MP2918

—4V TO 40V SYNCHRONOUS STEP-DOWN CONTROLLER

MP2918 Rev. 1.02

www.MonolithicPower.com

22

5/31/2017

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Figure 7).

MP2918

INTERNAL

LDO

4.5V

IN

VCC1

VCC2

Figure 7: Configuration of VCC2 Connecting to

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 also has a larger physical

size,

higher

series

resistance,

and

lower

saturation current. Choose the inductor ripple

current

to

be

approximately

30%

of

the

maximum load current. The inductance value

can then be calculated with Equation (7):

OUT

IN

OUT

IN

L

S

V

(V - V

)

L

V

ΔIf







(7)

frequency, and

∆I

ripple current.

The maximum inductor peak current can be

calculated with Equation (8):

L

L(MAX)

LOAD

ΔI

I

=I

+

2

(8)

Selecting the Input Capacitor

Since the input capacitor absorbs the input

switching current, it requires an adequate ripple

current rating. The selection of the input

capacitor is based mainly on its maximum ripple

current capability. The RMS value of the ripple

current flowing through the input capacitor can

be calculated with Equation (9):

OUT

OUT

RMS

LOAD

IN

IN

VV

I

=I

(1-

)

VV

(9)

shown in Equation (10):

(10)

The

input

capacitor

must

be

capable

of

handling this ripple current.

Output Capacitor Selection

The output capacitor impedance should be low

at the switching frequency. The output voltage

ripple can be estimated with Equation (11):

OUT

OUT

OUT

ESR

S

IN

S

O

VV

1

ΔV

1

R

f

L

V

8 f

C







 











 

 

(11)

value of the output capacitor.

For

tantalum

or

electrolytic

capacitor

applications,

the

ESR

dominates

the

impedance at the switching frequency. The

output voltage ripple can be approximated with

Equation (12):

OUT

OUT

OUT

ESR

S

IN

VV

ΔV

1

R

f

L

V





 









(12)

Power MOSFET Selection

Two N-channel MOSFETs must be selected for

the controller: one for the high-side switch, and

one for the low-side switch.

The driver level of the HS-FET and LS-FET is

selected MOSFETs must be no higher than this

value.

The

input voltage

charge (Qg), and thermal-related parameters

should be considered when choosing the power

MOSFETs.

maximum applied voltage between the drain