LTC1758-1EMS8 Linear Technology, LTC1758-1EMS8 Datasheet - Page 10

• 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
LTC1758-1/LTC1758-2
Determining External Loop Gain and Bandwidth
The external loop voltage gain contributed by the RF chan-
nel and directional coupler network should be measured in
a closed loop configuration. A voltage step is applied to
PCTL and the change in V
detected voltage is 0.6 • PCTL for PCTL < 640mV and
1.18PCTL – 0.38V for PCTL > 640mV. The external voltage
gain contributed by the RF power amplifier and directional
coupler network is 0.6 • V
0.38V) • V
closed loop configuration accounts for the nonlinear de-
tector gain that is dependent on RF input voltage and
frequency.
The LTC1758 unity gain bandwidth specified in the data
sheet assumes that the net voltage gain contributed by the
RF power amplifier and directional coupler is unity. The
bandwidth is calculated by measuring the rise time be-
tween 10% and 90% of the voltage change at V
for a small step in voltage applied to PCTL.
The LTC1758 control amplifier unity gain bandwidth (BW1)
is typically 250kHz. The phase margin of the control
amplifier is typically 90 .
For example, to determine the external RF channel loop
voltage gain with the loop closed, apply a 100mV step to
PCTL from 300mV to 400mV. V
10
Figure 3. Measured Open Loop Gain and Phase, PCTL < 640mV
BW1 = 0.35/rise time
–10
–20
–30
–40
–50
–60
60
50
40
30
20
10
80
70
0
PCTL
100
/ V
1k
U
PCA
FREQUENCY (Hz)
. Measuring voltage gain in the
10k
U
PCA
PCTL
PHASE
GAIN
(or V
100k
PCA
/ V
R
C
LOAD
LOAD
PCB
W
VPCA
(or V
1M
= 33pF
= 2k
) is measured. The
PCB
and (1.18PCTL –
1758
10M
F03
) will increase
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
PCA
U
or V
PCB
to supply enough feedback voltage to the RF pin to cancel
this 100mV step which would be the required detected
voltage of 60mV. V
create the RF output power change required. The net
external voltage gain contributed by the RF power ampli-
fier and directional coupler network can be calculated by
dividing the 60mV change at the RF pin by the 42mV
change at the V
then be approximately 1.4. The loop bandwidth extends to
1.4 • BW1. If BW1 is 250kHz, the loop bandwidth in-
creases to approximately 350kHz. The phase margin is
extracted from Figure 3. Repeat the above voltage gain
measurement over the full power and frequency range.
The phase margin degradation, due to external and inter-
nal pole combinations, is difficult to determine since
complex poles are present. Gain peaking may occur,
resulting in higher bandwidth and lower phase margin
than predicted from the open loop Bode plot. A low
frequency AC SPICE model of the LTC1758 power control-
ler is included to better determine pole and zero interac-
tions. The user can apply external gains and poles to
determine bandwidth and phase margin. DC, transient and
RF information cannot be extracted from the present
model. The model is suitable for external gain evaluations
up to 6 . The 350kHz PCTL input filter limits the band-
width, therefore, use the RF input as demonstrated in the
model.
Figure 4. Measured Open Loop Gain and Phase, PCTL > 640mV
–20
–30
–40
–50
–60
–10
60
50
40
30
20
10
80
70
0
100
PCA
pin. The net external voltage gain would
PCA
1k
changed from 1.498V to 1.540V to
FREQUENCY (Hz)
10k
PHASE
GAIN
100k
R
C
LOAD
LOAD
1M
= 33pF
= 2k
1758
10M
F04
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10