MP2229 – 21V, 6A SYNCHRONOUS STEP-DOWN CONVERTER

MP2229 Rev. 1.0

www.MonolithicPower.com

16

2/12/2015

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Generally, choose R6 to be around 100kΩ, then

R7 is:

LPM

CC

LPM

VR6

R7

VV

×

=

−

Refer to Figure 8 when setting the LPM voltage.

0.2

0.24

0.28

0.32

0.36

0.4

02468

10

FIGURE 8. Recommended LPM Selection for

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous,

and

therefore

requires

a

capacitor to supply the AC current while

maintaining the DC input voltage. Use low ESR

capacitors for the best performance. Ceramic

capacitors with X5R or X7R dielectrics are

recommended highly because of their low ESR

and small temperature coefficients. For most

applications, a 22µF and a 10µF capacitor are

sufficient.

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

where:

LOAD

C1

I

I

2

=

For simplification, choose an input capacitor

with a 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, place a small, high-quality ceramic

capacitor (e.g. 0.1μF) as close to the IC as

possible. When using ceramic capacitors, make

sure that they have enough capacitance to

provide sufficient charge in order to prevent

excessive voltage ripple at the input. The input-

voltage ripple caused by the capacitance can

be estimated by:

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

Selecting the Output Capacitor

The device requires an output capacitor (C2) to

maintain the DC output voltage. Use ceramic,

tantalum, or low ESR electrolytic capacitors.

Use low ESR capacitors to limit the output-

voltage ripple. Estimate the output-voltage

ripple with:

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2

⎛⎞

⎛⎞

Δ=

× −

×

+

⎜⎟

⎜⎟

×× ×

⎝⎠ ⎝⎠

equivalent series resistance (ESR) value of the

output capacitor.

For ceramic capacitors, the impedance at the

switching frequency is dominated by the

capacitance. The output-voltage ripple is

caused

mainly

by

the

capacitance.

For

simplification, the output-voltage ripple can be

estimated by:

OUT

OUT

OUT

2

IN

S1

VV

ΔV1

V

8f

L

C2

⎛⎞

=× −

⎜⎟

×× ×

⎝⎠

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated to:

OUT

OUT

OUT

ESR

IN

S1

VV

ΔV1

R

fL

V

⎛⎞

=× −

×

⎜⎟

×

⎝⎠

The characteristics of the output capacitor

affect the stability of the regulatory system. The

MP2229 can be optimized for a wide range of

capacitance and ESR values.