MP2249 – 6V, 3A, 1MHZ, SYNCHRONOUS STEP-DOWN CONVERTER
APPLICATION INFORMATION
Output Voltage Setting
The external resistor divider sets the output
voltage (see Page 1, Schematic Diagram). The
feedback resistor R1 also set the feedback loop
bandwidth with the internal compensation.
The feedback loop bandwidth (f
C
) is no higher
than 1/10
th
of switching frequency of MP2249. In
the case of output ceramic capacitor as C
O
, it is
usually set in the range of 50kHz and 100kHz for
optimal transient performance and good phase
margin. If an electrolytic capacitor is used, the
loop bandwidth is no higher than 1/4 of the ESR
zero frequency (f
ESR
). f
ESR
is given by:
where
∆I
L
is Inductor Ripple Current. Choose
inductor ripple current approximately 30% of the
maximum load current 3A.
The maximum inductor peak current is:
I
L(MAX)
= I
LOAD
+
∆I
L
2
SC
(A)
6
6.5
LxWxH
3
(mm )
7.3×7.3×4.5
7×6.9×3
Table 2—Suggested Surface Mount Inductors
Vendor
WURTH
744777002
744310200
TDK
RLF7030T-
1R5N6R1-T
1.5
8
6.5
7.8×6.8×3.2
2.2
2
13
14.2
Part
Number
L
(µH)
DCR
(mΩ)
f
ESR
=
1
2
π ×
R
ESR
×
C
O
For example, choose f
C
=80kHz with a ceramic
capacitor, C
O
=47µF, R1 is estimated to be
150kΩ. R2 is then given by:
R2
=
R1
V
OUT
−
1
0.6V
Table 1—Resistor Selection vs.
Output Voltage Setting
V
OUT
1.2V
1.5V
1.8V
2.5V
3.3V
R1
150kΩ
150kΩ
150kΩ
150kΩ
150kΩ
R2
150kΩ
100kΩ
75kΩ
47.5kΩ
33.2kΩ
L
1.5µH-2.2µH
1.5µH-2.2µH
1.5µH-2.2µH
1.5µH-2.2µH
1.5µH-2.2µH
C
OUT
(Ceramic)
22µF x 2
22µF x 2
22µF x 2
22µF x 2
22µF x 2
Input Capacitor Selection
The input capacitor reduces the surge current
drawn from the input and the 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
source. 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.
Output Capacitor Selection
The output capacitor keeps output voltage ripple
small and ensures a stable regulation loop. The
output capacitor impedance shall be low at the
switching frequency. Ceramic capacitors with
X5R or X7R dielectrics are recommended. If an
electrolytic capacitor is used, pay attention to
output ripple voltage, extra heating, and the
selection of feedback resistor R1 (refer to “Output
Voltage Setting” section) due to the large ESR of
electrolytic capacitor. The output ripple
∆V
OUT
is
approximately:
∆
V
OUT
≤
V
OUT
x(V
IN
- V
OUT
)
1
x(ESR +
)
V
IN
xf
OSC
xL
8xf
OSC
xC
O
Inductor Selection
A 1.5µH to 2.2µH 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
<20mΩ. See Table 2 for recommended inductors
and Vendors. For most designs, the inductance
value can be derived from the following equation:
V
OUT
x(V
IN
- V
OUT
)
L=
V
IN
x∆I
L
xf
OSC
MP2249 Rev.1.0
11/2/2010
www.MonolithicPower.com
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9
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