MBRS130 Ver la hoja de datos (PDF) - Monolithic Power Systems
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MBRS130 Datasheet PDF : 11 Pages
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TM
MP1570 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
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:
fESR
=
1
2π × C2 × RESR
In this case (as shown in Figure 2), 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:
fP3
=
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-tenth
of the switching frequency. Switching frequency
for the MP1570 is 340KHz, so the desired
crossover frequency is 34KHz.
Table 3 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 3—Compensation Values for Typical
Output Voltage/Capacitor Combinations
VOUT
L
1.8V 4.7µH
2.5V
3.3V
5V
12V
1.8
4.7-
6.8µH
6.8-
10µH
10-
15µH
15-
22µH
4.7µH
2.5V
3.3V
5V
2.5V
3.3V
5V
12V
4.7-
6.8µH
6.8-
10µH
10-
15µH
4.7-
6.8µH
6.8-
10µH
10-
15µH
15-
22µH
C2
100µF
Ceramic
47µF
Ceramic
22µFx2
Ceramic
22µFx2
Ceramic
22µFx2
Ceramic
100µF
SP-CAP
47µF
SP-CAP
47µF
SP-CAP
47µF
SP CAP
560µF Al.
30mΩ ESR
560µF Al
30mΩ ESR
470µF Al.
30mΩ ESR
220µF Al.
30mΩ ESR
R3 C3 C6
5.6kΩ 3.3nF None
4.7kΩ 4.7nF None
5.6kΩ 3.3nF None
7.5kΩ 3.3nF None
10kΩ 1.2nF None
10kΩ 2.2nF 100pF
5.6kΩ 3.3nF None
6.8kΩ 2.2nF None
10kΩ 2.2nF None
10kΩ 7.5nF 1.5nF
10kΩ 10nF 1.5nF
15kΩ 7.5nF 1nF
15kΩ 10nF 390pF
To optimize the compensation components for
conditions not listed in Table 2, the following
procedure can be used.
1. Choose the compensation resistor (R3) to set
the desired crossover frequency. Determine the
R3 value by the following equation:
R3 = 2π × C2 × fC × VOUT
GEA × GCS VFB
Where fC is the desired crossover frequency,
34KHz.
MP1570 Rev. 1.5
www.MonolithicPower.com
8
1/31/2006
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