MP2303A – 3A, 28V, 360kHz SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER

MP2303A Rev.1.1

www.MonolithicPower.com

10

7/25/2012

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2012 MPS. All Rights Reserved.

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 are preferred, but tantalum or

low-ESR electrolytic capacitors may also suffice.

Choose X5R or X7R dielectrics when using

ceramic capacitors.

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:

)

V

V

1

(

V

V

I

I

IN

OUT

IN

OUT

LOAD

1

C









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:

























IN

OUT

IN

OUT

S

LOAD

IN

V

V

1

V

V

1

C

f

I

V

Output Capacitor

The output capacitor 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:











































2

C

f

8

1

R

V

V

1

L

f

V

V

S

ESR

IN

OUT

S

OUT

OUT

Where C2 is the output capacitance value and

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:

























IN

OUT

2

S

OUT

OUT

V

V

1

2

C

L

f

8

V

ΔV

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:

ESR

IN

OUT

S

OUT

OUT

R

V

V

1

L

f

V

ΔV























The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP2303A can be optimized for a wide range of

capacitance and ESR values.

Compensation Components

MP2303A employs current mode control for

easy compensation and fast transient response.

The system stability and transient response are

controlled through the COMP pin. COMP pin is

the output of the internal transconductance

error amplifier. A series capacitor-resistor

combination sets a pole-zero combination to

control the characteristics of the control system.

The DC gain of the voltage feedback loop is

given by:

OUT

FB

VEA

CS

LOAD

VDC

V

V

A

G

R

A









2 poles of importance. One is due to the

compensation capacitor (C3) and the output

resistor of error amplifier, and the other is due

to the output capacitor and the load resistor.

These poles are located at:

VEA

EA

1

P

A

3

C

2

G

f







