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:
Table 3—Compensation Values for Typical
Output Voltage/Capacitor Combinations
VOUT
L
C2
R3
C3
C6
1.8V 4.7µH
100µF
Ceramic
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
1
2.5V
3.3V
5V
4.7-
6.8µH
47µF
Ceramic
fESR
=
2π × C2× RESR
6.8-
10µH
22µFx2
Ceramic
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:
10-
22µFx2
Ceramic
15µH
12V
1.8
15-
22µH
22µFx2
Ceramic
4.7µH
100µF
SP-CAP
1
fP3
=
2π× C6×R3
2.5V
3.3V
5V
4.7-
6.8µH
47µF
SP-CAP
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.
6.8-
10µH
47µF
SP-CAP
10-
47µF
SP CAP
15µH
2.5V
3.3V
5V
4.7-
560µF Al.
6.8µH 30mΩ ESR
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.
6.8-
10µH
560µF Al
30mΩ ESR
10-
470µF Al.
15kΩ 7.5nF
1nF
15µH
30mΩ ESR
12V
15-
220µF Al.
15kΩ 10nF 390pF
22µH
30mΩ ESR
To optimize the compensation components for
conditions not listed in Table 2, the following
procedure can be used.
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.
1. Choose the compensation resistor (R3) to set
the desired crossover frequency. Determine the
R3 value by the following equation:
2π × C2× fC VOUT
R3 =
×
GEA × GCS
VFB
Where fC is the desired crossover frequency,
34KHz.
MP1570 Rev. 1.5
1/31/2006
www.MonolithicPower.com
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2006 MPS. All Rights Reserved.
8
MPS [ MONOLITHIC POWER SYSTEMS ]
