MP9472–0.45A, 18V, NON-SYNCHRONOUS, RECTIFIED, STEP-DOWN CONVERTER

MP9472 Rev. 1.0

www.MonolithicPower.com

12

12/9/2015

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© 2015 MPS. All Rights Reserved.

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, add a small, high-quality ceramic

capacitor (i.e.: 0.1μF) placed as close to the IC

as possible. When using ceramic capacitors,

ensure that they have enough capacitance to

provide a sufficient charge to prevent excessive

voltage ripple at the input. The input voltage

ripple for low ESR capacitors can be estimated

with Equation (8):

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

=

Δ

IN

OUT

IN

OUT

S

LOAD

IN

V

V

1

V

V

f

1

C

I

V

(8)

Where C1 is the input capacitance value.

Selecting the 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 recommended to keep

the output voltage ripple low. The output voltage

ripple can be estimated with Equation (9):

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

+

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

Δ

2

C

f

8

1

R

V

V

1

L

f

V

V

S

ESR

IN

OUT

S

OUT

OUT

(9)

Where C2 is the output capacitance value and

value of the output capacitor.

With 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 with

Equation (10):

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

×

×

=

IN

OUT

2

S

OUT

OUT

V

V

1

2

C

L

f

8

V

ΔV

(10)

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 with Equation (11):

ESR

IN

OUT

S

OUT

OUT

R

V

V

1

L

f

V

ΔV

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

×

=

(11)

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP9472 can be optimized for a wide range of

capacitance and ESR values.

Compensation Components

The MP9472 employs current-mode control for

easy compensation and fast transient response.

The system stability and transient response are

controlled through COMP. COMP is the output of

the internal transconductance error amplifier. A

series resistor-capacitor combination sets a pole-

zero combination to control the characteristics of

the control system.

The DC gain of the voltage feedback loop can be

calculated with Equation (12):

OUT

FB

EA

CS

LOAD

VDC

V

V

A

G

R

A

×

×

×

=

(12)

The system has two important poles. 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 determined with

Equation (13) and Equation (14):

VEA

EA

1

P

A

3

C

2

G

f

×

×

π

=

(13)

LOAD

2

P

R

2

C

2

1

f

×

×

π

=

(14)

The system has one important zero due to the

compensation

capacitor

(C3)

and

the

compensation resistor (R3). This zero can be

determined with Equation (15):

3

R

3

C

2

1

Z

×

×

π

=

(15)

The system may have another zero if the output

capacitor has a large capacitance or a high ESR

value. This zero is due to the ESR and

capacitance of the output capacitor and can be

determined with Equation (16):

ESR

ESR

R

2

C

2

1

f

×

×

π

=

(16)