MP9403 – 3A, 7V, 250kHz INTEGRATED SYNCHRONOUS STEP-DOWN CONVERTER

MP9403 Rev.0.9

www.MonolithicPower.com

8

2/9/2010

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© 2010 MPS. All Rights Reserved.

Optional Schottky Diode

During the transition between high-side switch

and low-side switch, the body diode of the low-

side power MOSFET conducts the inductor

current. The forward voltage of this body diode is

high. An optional Schottky diode may be

paralleled between the SW pin and GND pin to

improve overall efficiency. Table 2 lists example

Schottky diodes and their Manufacturers.

Table 2—Diode Selection Guide

Part Number

Voltage/Current

Rating

Vendor

B320

20V, 3A

Diodes, Inc.

B320A

20V, 3A

Diodes, Inc.

SS32

20V, 3A

Vishay, Inc.

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 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 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. 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:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

=

Δ

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:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

+

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

Δ

2

C

f

8

1

R

V

V

1

L

f

V

V

S

ESR

IN

OUT

S

OUT

OUT

is the 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:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

×

=

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

MP9403 can be optimized for a wide range of

capacitance and ESR values.

Compensation Components

MP9403 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

×

×

×

=