LM2743MTCSLASHNOPB datasheet(25/44 Pages) TI2

25 / 44 page

CC2 =

AEA x 10,000

- CC1 = 882 pF

fZ1

CC1 =

AEA x 10,000 x fP2

= 27 pF

100

10k

100k

FREQUENCY (Hz)

-100

-70

-40

-10

100

10k

100k

FREQUENCY (Hz)

GEA x OPG

HEA =

1 + GEA + OPG

2S x 9 MHz

OPG =

LM2743

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SNVS276I – APRIL 2004 – REVISED FEBRUARY 2019

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Typical Applications (continued)

In this equation, the variable AEA is a ratio of the values of the capacitance and resistance of the compensation

components, arranged as shown in Figure 32. AEA is selected to provide the desired bandwidth. A starting value

of 80,000 for AEA should give a conservative bandwidth. Increasing the value will increase the bandwidth, but will

also decrease phase margin. Designs with 45° to 60° are usually best because they represent a good trade-off

between bandwidth and phase margin. In general, phase margin is lowest and gain highest (worst-case) for

maximum input voltage and minimum output current. One method to select AEA is to use an iterative process

beginning with these worst-case conditions.

1. Increase AEA

2. Check overall bandwidth and phase margin

3. Change VIN to minimum and recheck overall bandwidth and phase margin

4. Change IO 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 3.6 V and output current can vary from 0 to 4 A, and after a few iterations a

moderate gain factor of 101 dB is used.

The error amplifier of the LM2743 has a unity-gain bandwidth of 9 MHz. In order to model the effect of this

limitation, the open-loop gain can be calculated as:

(35)

The new error amplifier transfer function that takes into account unity-gain bandwidth is:

(36)

The gain and phase of the error amplifier are shown in Figure 36.

Figure 35. Gain vs Frequency

Figure 36. Error Amplifier Gain and Phase

In VM regulators, the top feedback resistor RFB2 forms a part of the compensation. Setting RFB2 to 10 kΩ, ±1%

usually gives values for the other compensation resistors and capacitors that fall within a reasonable range.

(Capacitances > 1 pF, resistances < 1 M

Ω) CC1, CC2, CC3, RC1, and RC2 are selected to provide the poles and

zeroes at the desired frequencies, using the following equations:

(37)

(38)

LM2743MTC/NOPB Datasheet(PDF) 25 Page - Texas Instruments

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