![](http://datasheet.mmic.net.cn/280000/MP2106_datasheet_16098239/MP2106_8.png)
MP2106 – 1.5A, 15V, 800KHz SYNCHRONOUS BUCK CONVERTER
MP2106 Rev. 1.6
2/22/2006
www.MonolithicPower.com
8
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
2006 MPS. All Rights Reserved.
TM
Calculate the required inductance value by the
equation:
(
V
IN
×
)
I
f
V
V
V
L
SW
OUT
IN
OUT
×
×
=
Where
I is the peak-to-peak inductor ripple
current. It is recommended to choose
I to be
30%~40% of the maximum load current.
Compensation
The system stability is controlled through the
COMP pin. COMP is the output of the internal
transconductance error amplifier. A series
capacitor-resistor combination sets a pole-zero
combination to control the characteristics of the
control system.
The DC loop gain is:
LOAD
CS
VEA
OUT
V
FB
VDC
R
G
A
V
A
×
×
×
=
Where V
FB
is the feedback voltage, A
VEA
is the
transconductance error amplifier voltage gain,
G
CS
is the current sense transconductance
(roughly the output current divided by the
voltage at COMP) and R
LOAD
is the load
resistance:
OUT
OUT
V
LOAD
I
R
=
Where I
OUT
is the output load current.
The system has 2 poles of importance, one is
due to the compensation capacitor (C3), and
the other is due to the load resistance and the
output capacitor (C2), where:
3
C
A
2
G
f
VEA
EA
1
P
×
×
π
=
P1 is the first pole, and G
EA
is the error amplifier
transconductance (300μA/V) and
2
C
R
2
1
f
LOAD
2
P
×
×
π
=
The system has one zero of importance, due to
the compensation capacitor (C3) and the
compensation resistor (R3). The zero is:
3
C
3
R
2
1
f
1
Z
×
×
π
=
If large value capacitors with relatively high
equivalent-series-resistance (ESR) are used,
the zero due to the capacitance and ESR of the
output capacitor can be compensated by a third
pole set by R3 and C4. The pole is:
4
C
3
R
2
1
f
3
P
×
×
π
=
The system crossover frequency (the frequency
where the loop gain drops to 1, or 0dB, is
important. Set the crossover frequency to below
one tenth of the switching frequency to insure
stable operation. Lower crossover frequencies
result in slower response and worse transient
load recovery. Higher crossover frequencies
degrade the phase and/or gain margins and
can result in instability.
Table 1—Compensation Values for Typical
Output Voltage/Capacitor Combinations
V
OUT
C2
R3
C3
C4
1.8V 22μF Ceramic
2.5V 22μF Ceramic
3.3V 22μF Ceramic
47μF Tantalum
(300m
)
47μF Tantalum
(300m
)
47μF Tantalum
(300m
)
6.8k
9.1k
12k
3.3nF
2.2nF
1.8nF
None
None
None
1.8V
13k
2nF
1nF
2.5V
18k
1.2nF 750pF
3.3V
24k
1nF
560pF
Choosing the Compensation Components
The values of the compensation components
given in Table 1 yield a stable control loop for
the given output voltage and capacitor. To
optimize the compensation components for
conditions not listed in Table 1, use the
following procedure.