SGM6132 Datasheet, PDF (10/14 Pages) SG Micro Corp – 1.4MHz Step-Down Converter

SGM6132
3A, 28.5V, 1.4MHz Step-Down Converter
APPLICATION INFORMATION
The system has two poles of importance. One is due to
the compensation capacitor (C3) and the output resistor
of error amplifier, and the other is due to the output
capacitor and the load resistor. These poles are located
at:
f_P1

G_EA
2π  C3  A_EA
f_P2

1
2π  C_OUT
 R_LOAD
G_EA is the error amplifier transconductance, 800µA/V.
The system has one zero of importance, due to the
compensation capacitor (C3) and the compensation
resistor (R3). This zero is located at:
f _Z1

1
2π  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:
f_ESR

1
2π  C_OUT
 R_ESR
In this case, a third pole set by the compensation
capacitor (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:
f_P3

1
2π  C6  R3
The goal of compensation design is to shape the
converter 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 unstable. A good rule of
thumb is to set the crossover frequency to approximately
one-thirtieth of the switching frequency. Switching
frequency for the SGM6132 is 1.4MHz, so the desired
crossover frequency is around 47kHz.
Table 2 lists the typical values of compensation
components for some standard output voltages with
various output capacitors and inductors. The values of
the compensation components have been optimized for
fast transient responses and good stability at given
conditions.
Table 2. Compensation Values for Typical Output Voltage/
Capacitor Combinations
V_OUT
(V)
L
(µH)
C_OUT
(µF)
R3 C3 R1 R2
(kΩ) (nF) (kΩ) (kΩ)
0.8
2.2
47/22×2 1.2 3.3
0 10.5
1.2
2.2
47/22×2 3
3.3 4.99 10
1.8
2.2
47/22×2 3.9 3.3 10.2 8.2
2.5 2.2 - 4.7 47/22×2 6.49 4.7 22.6 10.7
3.3 2.2 - 4.7 47/22×2 10 5.6 33 10.5
5
4.7 - 6.8 47/22×2 15 4.7 52.3 10
12 6.8 - 10 47/22×2 39 2.2 140 10
INPUT
ENABLE
CIN
10μF
×2
R4
C5
10Ω or shorted 10nF
IN
EN
BS
SW
SGM6132
SS
GND
FB
COMP
C4
0.1μF
C6
C3
Optional
R3
L
R1
D1
R2
B340A
OUTPUT
COUT
Figure 2. Typical Application Circuit
SG Micro Corp
10
www.sg-micro.com