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

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© 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)

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:

(V)

R7

R4

C4

R1

R2

(V)

(Ω)

(Ω)

(F)

(Ω)

(Ω)

(Hz)

12

1.2

301k 806k 220p 17.4k 40.2k 440k

(V)

R7

R4

C4

R1

R2

(V)

(Ω)

(Ω)

(F)

(Ω)

(Ω) (Hz)

12

1.8 402k 649k 220p 30k 24.3k 500k

(V)

R7

R4

C4

R1

R2

(V)

(Ω)

(Ω)

(F)

(Ω)

(Ω)

(Hz)

12 2.5 499k 499k 330p 21.5k 10k 544k

(V)

R7

R4

C4

R1

(V)

(Ω)

(Ω)

(F)

(Ω)

(Ω)(Hz)

12 3.3 680k 806k

330p 31.6k 10k 520k

(V)

R7

R4

C4

R1

(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

U1

33nF

C6

VOUT

4.2V-20V

Figure 13: Detailed Application Schematic