MPQ28261 – 21V, 3A, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER

MPQ28261 Rev. 1.12

www.MonolithicPower.com

16

10/10/2013

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Selecting the Input Capacitor

The MPQ28261 requires an input capacitor (C1) to

supply the AC current to the step-down

converter while maintaining the DC input

voltage, because the input current to the step-

down

converter

is

discontinuous.

Use

capacitors with low equivalent series resistance

(ESR)—such as ceramic capacitors with X5R or

X7R dielectrics with low ESR and small

temperature coefficients. For most applications,

use a 22µF capacitor.

Since C1 absorbs the input switching current, it

must withstand significant ripple current. 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 an input capacitor

with an RMS current rating greater than half of

the maximum load current.

The input capacitor can be electrolytic, tantalum

or ceramic. Include a small ceramic capacitor

with a value of ~0.1µF as close to the IC as

possible when using electrolytic or tantalum

capacitors.

Use

sufficiently-large

ceramic

capacitors that can provide sufficient charge to

prevent excessive voltage ripple. The capacitor-

induced input voltage ripple can be estimated

by:

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

Selecting the Output Capacitor

The output capacitor (C2) maintains the DC

output voltage. Use ceramic, tantalum, or low-

ESR electrolytic capacitors to minimize the

output voltage. The output voltage ripple can be

estimated as:

OUT

OUT

OUT

ESR

SIN

S

VV

1

V1

R

fL1

V

8 f

C2

⎛⎞

⎛⎞

Δ=

× −

×

+

⎜⎟

⎜⎟

×× ×

⎝⎠ ⎝⎠

ESR value of the output capacitor.

For

ceramic

capacitors,

the

capacitor

dominates the impedance at the switching

frequency, and subsequently dominates the

output voltage ripple. For simplicity, the output

voltage ripple can be estimated as:

OUT

OUT

OUT

2

SIN

VV

ΔV1

8f

L1 C2

V

⎛⎞

=× −

⎜⎟

×× ×

⎝⎠

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. Subsequently, the output ripple can

be approximated as:

OUT

OUT

OUT

ESR

IN

S

VV

ΔV1

R

fL1

V

⎛⎞

=× −

×

⎜⎟

×

⎝⎠

The characteristics of the output capacitor also

affect the system regulation stability. The

MPQ28261 can be optimized for a wide range

of capacitances and ESR values.

The maximum capacitance is 1100μF. (Tested

on Chroma 63030, CCH mode, 3A load current,

External Bootstrap Diode

An external bootstrap diode may enhance the

efficiency of the regulator under the following

applicable conditions:

Duty cycle is high: D=

IN

OUT

V

V

>65%

In these cases, an external BST diode is

recommended from the VCC pin to BST pin, as

shown in Figure 6.

MPQ28261

SW

BST

C

L

VCC

BST

External BST Diode

IN4148

+

Figure 6: Add Optional External Bootstrap Diode

to Enhance Efficiency

The recommended external BST diode is

IN4148 where the BST capacitor value is

between 0.1 and 1μF.