EV2106DQ/DK-00A View Datasheet(PDF) - Monolithic Power Systems
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EV2106DQ/DK-00A Datasheet PDF : 11 Pages
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TM
MP2106 – 1.5A, 15V, 800KHz SYNCHRONOUS BUCK CONVERTER
Calculate the required inductance value by the
equation:
( ) L = VOUT × VIN − VOUT
VIN × fSW × ∆I
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:
A VDC
=
⎜⎜⎝⎛
VFB
VOUT
⎟⎟⎠⎞ × A VEA
× GCS
× RLOAD
Where VFB is the feedback voltage, AVEA is the
transconductance error amplifier voltage gain,
GCS is the current sense transconductance
(roughly the output current divided by the
voltage at COMP) and RLOAD is the load
resistance:
R LOAD
=
VOUT
IOUT
Where IOUT 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:
fP1
=
2π ×
GEA
A VEA
× C3
P1 is the first pole, and GEA is the error amplifier
transconductance (300µA/V) and
fP2
=
1
2π × RLOAD
× C2
The system has one zero of importance, due to
the compensation capacitor (C3) and the
compensation resistor (R3). The zero is:
f Z1
=
1
2π × R3 × C3
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:
fP3
=
1
2π × R3 × C4
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
VOUT
C2
1.8V 22µF Ceramic
2.5V 22µF Ceramic
3.3V 22µF Ceramic
1.8V
47µF Tantalum
(300mΩ)
2.5V
47µF Tantalum
(300mΩ)
3.3V
47µF Tantalum
(300mΩ)
R3
6.8kΩ
9.1kΩ
12kΩ
13kΩ
C3
3.3nF
2.2nF
1.8nF
2nF
C4
None
None
None
1nF
18kΩ 1.2nF 750pF
24kΩ 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.
MP2106 Rev. 1.6
www.MonolithicPower.com
8
2/22/2006
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2006 MPS. All Rights Reserved.
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