MP9928—4V TO 60V SYNCHRONOUS STEP-DOWN CONTROLLER

MP9928 Rev. 1.0

www.MonolithicPower.com

20

5/20/2016

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MP9928

Internal

LDO

4.7V

VCC1

VCC2

Selecting the Inductor

An inductor with a DC current rating at least 25%

higher

than

the

maximum

load

current

is

recommended for most applications. A larger

value inductor results in less ripple current and a

lower output ripple voltage. However, the larger

value inductor has a larger physical size, higher

series resistance, and lower saturation current.

Generally, choose the inductor ripple current

approximately 30% of the maximum load current.

Then the inductance value can be then be

calculated by:

OUT

IN

OUT

IN

L

S

V

(V - V

)

L

V

ΔI f

×

=

××

L

ΔI is

the peak-to-peak inductor ripple current.

The maximum inductor peak current is:

L

L(MAX)

LOAD

ΔI

I

=I

+

2

Input Capacitor Selection

Since the input capacitor absorbs the input

switching current, it requires an adequate ripple

current

rating.

The

selection

of

the

input

capacitor is mainly based on its maximum ripple

current capability. The RMS value of the ripple

current flowing through the input capacitor can be

described as:

OUT

OUT

RMS

LOAD

IN

IN

VV

I

=I

(1-

)

VV

selected must be capable of handling this ripple

current.

Output Capacitor Selection

The output capacitor keeps the output voltage

ripple small and ensures regulation loop stability.

The output capacitor impedance should be low at

the switching frequency. The output voltage

ripple can be estimated by:

OUT

OUT

OUT

ESR

S

IN

S

O

VV

1

ΔV

1R

f

L

V

8f

C







=

×−

×

+







×

××

 



value of the output capacitor.

For tantalum or electrolytic capacitor application,

the ESR dominates the impedance at the

switching frequency. So the above formula can

be approximated as:

OUT

OUT

OUT

ESR

S

IN

VV

ΔV

1R

f

L

V



=

×−

×



×



Compensation Components

The MP9928 employs current-mode control for

easy compensation and fast transient response.

The COMP pin controls system stability and

transient response. The COMP pin is the output

of the internal error amplifier. A series capacitor-

resistor

combination

sets

a

pole-zero

combination to control the control system’s

characteristics. The DC gain of the voltage

feedback loop is:

OUT

FB

O

CS

LOAD

VDC

V

V

A

G

R

A

×

×

×

=

3000V/V,

is

the

current-sense

transconductance,

(A/V),

and