ame5268 AME, Inc., ame5268 Datasheet - Page 13

• Manufacturer Part Number: ame5268
• Description: 3a, 28v, 340khz Synchronous Rectified Step-down Converter
• Manufacturer: AME, Inc.

n Detailed Description (Contd.)
Rev. A.01
AME5268
tion and fast transient response. The system stability and
transient response are controlled through the C
C
amplifier. A series capacitor-resistor combination sets a
pole-zero combination to govern the characteristics of the
control system. The DC gain of the voltage feedback loop
is given by:
error amplifier voltage gain, G
transconductance and R
system has two poles of importance. One is due to the
output capacitor and the load resistor, and the other is due
to the compensation capacitor (C3) and the output resistor
of the error amplifier. These poles are located at:
The system has one zero of importance, due to the com-
pensation capacitor (C3) and the compensation resistor
(R3). This zero is located at:
output capacitor has a large capacitance and/or a high ESR
value. The zero, due to the ESR and capacitance of the
output capacitor, is located at:
OMP
Compensation Components
AME5268 has current mode control for easy compensa-
Where G
The system may have another zero of importance, if the
Where V
A
f
f
f
f
P
P
Z
ESR
VDC
1
2
1
is the output of the internal transconductance error
2
2
2
2
R
FB
EA
AME
LOAD
is the error amplifier transconductance.
is the feedback voltage (0.925V), A
C
C
C
G
1
C
3
3
2
EA
1
1
2
G
R
A
R
CS
R
VEA
3
LOAD
ESR
LOAD
A
EA
is the load resistor value. The
V
CS
V
OUT
FB
is the current sense
VEA
OMP
is the
pin.
i e
C
pacitor (C6) and the compensation resistor (R3) is used
to compensate the effect of the ESR zero on the loop
gain. This pole is located at:
verter transfer function to get a desired loop gain. The
system crossover frequency where the feedback loop has
the unity gain is important. Lower crossover frequencies
result in slower line and load transient responses, while
higher crossover frequencies could cause system insta-
bility. A good standard is to set the crossover frequency
below one-tenth of the switching frequency. To optimize
the compensation components, the following procedure
can be used.
1. Choose the compensation resistor (R3) to set
the desired crossover frequency.
Determine R3 by the following equation:
typically below one tenth of the switching frequency.
2. Choose the compensation capacitor (C3) to achieve
the desired phase margin. For applications with typical
inductor values, setting the compensation zero (fZ1) be-
low one-forth of the crossover frequency provides suffi-
cient phase margin.
Determine C3 by the following equation:
C
R
f
Rectified Step-Down Converter
P
In this case, a third pole set by the compensation ca-
The goal of compensation design is to shape the con-
3
Where f
Where R3 is the compensation resistor.
3A, 28V, 340KHz Synchronous
3
3
2
2
2
G
EA
C
C
is the desired crossover frequency which is
C
R
4
1
2
G
3
6
CS
f
R
C
f
C
3
V
V
OUT
FB
2
G
C
EA
2
G
0
1 .
CS
fs
V
V
OUT
FB
13