MP2307 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER

MP2307 Rev. 1.7

www.MonolithicPower.com

8

3/14/2006

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2006 MPS. All Rights Reserved.

TM

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

input capacitor with a 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 for low ESR capacitors can

be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

=

∆

IN

OUT

IN

OUT

S

LOAD

IN

V

V

1

V

V

f

1

C

I

V

Where C1 is the input capacitance value.

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:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

+

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

∆

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.

When using ceramic capacitors, the impedance

at the switching frequency is dominated by the

capacitance which is the main cause for the

output voltage ripple. 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

∆V

When using 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

∆V

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP2307 can be optimized for a wide range of

capacitance and ESR values.

Compensation Components

MP2307 employs current mode control for easy

compensation and fast transient response. The

system stability and transient response are

controlled through the COMP pin. COMP is the

output of the internal transconductance error

amplifier.

A

series

capacitor-resistor

combination sets a pole-zero combination to

govern the characteristics of the control system.

The DC gain of the voltage feedback loop is

given by:

OUT

FB

EA

CS

LOAD

VDC

V

V

A

G

R

A

×

×

×

=

is the load resistor value.

The system has two poles of importance. One

is due to the compensation capacitor (C3) and

the output resistor of the 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

×

×

π

=

LOAD

2

P

R

2

C

2

1

f

×

×

π

=

Where

is

the

error

amplifier

transconductance.