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MP28248 Datasheet(PDF) 17 Page - Monolithic Power Systems

Part No. MP28248
Description  High-Efficiency, Fast-Transient, 3A, 4.2V-20V Input Synchronous Step-down Converter in a QFN12 (2x3mm) Package
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Maker  MPS [Monolithic Power Systems]
Homepage  http://www.monolithicpower.com
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MP28248 Datasheet(HTML) 17 Page - Monolithic Power Systems

 
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MP28248 – 3A, 4.2V-20V INPUT, FAST-TRANSIENT SYNCHRONOUS STEP-DOWN CONVERTER IN QFN12 (2X3mm)
MP28248 Rev. 1.0
www.MonolithicPower.com
17
1/5/2012
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
Inductor
The inductor supplies constant current to the
output load while being driven by the switched
input voltage. A larger-value inductor will result in
less ripple current that will result in lower output
ripple voltage. However, a larger-value inductor
will have a larger physical footprint, higher series
resistance, and/or lower saturation current. A
good rule for determining the inductance value is
to design the peak-to-peak ripple current in the
inductor to be in the range of 30% to 40% of the
maximum output current, and that the peak
inductor current is below the maximum switch
current limit. The inductance value can be
calculated by:
OUT
OUT
SW
L
IN
VV
L=
(1-
)
f
ΔIV
(25)
Where ΔIL is the peak-to-peak inductor ripple
current.
The inductor should not saturate under the
maximum inductor peak current, where the peak
inductor current can be calculated by:

 
OUT
OUT
LP
OUT
SW
IN
VV
II
(1
)
2f
L
V
(26)
Design Example
Some design examples with typical outputs are
provided in the following tables:
Table 2: 1.2V VOUT (L = 2μH)
VIN VOUT
(V)
R7
R4
C4
R1
R2
FSW
(V)
(Ω)
(Ω)
(F)
(Ω)
(Ω)
(Hz)
12
1.2
301k 806k 220p 17.4k 40.2k 440k
Table 3: 1.8V VOUT (L = 2μH)
VIN
VOUT
(V)
R7
R4
C4
R1
R2
FSW
(V)
(Ω)
(Ω)
(F)
(Ω)
(Ω) (Hz)
12
1.8 402k 649k 220p 30k 24.3k 500k
Table 4: 2.5V VOUT (L = 2μH)
VIN VOUT
(V)
R7
R4
C4
R1
R2
FSW
(V)
(Ω)
(Ω)
(F)
(Ω)
(Ω)
(Hz)
12 2.5 499k 499k 330p 21.5k 10k 544k
Table 5: 3.3V VOUT (L = 4.7μH)
VIN VOUT
(V)
R7
R4
C4
R1
R2 FSW
(V)
(Ω)
(Ω)
(F)
(Ω)
(Ω)(Hz)
12 3.3 680k 806k
330p 31.6k 10k 520k
Table 6: 5V VOUT (L = 8μH)
VIN
VOUT
(V)
R7
R4
C4
R1
R2 FSW
(V)
(Ω)
(Ω)
(F)
(Ω)
(Ω) (Hz)
12
5
1M
1.2M 220p 53.6k 10k 544k
The detailed application schematic is shown in
Figure 13. The typical performance and circuit
waveforms have been shown in the Typical
Performance Characteristics section. For more
possible applications of this device, please refer
to related Evaluation Board Data Sheets.
Typical Application Schematic
EN
VIN
25V
C1A
C5
301k
R7
806K
R4
SW
220pF
C4
1210
C2B
NS
C2D
NS
J1
SS
1.2V@3A
GND
VCC
GND
NS
R8
NS
D1
C3
40.2k
R2
1210
0603
C2A
C2C
7443552200
499k
R5
1nF
C7
0
R3
0
R9
17.4k
R1
VOUT
L1
MP28248
SS
6
VCC
4
FREQ
8
EN
5
IN
9
SW 2
FB 7
BST 3
SW 10
U1
33nF
C6
VOUT
4.2V-20V
Figure 13: Detailed Application Schematic


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