AME
2A, 28V, 340KHz Synchronous
Rectified Step-Down Converter
AME5269
n Detailed Description (Contd.)
Compensation Components
In this case, a third pole set by the compensation ca-
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:
AME5269 has current mode control for easy compensa-
tion and fast transient response. The system stability and
transient response are controlled through the C
pin.
OMP
COMP is the output of the internal transconductance error
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:
1
fP3 =
2p ´ C6´ R3
The goal of compensation design is to shape the con-
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.
V
FB
A
VDC = RLOAD ´ GCS ´ AEA´
V
OUT
Where VFB is the feedback voltage (0.925V), AVEA is the
error amplifier voltage gain, G is the current sense
CS
transconductance and RLOAD is the load resistor value. The
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:
1. Choose the compensation resistor (R3) to set
the desired crossover frequency.
GEA
Determine R3 by the following equation:
f
P1
=
=
2p ´ C3´ AVEA
2p ´ C2´ f
C
VOUT 2p ´ C2´ 0.1´ fs VOUT
1
R3 =
´
<
´
f
P2
G
EA´ GCS
VFB
G
EA´ GCS
VFB
2p ´ C2´ RLOAD
C
Where GEA is the error amplifier transconductance.
The system has one zero of importance, due to the com-
pensation capacitor (C3) and the compensation resistor
Where fC is the desired crossover frequency which is
typically below one tenth of the switching frequency.
i e
(R3). This zero is located at:
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.
1
f =
Z1
2p ´ C3´ R3
The system may have another zero of importance, if the
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:
Determine C3 by the following equation:
4
C3 >
2p ´ R3´ f
C
1
f
ESR
=
2p ´ C2´ RESR
Where R3 is the compensation resistor.
Rev. A.01
11
AME [ ANALOG MICROELECTRONICS ]
