MP2314 – 24V, 2A SYNC STEP DOWN CONVERTER

MP2314 Rev. 1.02

www.MonolithicPower.com

13

9/17/2014

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Figure 6: AAM Selection for Common Output

Selecting the 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 with X5R or

X7R dielectrics are highly recommended

because

of

their

low

ESR

and

small

temperature coefficients. For most applications,

a 22µF capacitor is 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

The worse case condition occurs at VIN =

2VOUT, 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:

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

MP1495 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%