LM2743MTC datasheet(17/23 Pages) NSC

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Application Information (Continued)

point is one-fifth of the switching frequency, or 60kHz for this

example. The generic equation for the error amplifier transfer

function is:

In this equation the variable A

EA is a ratio of the values of the

capacitance and resistance of the compensation compo-

nents, arranged as shown in Figure 11.A

EA is selected to

provide the desired bandwidth. A starting value of 80,000 for

EA should give a conservative bandwidth. Increasing the

value will increase the bandwidth, but will also decrease

phase margin. Designs with 45-60˚ are usually best because

they represent a good tradeoff between bandwidth and

phase margin. In general, phase margin is lowest and gain

highest (worst-case) for maximum input voltage and mini-

mum output current. One method to select A

EA is to use an

iterative process beginning with these worst-case conditions.

Increase A

Check overall bandwidth and phase margin

Change V

IN to minimum and recheck overall bandwidth

and phase margin

Change I

O to maximum and recheck overall bandwidth

and phase margin

The process ends when the both bandwidth and the phase

margin are sufficiently high. For this example input voltage

can vary from 3.0 to 3.6V and output current can vary from 0

to 4A, and after a few iterations a moderate gain factor of

110,000 is used.

The error amplifier of the LM2743 has a unity-gain band-

width of 9MHz. In order to model the effect of this limitation,

the open-loop gain can be calculated as:

The new error amplifier transfer function that takes into

account unity-gain bandwidth is:

The gain and phase of the error amplifier are shown in

Figure 13.

In VM regulators, the top feedback resistor R

FB2 forms a part

of the compensation. Setting R

FB2 to 10k

Ω, ±1% usually

gives values for the other compensation resistors and ca-

pacitors that fall within a reasonable range. (Capacitances >

1pF, resistances < 1M

Ω)C

C1,CC2,CC3,RC1, and RC2 are

selected to provide the poles and zeroes at the desired

frequencies, using the following equations:

20095274

20095275

FIGURE 13. Error Amp. Gain and Phase

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