MP1583 – 3A, 23V, 385KHz STEP-DOWN CONVERTER

MP1583 Rev. 3.1

www.MonolithicPower.com

7

6/20/2011

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© 2011 MPS. All Rights Reserved.

Output Rectifier Diode

The output rectifier diode supplies the current to

the inductor when the high-side switch is off.

Use a Schottky diode to reduce losses due to

the diode forward voltage and recovery times.

Choose a diode whose maximum reverse voltage

rating is greater than the maximum input voltage,

and whose current rating is greater than the

maximum load current. Table 2 lists example

Schottky diodes and manufacturers.

Table 2—Diode Selection Guide

Diode

Voltage/Current

Rating

Manufacture

SK33

30V, 3A

Diodes Inc.

SK34

40V, 3A

Diodes Inc.

B330

30V, 3A

Diodes Inc.

B340

40V, 3A

Diodes Inc.

MBRS330

30V, 3A

On Semiconductor

MBRS340

40V, 3A

On Semiconductor

Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required

to supply the AC current to the step-down

converter while maintaining the DC input

voltage. Use low ESR capacitors for the best

performance. Ceramic capacitors are preferred,

but tantalum or low-ESR electrolytic capacitors

will also suffice.

Since the input capacitor absorbs the input

switching current it requires an adequate ripple

current rating. The RMS current in the input

capacitor can be estimated by:

⎟⎟

⎟

⎠

⎞

⎜⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

C

V

V

1

V

V

I

I

1

where:

2

1

LOAD

C

I

I

=

For simplification, choose an input capacitor

whose RMS current rating is greater than half of

the maximum load current.

The input capacitor can be electrolytic, tantalum

or ceramic. When using electrolytic or tantalum

capacitors, a small, high quality ceramic

capacitor (i.e. 0.1μF) should be placed as close

to the IC as possible.

When using ceramic capacitors, make sure that

they have enough capacitance to provide

sufficient charge to prevent excessive voltage

ripple at the input. The input voltage ripple

caused by capacitance can be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

=

Δ

IN

OUT

IN

OUT

S

LOAD

IN

V

V

1

V

V

C

f

I

V

1

Where C1 is the input capacitance value.

Output Capacitor

The output capacitor is required to maintain the

DC output voltage. Ceramic, tantalum or low

ESR electrolytic capacitors are recommended.

Low ESR capacitors are preferred so as to

keep the output voltage ripple low. The output

voltage ripple can be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

+

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

Δ

2

8

1

1

C

f

R

V

V

L

f

V

V

S

ESR

IN

OUT

S

OUT

OUT

Where L is the inductor value, C2 is the output

series resistance (ESR) value of the output

capacitor.

In the case of ceramic capacitors, the

impedance at the switching frequency is

dominated by the capacitance, which is the

main cause for the output voltage ripple. For

simplification, the output voltage ripple can be

estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

×

=

IN

OUT

S

OUT

OUT

V

V

1

C

L

f

V

ΔV

2

8

2

In the case of tantalum or electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency. For simplification, the

output ripple can be approximated to:

ESR

IN

OUT

S

OUT

OUT

R

V

V

1

L

f

V

ΔV

×

⎟

⎠

⎞

⎜

⎝

⎛ −

×

×

=