MP28258 – 3A, 4.2V-20V INPUT, FAST TRANSIENT SYNCHRONOUS STEP-DOWN CONVERTER IN QFN12 (2X3mm)

MP28258 Rev 1.14

www.MonolithicPower.com

15

9/25/2012

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2012 MPS. All Rights Reserved.

soft-start time, and then it will fail to regulate. The

limited approximately by:

O_ MAX

LIM _ AVG

OUT

ss

OUT

C(I

I

) T / V

=−

×

(23)

Inductor

The inductor is necessary to supply 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

)

FI

V

=× −

×Δ

(24)

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

=+

× −

×

(25)

Recommend Design Example

Some design examples and recommended

maximum output capacitor value with typical

outputs are provided below when the ceramic

capacitors is applied:

(V)

(V)

C1

R7

(Ω)

R4

(Ω)

C4

(F)

R1

(Ω)

R2

(Ω)

(Hz)

12

1.2 10μF*1 300k 499k 220p 12.1k

24.3k 450k

5

1.2 10μF*1 300k 390k 220p 12.1k

24.3k 440k

(V)

(V)

C1

R7

(Ω)

R4

(Ω)

C4

(F)

R1

(Ω)

R2

(Ω)

(Hz)

12

1.8 10μF*1 402k 499k 220p

30k

24.3k 480k

51.8 10μF*1 402k 390k 220p

30k

24.3k 460k

(V)

(V)

C1

R7

(Ω)

R4

(Ω)

C4

(F)

R1

(Ω)

R2

(Ω)

(Hz)

12

2.5 10μF*1 500k 453k 390p 21.5k

10k

500k

52.5 10μF*1 500k 453k 390p 21.5k

10k

500k

(V)

(V)

C1

R7

(Ω)

R4

(Ω)

C4

(F)

R1

(Ω)

R2

(Ω)

(Hz)

12

3.3 10μF*1 680k 470k 330p 31.6k

10k

500k

53.3 10μF*1 680k 470k 330p 31.6k

10k

500k

(V)

(V)

C1

R7

(Ω)

R4

(Ω)

C4

(F)

R1

(Ω)

R2

(Ω)

(Hz)

12

5

10μF*1

1M

750k 330p 53.6k

10k

500k

Table 6—Recommended maximum output

(V)

1.2

1.8

2.5

3.3

5

(μF)

680

570

390

330

220

The detailed application schematic is shown in

Figure 13 when large ESR caps are used, and

Figure14 and Figure 15 when low ESR caps are

applied. 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.