MP2227 – 24V, 3A, 1.3MHz SYNCHRONOUS STEP-DOWN CONVERTER
MP2227 Rev. 1.1 www.MonolithicPower.com 11
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© 2011 MPS. All Rights Reserved.
APPLICATION INFORMATION
Setting the Output Voltage
The external resistor divider sets the output
voltage (see Typical Application Schematic). The
feedback resistor R1 also sets the feedback loop
bandwidth with the internal compensation
capacitor (see Figure 1). The relation between
R1 and feedback loop bandwidth (f
C)
, output
capacitance (C
O
) is as follows:
6
CO
1.24 10
R1(K )
f(kHz) C(F)
.
The feedback loop bandwidth (f
C
) is no higher
than 1/10
th
of switching frequency of MP2227. In
the case of ceramic capacitor as C
O
, it’s usually
set to be in the range of 50kHz and 150kHz for
optimal transient performance and good phase
margin. If electrolytic capacitor is used, the loop
bandwidth is no higher than 1/4
th
of the ESR zero
frequency (
f
ESR
). f
ESR
is given by:
ESR
ESR O
1
f
2R C
For example, choose f
C
=70kHz with ceramic
capacitor, C
O
=47uF, R1 is estimated to be
400KΩ. R2 is then given by:
OUT
R1
R2
V
1
0.8V
Table 1—Resistor Selection vs. Output
Voltage Setting
V
OUT
(V) R1 (k) R2 (k) L (H)
C
OUT
(ceramic)
1.2 400 806 0.47μH-1μH 47μF
1.5 400 453 0.47μH-1μH 47μF
1.8 400 316 0.47μH-1μH 47μF
2.5 400 187 0.47μH-1μH 47μF
3.3 400 127 0.47μH-1μH 47μF
Selecting the Inductor
A 0.47µH to 1µH inductor with DC current rating
at least 25% higher than the maximum load
current is recommended for most applications.
For best efficiency, the inductor DC resistance
shall be <10mΩ. See Table 2 for recommended
inductors and manufacturers. For most designs,
the inductance value can be derived from the
following equation:
OUT IN OUT
IN L OSC
Vx(V V)
L
VxIxf
Where ∆IL is Inductor Ripple Current. Choose
inductor ripple current approximately 30% of the
maximum load current, 3A.The maximum
inductor peak current is:
L
L(MAX) LOAD
I
II
2
Under light load conditions, larger inductance is
recommended for improved efficiency
Input Capacitor Selection
The input capacitor reduces the surge current
drawn from the input and switching noise from
the device. The input capacitor impedance at the
switching frequency shall be less than input
source impedance to prevent high frequency
switching current passing to the input. Ceramic
capacitors with X5R or X7R dielectrics are highly
recommended because of their low ESR and
small temperature coefficients. For most
applications, a 47µF capacitor is sufficient.
