LTC4402-1EMS8 Linear Technology, LTC4402-1EMS8 Datasheet - Page 7

• Manufacturer Part Number: LTC4402-1EMS8
• Description: IC CTRLR RF PWR MULTIBAND 8MSOP
• Manufacturer: Linear Technology

Specifications of LTC4402-1EMS8

• Rf Type: Cellular, GSM, GPRS, PCS, Wireless Modem, TDMA
• Frequency: 300MHz ~ 2.7GHz
• Features: Single Output
• Package / Case: 8-TSSOP, 8-MSOP (0.118", 3.00mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
Demo Board
The LTC4402-X demo board is available upon request. The
demo board has a 900MHz and an 1800MHz RF channel
and V
for SHDN are generated on the board using a 13MHz
crystal oscillator reference. The PCTL power control pin is
driven by a 10-bit DAC and the DAC profile can be loaded
via a serial port. The serial port data is stored in a flash
memory which is capable of storing eight ramp profiles.
The board is supplied preloaded with four GSM power
profiles and four DCS power profiles covering the entire
power range. External timing signals can be used in place
of the internal crystal controlled timing. A power ramp
software package is available which allows the user to
create power control ramps.
LTC4402 Control Loop Stability
There are several factors that can improve or degrade loop
frequency stability.
1) The additional voltage gain supplied by the RF power
amplifier increases the loop gain, raising poles normally
SHDN
–10
–20
–30
–40
–50
–60
–70
–80
10
0
HOLD
START
200mV
CODE
–28
controlled by the LTC4402-X. Timing signals
START
PULSE
Figure 1. LTC4402 Ramp Timing
11 s MINIMUM, ALLOWS TIME
FOR AUTOZERO TO SETTLE
–18
U
–10
U
0
TIME ( s)
W
543
553
561
U
571
ZERO
CODE
4402 F01
below the 0dB axis. The extra voltage gain can vary
significantly over input/output power ranges, frequency,
power supply, temperature and manufacturer. RF power
amplifier gain control transfer functions are often not
available and must be generated by the user. Loop oscil-
lations are most likely to occur in the midpower range
where the external voltage gain associated with the RF
power amplifier typically peaks. It is useful to measure the
oscillation or ringing frequency to determine whether it
corresponds to the expected loop bandwidth and thus is
due to high gain bandwidth.
2) Loop voltage losses supplied by the coupler network
will improve phase margin. The larger the coupler loss the
more stable the loop will become. However, larger losses
reduce the RF signal to the LTC4402-X and detector
performance may be degraded at low power levels. (See
RF Detector Characteristics.)
3) Additional poles within the loop due to filtering or the
turn-on response of the RF power amplifier can degrade
the phase margin if these pole frequencies are near the
effective loop bandwidth frequency. Generally loops using
RF power amplifiers with fast turn-on times have more
phase margin. Extra filtering below 16MHz should never
be placed within the control loop, as this will only degrade
phase margin.
4) Control loop instability can also be due to open loop
issues. RF power amplifiers should first be characterized
in an open loop configuration to ensure self oscillation is
not present. Self-oscillation is often related to poor power
supply decoupling, ground loops, coupling due to poor
layout and extreme V
quency is generally in the 100kHz to 10MHz range. Power
supply related oscillation suppression requires large value
ceramic decoupling capacitors placed close to the RF
power amp supply pins. The range of decoupling capacitor
values is typically 1nF to 3.3 F.
5) Poor layout techniques associated with the coupler
network may result in high frequency signals bypassing
the coupler. This could result in stability problems due to
the reduction in the coupler loss.
LTC4402-1/LTC4402-2
SWR
conditions. The oscillation fre-
7
4402f