LTC4366 datasheet(14/22 Pages) AD

14 / 22 page

LTC4249

Rev. 0

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• RM: the IMON sense and overcurrent setpoint resistor

(10kΩ)

• CF: the feedback capacitance used to provide a stable

loop (330pF)

• RG: resistance to limit op amp overshoot due to PA

input capacitance (100Ω)

• CG: PA input capacitance (150pF)

28V

COUT

10k

RON

IOUT

LTC4249

RF_PA

4249 F11

–

Figure 11. PA Servo Loop Components

It is important to understand how the loop components

affect stability and the closed loop response IOUT/VC.

The LTC4249 current monitor GM and RM components

may be converted to a Thevenin equivalent. CF is tem-

porarily removed for analysis. GMPA is the PA transcon-

ductance. The loop is transformed to a block diagram

equivalent in Figure 12, where A(s) is the op amp open

loop transfer function and the canonical G(s) and H(s)

blocks are outlined.

A(s)

(s • RG • CG + 1)

GMPA

RON

(s • RON • COUT + 1)

IOUT

GM • RM

G(s)

H(s)

4249 F12

–

Figure 12. Servo Loop Block Diagram without CF

Figure 13 shows the magnitude and phase of the uncom-

pensated loop gain G(s)H(s). It is easy to see inadequate

phase margin (~0°) at the crossover frequency. The

region near crossover consists of multiple deleterious

poles including contributions from RON • COUT, RG • CG

and high frequency op amp poles.

CROSSOVER

LOOP GAIN

PHASE

FREQUENCY (Hz)

100

10k

100k

10M

–20

–180

–135

–90

–45

4249 F13

Figure 13. Uncompensated Loop Gain and Phase

Crossing the system over at a lower frequency can help

achieve loop stability. This is easily done by adding feed-

back elements around the op amp. If we add a generalized

feedback impedance ZF around the op amp, the loop can

be stabilized and sensitivity to the op amp transfer func-

tion can be eliminated. Figure 14 shows the block diagram

with ZF added.

(s • RG • CG + 1)

GMPA

RON

(s • RON • COUT + 1)

IOUT

GM • (RM || ZF)

G(s)

H(s)

4249 F14

–

RM + ZF

Figure 14. Servo Loop Block Diagram with

Generalized Impedance ZF around Op Amp

With ZF = 1/sCF, the crossover frequency is lower

(~10kHz) and phase margin improves to almost 90° as

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