALC)
In TX mode, the PCIC disables the receiver, turns on the transmitter, and controls the TX power
level. The automatic level control (ALC) circuit operates as follows:
The power level is set by programming an internal DAC to a calibrated reference voltage. D/A
settings for the power set points were determined during radio tuning and stored in EEPROM. An
internal op-amp compares the D/A reference voltage to the detector voltage at pin 1(RFIN) (TP101)
and produces an error signal output. This signal is buffered by another op-amp, configured as a low-
pass filter, or integrator, to produce the INT output at pin 4 (TP111).
This INT output supplies voltage to drive the gain control pins of amplifiers U102 and Q107.
Resistors R105 and R106 determine the voltage ratio between U102 pin 1 (VCNTRL) and the Q107
gate. Transient response during key-up and key-down is controlled by the power amplifier rise and
fall times. External capacitors at pins CI, CJ, and CL, along with internal programmable resistors,
determine the ALC time constants.
2.4.3.8.3 Temperature Cut Back
The PCIC contains a temperature cut-back circuit to protect the power amplifier (PA) from thermal
damage that might result from incorrect assembly of the radio. External sensor U103 is a linear
temperature-to-voltage transducer, placed near the hottest spot in the radio: power module Q107.
18 VAR2 Buffered D/A output
19 VLIM Test point for internal D/A No.2 voltage
20 VAR1 Buffered D/A output
21 RS Asynchronous reset input
22 NA Spare pin
23 RX RX/TX mode control-bit output
24 VAR3 Buffered D/A output
25 GND2 Ground
26 CLK SPI clock input
27 BPOS Power supply input
28 DATA SPI data input/output
29 CEX SPI chip select input
30 TEMP Temperature sensor input
31 RSET External resistor; used to set the temperature cutback rate
32 ANO Switched BPOS output
Table 2-8. Power Control IC (U104) Pin Descriptions (Continued)
Pin Name Description