ACPM-7891-BLK HP [Agilent(Hewlett-Packard)], ACPM-7891-BLK Datasheet - Page 22

• Manufacturer Part Number: ACPM-7891-BLK
• Description: Agilent ACPM-7891 Tri-Band Power Amplifier Module EGSM, DCS and PCS Multi-slot GPRS
• Manufacturer: HP [Agilent(Hewlett-Packard)]

22
Appendix C
LTC1758 Theory of Operation
The LTC1758-2 is a dual band RF
power controller for RF power
amplifiers operating in the
850 MHz to 2 GHz range.
RF power is controlled by driv-
ing the RF amplifier power
control pins and sensing the
resultant RF output power via a
directional coupler. The RF
sense voltage is peak detected
using an on-chip Schottky diode.
This detected voltage is com-
pared to the DAC voltage at the
PCTL pin to control the output
power. The RF power amplifier
is protected against high supply
current and high power control
pin voltages. Internal and exter-
nal offsets are cancelled over
temperature by an autozero con-
trol loop, allowing accurate low
power programming. The shut-
down feature disables the part
and reduces the supply current
to <1_A.
Modes of Operation
The LTC1758-2 supports three
operating modes: shutdown,
autozero and enable.
In shutdown mode (SHDN =
Low) the part is disabled and
supply currents will be reduced
to <1_A. VPCA and VPCB will be
connected to ground via 100_
switches.
In autozero mode (SHDN = High,
TXEN = Low) VPCA and VPCB
will remain connected to ground
and the part will be in the
autozero mode. The part must
remain in autozero for at least
50_s to allow for the autozero
circuit to settle.
In enable mode (SHDN = High,
TXEN = High) the control loop
and protection functions will be
operational. When TXEN is
switched high, acquisition will
begin. The control amplifier will
start to ramp the control voltage
to the RF power amplifier. The
RF amplifier will then start to
turn on. The feedback signal
from the directional coupler and
the output power will be
detected by the LTC1758-2 at the
SHDN
Figure C1. LTC-1758-2 Pinout.
SHDN
Figure C2. LTC1758-2 Timing Diagram.
TXEN
tS: autozero settling time, 50µs minimum
t1: BSEL change prior to TXEN, 200ns typical
t2: BSEL change after TXEN, 200ns typical
Note 1: The external DAC driving the PCTL pin can be enabled during autozero. The autozero system
BSEL
BSEL
PCTL
GND
V
V
MODE
Shutdown
Autozero
Enable
V
RF
PCA
PCB
IN
1
2
3
4
5
10-Lead Plastic MSOP
will cancel the DAC transient. the DAC must be settled to an offset ≥ 400mv before TXEN
is asserted high.
Shutdown
MS10 Package
TOP VIEW
SHDN
Low
High
High
Voltage
Autozero
Start
t
S
TXEN
Low
Low
High
10
9
8
7
6
V
V
V
TXEN
PCTL
CC
PCA
PCB
Enable
OPERATION
Disabled
Autozero
Power Control
t
1
RF pin. The loop closes and the
amplifier output tracks the DAC
voltage ramping at PCTL. The RF
power output will then follow
the programmed power profile
from the DAC.
The LTC1758 datasheet provides
more detailed description of the
part’s operation and can be
downloaded from Linear
Technology’s website.
t
2
Voltage
Note 1
Start