Datasheet
Z
OUT ESR
1
C R 2
=
´ ´ p
f
P
OUT L
1
f
C R 2
=
´ ´ p
ps L
Adc = gm R´
Z
OUT
C
P
s
1
2
V
Adc
V
s
1
2
æ ö
+
ç ÷
p ´
è ø
= ´
æ ö
+
ç ÷
p ´
è ø
f
f
V
O
R
ESR
C
OUT
R
L
VC
gm
ps
fp
fz
Adc
TPS54340
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SLVSBK0B –OCTOBER 2012–REVISED MARCH 2014
Feature Description (continued)
As the load current increases and decreases, the low-frequency gain decreases and increases, respectively. This
variation with the load may seem problematic at first glance, but fortunately the dominant pole moves with the
load current (see Equation 11). The combined effect is highlighted by the dashed line in the right half of
Figure 31. As the load current decreases, the gain increases and the pole frequency lowers, keeping the 0-dB
crossover frequency the same with varying load conditions. The type of output capacitor chosen determines
whether the ESR zero has a profound effect on the frequency compensation design. Using high ESR aluminum
electrolytic capacitors may reduce the number frequency compensation components needed to stabilize the
overall loop because the phase margin is increased by the ESR zero of the output capacitor (see Equation 12).
Figure 31. Simple Small Signal Model and Frequency Response for Peak Current Mode Control
(9)
(10)
(11)
(12)
8.3.16 Small Signal Model for Frequency Compensation
The TPS54340 uses a transconductance amplifier for the error amplifier and supports three of the commonly-
used frequency compensation circuits. Compensation circuits Type 2A, Type 2B, and Type 1 are shown in
Figure 32. Type 2 circuits are typically implemented in high bandwidth power-supply designs using low ESR
output capacitors. The Type 1 circuit is used with power-supply designs with high-ESR aluminum electrolytic or
tantalum capacitors. Equation 13 and Equation 14 relate the frequency response of the amplifier to the small
signal model in Figure 32. The open-loop gain and bandwidth are modeled using the R
O
and C
O
shown in
Figure 32. See the application section for a design example using a Type 2A network with a low ESR output
capacitor.
Equation 13 through Equation 22 are provided as a reference. An alternative is to use WEBENCH software tools
to create a design based on the power supply requirements.
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