MP1591 – 2A, 32V, 330KHz STEP-DOWN CONVERTER

MP1591 Rev. 2.3

www.MonolithicPower.com

6

9/27/2006

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© 2006 MPS. All Rights Reserved.

The input capacitor value should be greater

than 10µF. The capacitor can be electrolytic,

tantalum or ceramic. However, since it absorbs

the input switching current it requires an

adequate ripple current rating. Its RMS current

rating should be greater than approximately 1/2

of the DC load current.

For insuring stable operation C1 should be

placed as close to the IC as possible.

Alternately, a smaller high quality ceramic

0.1µF capacitor may be placed closer to the IC

and a larger capacitor placed farther away. If

using this technique, it is recommended that the

larger capacitor be a tantalum or electrolytic

type. All ceramic capacitors should be placed

close to the MP1591.

Output Capacitor (C5)

The output capacitor is required to maintain the

DC output voltage. Low ESR capacitors are

preferred to keep the output voltage ripple low.

The characteristics of the output capacitor also

affect the stability of the regulation control

system. Ceramic, tantalum or low ESR

electrolytic capacitors are recommended. In the

case of ceramic capacitors, the impedance at

the switching frequency is dominated by the

capacitance, and so the output voltage ripple is

mostly independent of the ESR. The output

voltage ripple is estimated to be:

2

SW

LC

IN

RIPPLE

f

f

V

4

.

1

V

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

≅

switching frequency.

In the case of tantalum or low-ESR electrolytic

capacitors, the ESR dominates the impedance

at the switching frequency, and so the output

ripple is calculated as:

ESR

RIPPLE

R

I

V

×

∆

≅

output capacitors.

Output Rectifier Diode (D1)

The output rectifier diode supplies the current to

the inductor when the high-side switch is off. To

reduce losses due to the diode forward voltage

and recovery times, use a Schottky rectifier.

Table 2 provides some recommended Schottky

rectifiers based on the maximum input voltage

and current rating.

Table 2—Diode Selection Guide

2A Load Current

3A Load Current

(Max)

Part

Number

Vendor

Part

Number

Vendor

15V

30BQ15

4

B220

1

B320

1

SK23

6

SK33

1, 6

20V

SR22

6

SS32

3

20BQ030

4

B330

1

B230

1

B340L

1

SK23

6

MBRD330

4, 5

SR23

3, 6

SK33

1, 6

30V

SS23

2, 3

SS33

2, 3

21DQ04

4

B340L

1

MBRS240L

5

MBRS340

4

SK24

6

SK34

1, 6

34V

SS24

2, 3

SS34

2, 3

Table 3 lists manufacturer’s websites.

Table 3—Schottky Diode Manufacturers

#

Vendor

Web Site

1

Diodes, Inc.

www.diodes.com

2

Fairchild Semiconductor

www.fairchildsemi.com

3

General Semiconductor

www.gensemi.com

4

International Rectifier

www.irf.com

5

On Semiconductor

www.onsemi.com

6

Pan Jit International

www.panjit.com.tw

Choose a rectifier whose maximum reverse

voltage rating is greater than the maximum

input voltage, and whose current rating is

greater than the maximum load current.