LTC4367 datasheet(15/22 Pages) AD

15 / 22 page

LTC4249

Rev. 0

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shown in Figure 15. Step response as shown on the front

page is clean with a rise time of about 10µs.

CROSSOVER

LOOP GAIN

PHASE

FREQUENCY (Hz)

100

10k

100k

–20

–180

–135

–90

–45

4249 F15

Figure 15. Compensated Loop Gain and Phase

(ZF = 1/sCF, CF = 330pF)

When COUT is large (closer to 100µF), phase margin may

be lower compared to the 10µF example. With large COUT,

another loop term should be considered for its phase

margin impact. During step changes in load current, the

LTC4249 current monitor output lags load current briefly.

Transport lag results in phase erosion that is a function of

frequency but does not impact the loop gain magnitude.

The phase erosion is equal to 360° • f • T, where T ≈ 2µs.

At the crossover frequency f = 10kHz, phase margin loss

due to transport lag is –7.2°.

In a low phase margin situation, a lead-lag network may

be required to control the step response. Adding a resis-

tor (RZ) in series with CF provides a leading zero and a

small capacitor (CP) around RZ provides a lagging pole

to restore high frequency roll-off (Figure 16). For analysis

purposes, simply condense the compensation network

around the op amp into a ZF impedance and use the model

in Figure 14 to obtain the new loop gain and phase (add

the transport lag loss term to the phase as needed).

10k

330pF

20k

33pF

4249 F16

Figure 16. Lead-Lag Network Example

RF PA Bias Sequencing

Some RF PAs have a strict bias sequence order. Use the

LTC4249 EN inputs, RDY outputs and IMON outputs to

simplify the sequencing process. A recommended bias

sequence during power up is the following:

1) Start with the PA VDD near ground. To achieve this,

begin with EN low (ECB off, OUT near ground). Drive

the PA VGG to a suitably negative voltage to guarantee

PA BIAS current = 0. RDY should be pulling low at this

time.

2) Raise EN above threshold (0.8V). The LTC4249 enters

its current limited operating phase and charges the

OUT capacitance with 1A. After 300µs, OUT is checked

for voltage above 1.2V. If it is less, the channel output

is deemed to be “shorted” and the corresponding ECB

shuts off. Assuming no short, the LTC4249 completes

current limited operating phase after 10ms and begins

low RON operation. RDY is now released and may be

pulled high. OUT voltage should now be close to IN.

3) After RDY release, increase PA VGG voltage until desired

BIAS current is achieved. BIAS current may be inferred

from the voltage at the IMON output.

4) Apply RFIN signal.

APPLICATIONS INFORMATION

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