8

LTC4402-1/LTC4402-2

4402f

APPLICATIO S I FOR ATIO

Determining External Loop Gain and Bandwidth

The external loop voltage gain contributed by the RF chan-

nel and coupler network should be measured in a closed

loop configuration. A voltage step is applied to PCTL and

voltage is 0.6 • PCTL and the external voltage gain contrib-

uted by the RF power amplifier and coupler network is

0.6 •

loop configuration accounts for the nonlinear detector

gain that is dependent on RF input voltage and frequency.

The LTC4402-X unity gain bandwidth specified in the data

sheet assumes that the net voltage gain contributed by the

RF power amplifier and coupler network is unity. The

bandwidth is calculated by measuring the rise time be-

for a small step in voltage applied to PCTL.

BW1 = 0.35/rise time

The LTC4402-X control amplifier unity gain bandwidth

(BW1) is typically 450kHz. For PCTL <80mV the phase

margin of the control amplifier is typically 90

°.

For PCTL voltages <80mV, the RF detected voltage is

0.6PCTL. For PCTL voltages >160mV, RF detected voltage

is 1.22PCTL – 0.1. This change in gain is due to an internal

compression circuit designed to extend the detector range.

For example, to determine the external RF channel loop

voltage gain with the loop closed, apply a 100mV step to

supply enough feedback voltage to the RF pin to cancel

this 100mV step which would be the required detected

to 1.528V to create the RF output power change required.

The net external voltage gain contributed by the RF power

amplifier and directional coupler network can be calcu-

lated by dividing the 60mV change at the RF pin by the

would then be approximately 2. The loop bandwidth

extends to 2 • BW1. If BW1 is 450kHz, the loop bandwidth

increases to approximately 0.9MHz. The phase margin

can be determined from Figures 2 and 3. Repeat the

above voltage gain measurement over the full power and

frequency range.

External pole frequencies within the loop will further

reduce phase margin. The phase margin degradation, due

to external and internal 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 LTC4402-X power

controller is included (Figures 6 and 7) to better determine

pole and zero interactions. The user can apply external

gains and poles to determine bandwidth and phase mar-

gin. DC, transient and RF information cannot be extracted

from this model. The model is suitable for external gain

evaluations up to 6

×. The 270kHz PCTL input filter limits

the bandwidth; therefore, use the RF input as demon-

strated in the model.

Figure 2. Measured Open Loop Gain and Phase, PCTL <80mV

Figure 3. Measured Open Loop Gain and Phase, PCTL >160mV

FREQUENCY (Hz)

100

–20

–10

0

20

10

40

30

1k

10k

100k

1M

10M

4402 F02

–30

–40

–50

–60

60

50

80

70

–20

0

20

60

40

100

80

–40

–60

–80

–100

–120

140

120

160

PHASE

GAIN

FREQUENCY (Hz)

100

–20

–10

0

20

10

40

30

1k

10k

100k

1M

10M

4402 F03

–30

–40

–50

–60

60

50

80

70

–20

0

20

60

40

100

80

–40

–60

–80

–100

140

120

180

160

PHASE

GAIN