MP2108DQ MPS [Monolithic Power Systems], MP2108DQ Datasheet - Page 9

• Manufacturer Part Number: MP2108DQ
• Description: 2A, 6V, 740KHz Synchronous Buck Converter
• Manufacturer: MPS [Monolithic Power Systems]

Calculate the required inductance value by the
equation:
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:
Where V
is the transconductance error amplifier voltage
gain, 400 V/V and G
transconductance, (roughly the output current
divided by the voltage at COMP), 4.5A/V.
R
Where I
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:
P1 is the first pole, and G
transconductance (450µA/V) and
The system has one zero of importance, due to
the compensation capacitor (C3) and the
compensation resistor (R3). The zero is:
MP2108 Rev 1.1
10/2/2006
LOAD
is the load resistance:
A
OUT
VDC
FB
is the output load current.
is the feedback voltage, 0.9V, A
=
L
⎛
⎜
⎜
⎝
f
f
P
=
P
V
2
f
V
1
Z
OUT
V
R
FB
=
1
=
OUT
LOAD
=
2
2
V
⎞
⎟
⎟
⎠
π
π
2
IN
×
×
×
π
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
×
×
CS
A
(
R
×
=
A
V
G
f
VEA
R
LOAD
SW
IN
VEA
EA
V
1
I
EA
1
is the current sense
3
OUT
OUT
−
×
is the error amplifier
×
×
×
V
C
×
∆
G
OUT
C
3
C
I
CS
3
2
)
×
R
MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER
LOAD
© 2006 MPS. All Rights Reserved.
www.MonolithicPower.com
VEA
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:
The system crossover frequency (the frequency
where the loop gain drops to 1dB or 0dB) is
important. Set the crossover frequency 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.
Choosing the Compensation Components
The values of the compensation components
listed in Table 1 yields 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.
Table 1—Compensation Values for Typical
1.8V 22µF Ceramic
2.5V 22µF Ceramic
3.3V 22µF Ceramic
1.8V
2.5V
3.3V
V
Output Voltage/Capacitor Combinations
OUT
47µF Tantalum
(300mΩ)
47µF Tantalum
(300mΩ)
47µF Tantalum
(300mΩ)
C2
f
P
3
=
2
π
×
R
6.8kΩ
9.1kΩ
12kΩ
13kΩ
18kΩ
24kΩ
1
3
R3
×
C
4
3.3nF
2.2nF
1.8nF
1.2nF
2nF
1nF
C3
750pF
560pF
None
None
None
1nF
C4
9