MP2305 – 2A, 23V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER

MP2305 Rev. 1.7

www.MonolithicPower.com

7

4/17/2006

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© 2006 MPS. All Rights Reserved.

TM

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 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 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

∆V

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

∆V

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP2305 can be optimized for a wide range of

capacitance and ESR values.

Compensation Components

MP2305 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

EA

CS

LOAD

VDC

V

V

A

G

R

A

×

×

×

=

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.

The system has one zero of importance, due to the

compensation capacitor (C3) and the compensation

resistor (R3). This zero is located at:

3

R

3

C

2

1

Z

×

×

π

=

The system may have another zero of

importance, if the output capacitor has a large

capacitance and/or a high ESR value. The zero,

due to the ESR and capacitance of the output

capacitor, is located at:

ESR

ESR

R

2

C

2

1

f

×

×

π

=