MP28372 — DUAL 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER

MP28372 Rev. 1.4

www.MonolithicPower.com

8

12/10/2007

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© 2007 MPS. All Rights Reserved.

TM

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 prevent excessive voltage

ripple at input. The input voltage ripple caused

by capacitance can be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

Δ

IN

OUT

IN

OUT

LOAD

IN

V

V

1

V

V

1

C

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

1

L

f

V

V

S

ESR

IN

OUT

S

OUT

OUT

Where L1 is the inductor value, C2 is the output

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

1

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

1

L

f

V

ΔV

×

⎟

⎠

⎞

⎜

⎝

⎛

−

×

×

=

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP28372 can be optimized for a wide range of

capacitance and ESR values.

Compensation Components

The MP28372 employs current mode control on

each channel 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

×

×

×

=

The system has two 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

×

×

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: