MP2308 – 18V, 4A SYNCHRONOUS STEP-DOWN CONVERTER

MP2308 Rev. 1.0

www.MonolithicPower.com

12

7/9/2012

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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:

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V







 







Selecting the Output Capacitor

The output capacitor (C2) 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:

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.

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:

OUT

OUT

OUT

2

IN

S1

VV

ΔV1

V

8f

L

C2



 



 



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:

OUT

OUT

OUT

ESR

IN

S1

VV

ΔV1

R

fL

V



 









The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP2308 can be optimized for a wide range of

capacitance and ESR values.

External Bootstrap Diode

An external bootstrap diode may enhance the

efficiency of the regulator, the applicable

conditions of external BST diode are:





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 5.

Figure 5—Add Optional External Bootstrap

Diode to Enhance Efficiency

The recommended external BST diode is

IN4148, and the BST cap is 0.1~1μF.

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,

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.