MP2321 Rev. 1.0

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17

10/26/2016

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Selecting the Inductor

An inductor is necessary to supply constant

current to the output load while being driven by

the switched input voltage. An inductor with a

larger value results in less ripple current and

lower output ripple voltage. However, a larger

inductor also has a larger physical footprint,

higher series resistance, and lower saturation

current. A good rule for determining the

inductance value is to design the peak-to-peak

ripple current in the inductor to be 30% to 40%

of the maximum output current and the peak

inductor current to be below the maximum

switch current limit. The inductance value can

be calculated with Equation (14):

OUT

OUT

SW

L

IN

VV

L(1

)

FI

V

 



(14)

current.

The inductor should not saturate under the

maximum inductor peak current. The peak

inductor

current

can

be

calculated

with

Equation (15):

OUT

OUT

LP

OUT

SW

IN

VV

II

(1

)

2F

L

V



 



(15)

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous

and

therefore

requires

a

capacitor to supply AC current to the step-down

converter while maintaining the DC input

voltage. Ceramic capacitors are recommended

for the best performance and should be placed

as close to VIN as possible. Capacitors with

X5R

and

X7R

ceramic

dielectrics

are

recommended because they are fairly stable

with temperature fluctuations.

The capacitors must also have a ripple current

rating greater than the maximum input ripple

current of the converter. The input ripple current

can be estimated with Equation (16):

OUT

OUT

CIN

OUT

IN

IN

VV

II

(1

)

VV



 

(16)

shown in Equation (17):

OUT

CIN

I

I

2



(17)

For simplification, choose an input capacitor

with an RMS current rating greater than half of

the maximum load current.

The input capacitance value determines the

input voltage ripple of the converter. If there is

an input voltage ripple requirement in the

system, choose an input capacitor that meets

the specification.

The input voltage ripple can be estimated with

Equation (18):

OUT

OUT

OUT

IN

SW

IN

IN

IN

IV

V

V(1

)

FC

V

V





 



(18)

shown in Equation (19):

OUT

IN

SW

IN

I

1

V

4F

C

 



(19)

Selecting the Output Capacitor

The output capacitor is required to maintain the

DC output voltage. Ceramic or POSCAP

capacitors are recommended. The output

voltage ripple can be estimated with Equation

(20):

OUT

OUT

OUT

ESR

SW

IN

SW

OUT

VV

1

V(1

) (R

)

FL

V

8 F

C



 









(20)

In the case of ceramic capacitors, the

impedance at the switching frequency is

dominated by the capacitance. The output

voltage ripple is caused mainly by the

capacitance. For simplification, the output

voltage ripple can be estimated with Equation

(21):

OUT

OUT

OUT

2

SW

OUT

IN

VV

V(1

)

8F

L C

V



 

 

(21)

The output voltage ripple caused by the ESR is

very small. Therefore, an external ramp is

needed to stabilize the system. The external

ramp can be generated through the capacitor

Cr.

In the case of POSCAP capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated with Equation (22):

OUT

OUT

OUT

ESR

SW

IN

VV

V(1

) R

FL

V



 





(22)