LTC1758-1EMS8 Linear Technology, LTC1758-1EMS8 Datasheet - Page 9

• Manufacturer Part Number: LTC1758-1EMS8
• Description: IC RF PWR CONTRLLR SINGLE 8-MSOP
• Manufacturer: Linear Technology

Specifications of LTC1758-1EMS8

• Rf Type: Cellular, GSM, GPRS, PCS, Wireless Modem, TDMA
• Frequency: 850MHz ~ 2GHz
• 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
to allow the autozero to cancel the step. Slow DAC rise
times will extend this time by the additional RC time
constants.
Another factor that affects power ramp sidebands is the
DAC signal to PCTL. The bandwidth of the LTC1758 may
not be low enough to adequately filter out steps associated
with the DAC. If the baseband chip does not have an
internal filter, it is recommended that a 1-stage external
filter be placed between the DAC output and the PCTL pin.
Resistor values should be kept below 2k since the PCTL
input resistance is 90k. A typical filter scheme is shown in
Figure 2.
The power control ramp should be started in the range of
1 s to 10 s after TXEN is asserted high.
Demo Board
The LTC1758 demo board is available upon request. The
demo board has a 900MHz and an 1800MHz RF channel
controlled by the LTC1758. Timing signals for TXEN are
generated on the board using a 13MHz crystal 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 variety of RF power amplifiers are available.
LTC1758 Control Loop Stability
The LTC1758 provides a stable control loop for several RF
power amplifier models from different manufacturers
over a wide range of frequencies, output power levels and
V
can improve or degrade loop frequency stability.
SWR
conditions. However, there are several factors that
DAC
U
Figure 2
U
2k
W
330pF
PTCL
LTC1758
1758 F02
U
1) The additional voltage gain supplied by the RF power
amplifier increases the loop gain raising poles normally
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 directional coupler
will improve phase margin. The larger the directional
coupler loss the more stable the loop will become. How-
ever, larger losses reduce the RF signal to the LTC1758
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 directional
coupler area may result in high frequency signals bypass-
ing the coupler. This could result in stability problems due
to the reduction in the coupler loss.
LTC1758-1/LTC1758-2
SWR
conditions. The oscillation fre-
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