MP28251 – 20V, 4A SYNCHRONOUS STEP-DOWN CONVERTER

MP28251 Rev. 1.16

www.MonolithicPower.com

12

4/17/2013

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For simplification, choose the 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, 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 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







 







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







 









 

 

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 mainly

caused 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 also

affect the stability of the regulatory system. The

MP28251 can be optimized for a wide range of

capacitance and ESR values.

External Bootstrap Diode

An external bootstrap diode can enhance the

efficiency of the regulator, given the following

conditions:





Duty cycle is high: D=

IN

OUT

V

V

>65%

Use an external BST diode from the VCC pin to

the BST pin—as shown in Figure 5—for such

cases.

SW

BST

MP28251

C

L

BST

External BST Diode

VCC

IN4148

Figure 5: Optional Bootstrap Diode

The recommended external BST diode is

IN4148, and the BST capacitor is 0.1µF to 1μF.

PC Board Layout

This PCB board layout is referring to the

schematic in Figure 6.

Place the high-current paths (GND, IN and SW)

very close to the device with short, direct and

wide traces. The input decoupling capacitor

needs to be placed as close as possible to the

IN and GND pins. The VCC decoupling

capacitor needs to be placed as close as

possible to the VCC pin and multiple VIAs

should be used on both the ground side of the

VCC decoupling capacitor and the GND pins to

connect to the inner and bottom ground plane.

Place the external feedback resistors next to

the FB pin. Keep the switching node SW short

and away from the feedback network.