MPQ4458 –INDUSTRIAL GRADE,1A, 4MHz, 36V STEP-DOWN CONVERTER

MPQ4458 Rev. 1.0

www.MonolithicPower.com

11

12/5/2012

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© 2012 MPS. All Rights Reserved.

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 3

lists

example

Schottky

diodes

and

manufacturers.

Table 3—Output Diodes

Manufacturer

Part

Number

Voltage

Rating

(V)

Current

Rating

(A)

Package

Diodes Inc.

B240A-

13-F

40V

2A

SMA

Diodes Inc.

B340A-

13-F

40V

3A

SMA

Central semi

CMSH2-

40M

40V

2A

SMA

Central semi

CMSH3-

40MA

40V

3A

SMA

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

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

where:

2

I

I

LOAD

1

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:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

=

Δ

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:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

+

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

Δ

2

C

f

8

1

R

V

V

1

1

L

f

V

V

S

ESR

IN

OUT

S

OUT

OUT

Where L is the inductor value, CO 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

1

L

f

8

V

ΔV

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

1

L

f

V

ΔV

×

⎟

⎠

⎞

⎜

⎝

⎛ −

×

×

=

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MPQ4458 can be optimized for a wide range of

capacitance and ESR values.