Datasheet

f
Z1
C
C1
=
A
EA
x 10,000 x f
P2
= 27 pF
100
1k 10k 100k 1M
FREQUENCY (Hz)
-100
-70
-40
-10
20
50
PHASE (
o
)
100
1k 10k 100k 1M
FREQUENCY (Hz)
0
12
24
36
48
60
GAIN (dB)
G
EA
x OPG
H
EA
=
1 + G
EA
+ OPG
2S x 9 MHz
OPG =
s
LM2743
SNVS276G –APRIL 2004–REVISED MARCH 2013 www.ti.com
1. Increase A
EA
2. Check overall bandwidth and phase margin
3. Change V
IN
to minimum and recheck overall bandwidth and phase margin
4. 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 101dB 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:
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 37.
Figure 37. Error Amp. Gain and Phase
In VM regulators, the top feedback resistor R
FB2
forms a part of the compensation. Setting R
FB2
to 10 kΩ, ±1%
usually gives values for the other compensation resistors and capacitors that fall within a reasonable range.
(Capacitances > 1pF, resistances < 1MΩ) C
C1
, C
C2
, C
C3
, R
C1
, and R
C2
are selected to provide the poles and
zeroes at the desired frequencies, using the following equations:
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