LTC1757A-1 LINER [Linear Technology], LTC1757A-1 Datasheet - Page 8

• Manufacturer Part Number: LTC1757A-1
• Description: Single/Dual Band RF Power Controllers
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1757A-1/LT1757A-2
External Termination
The LTC1757A has an internal 175 termination resistor
at the RF pin. If a directional coupler is used, it is
recommended that an external 68 termination resistor
be connected between the RF coupling capacitor (33pF),
and ground at the side connected to the directional
coupler. If the termination is placed at the LTC1757A RF
pin, then the 68 resistor must be connected to V
the detector is referenced to V
nents should be placed adjacent to the LTC1757A.
Power Ramp Profiles
The external gain associated with the RF channel can vary
significantly between RF power amplifier types. The
LTC1757A frequency compensation has been optimized
to be stable with several different power amplifiers and
manufacturers. This frequency compensation generally
defines the loop dynamics that impact the power/time
response and possibly (slow loops) the power ramp
sidebands. The LTC1757A operates open loop until an RF
signal appears at the RF pin, at which time the loop closes
8
U
U
IN
SHDN
TXEN
–10
–20
–30
–40
–50
–60
–70
–80
. Termination compo-
10
W
0
START
100mV
CODE
–28
START
PULSE
Figure 1. LTC1757A Ramp Timing
50 s MINIMUM, ALLOWS TIME FOR DAC
AND AUTOZERO TO SETTLE
U
–18
IN
–10
since
0
TIME ( s)
and the output power follows the DAC profile. The RF
power amplifier will require a certain control voltage level
(threshold) before an RF output signal is produced. The
LTC1757A V
old voltage in order to meet the power/time profile. To
reduce this time, the LTC1757A starts at 550mV. However,
at very low power levels the PCTL input signal is small, and
the V
reach the RF power amplifier threshold voltage. To reduce
this time, it may be necessary to apply a positive pulse at
the start of the ramp to quickly bring the V
the threshold voltage. This can generally be achieved with
DAC programming. The magnitude of the pulse is depen-
dent on the RF amplifier characteristics.
Power ramp sidebands and power/time are also a factor
when ramping to zero power. For RF amplifiers requiring
high control voltages, it may be necessary to further adjust
the DAC ramp profile. When the power is ramped down the
loop will eventually open at power levels below the
LTC1757A detector threshold. The LTC1757A will then go
open loop and the output voltage at V
PCA/B
543
553
outputs may take several microseconds to
PCA/B
561
outputs must quickly rise to this thresh-
571
ZERO
CODE
1757A F01
PCA
or V
PCA/B
PCB
outputs to
will stop