MP28259 – 2A, 4.2V-20V INPUT, FAST TRANSIENT SYNCHRONOUS STEP-DOWN CONVERTER

MP28259 Rev. 1.34

www.MonolithicPower.com

15

4/27/2012

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has an around 1ms soft-start time period. If the

output capacitor value is too high, the output

voltage can’t reach the design value during the

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

=−

×

(24)

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

L1

FI

V

⎛⎞

=× −

⎜⎟

×Δ

⎝⎠

(25)

current.

Choose an inductor that will not saturate under

the maximum inductor peak current. The peak

inductor current can be calculated by:

OUT

OUT

LP

OUT

SW

IN

VV

II

1

2F

L

V

⎛⎞

=+

× −

⎜⎟

×

⎝⎠

(26)

Design Examples

Some design examples and recommended

maximum output capacitor value with typical

outputs are provided below when the ceramic

capacitors is applied with R9=0ohm:

(V)

(V)

C1

R7

(Ω)

R4

(Ω)

R1

(Ω)

R2

(Ω)

(Hz)

12

1.2

10μF*1 300k

499k

12.1k 24.3k

450k

5

1.2

10μF*1 300k

390k

12.1k 24.3k

440k

(V)

(V)

C1

R7

(Ω)

R4

(Ω)

R1

(Ω)

R2

(Ω)

(Hz)

12

1.8

10μF*1 402k

499k

12.1k 24.3k

480k

5

1.8

10μF*1 402k

390k

12.1k 24.3k

460k

(V)

(V)

C1

R7

(Ω)

R4

(Ω)

R1

(Ω)

R2

(Ω)

(Hz)

12

2.5

10μF*1 536k

499k

12.1k

5.9k

490k

5

2.5

10μF*1 536k

390k

12.1k

5.9k

480k

(V)

(V)

C1

R7

(Ω)

R4

(Ω)

R1

(Ω)

R2

(Ω)

(Hz)

12

3.3

10μF*1 698k

560k

12.1k 3.92k

500k

5

3.3

10μF*1 698k

560k

12.1k 3.92k

485k

(V)

(V)

C1

R7

(Ω)

R4

(Ω)

R1

(Ω)

R2

(Ω)

(Hz)

12

5

10μF*1 887k

560k

12.1k 2.32k

600k

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.