Appendix C
LTC1758 Theory of Operation
In enable mode (SHDN = High,
TXEN = High) the control loop
and protection functions will be
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 LTC1758-2 is a dual band RF operational. When TXEN is
power controller for RF power
amplifiers operating in the
850 MHz to 2 GHz range.
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
The LTC1758 datasheet provides
more detailed description of the
part’s operation and can be
downloaded from Linear
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.
Technology’s website.
detected by the LTC1758-2 at the
TOP VIEW
VIN
VCC
VPCA
VPCB
1
2
3
4
5
10
9
8
7
6
RF
SHDN
BSEL
GND
TXEN
PCTL
MS10 Package
10-Lead Plastic MSOP
Figure C1. LTC-1758-2 Pinout.
MODE
SHDN
Low
TXEN
Low
Low
High
OPERATION
Shutdown
Autozero
Enable
Disabled
High
High
Autozero
Power Control
Modes of Operation
The LTC1758-2 supports three
operating modes: shutdown,
autozero and enable.
Shutdown Autozero
Enable
t1
SHDN
BSEL
TXEN
PCTL
t2
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.
tS
Note 1
Start
Voltage
V
PCA
PCB
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.
Start
Voltage
V
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
will cancel the DAC transient. the DAC must be settled to an offset ≥ 400mv before TXEN
is asserted high.
Figure C2. LTC1758-2 Timing Diagram.
22
HP [ HEWLETT-PACKARD ]
