MP1423 – 3A, 23V, 385KHz STEP-DOWN CONVERTER

MP1423 Rev. 1.1

www.MonolithicPower.com

6

1/6/2006

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TM

Output Rectifier Diode

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 diode.

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

may also suffice.

Since the input capacitor (C1) 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

1

C

V

V

1

V

V

I

I

The worst-case condition occurs at VIN = 2VOUT,

where:

2

I

I

LOAD

1

C =

For simplification, choose the input capacitor

whose RMS current rating 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 input. The input voltage ripple caused by

capacitance can be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

=

∆

IN

OUT

IN

OUT

S

LOAD

IN

V

V

1

V

V

1

C

f

I

V

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 to keep the

output voltage ripple low. The output voltage

ripple can be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

+

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

∆

2

C

f

8

1

R

V

V

1

L

f

V

V

S

ESR

IN

OUT

S

OUT

OUT

Where L is the inductor value, C2 is the output

capacitance value, and RESR is the equivalent

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. The output voltage ripple is mainly

caused by the capacitance. For simplification, the

output voltage ripple can be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

×

=

IN

OUT

2

S

OUT

OUT

V

V

1

2

C

L

f

8

V

∆V

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

∆V

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP1423 can be optimized for a wide range of

capacitance and ESR values.