MP2326 Rev. 1.1

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18

1/28/2016

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Equations 12 and 13 are the typical switching

frequency calculation formulas. The actual

frequency will change a little at different load

currents and with different input voltages.

Selecting the Inductor

The inductor is necessary to supply constant

current to the output load while being driven by

the switched input voltage. A larger value

inductor results in less ripple current, which

results in lower output-ripple voltage. However,

a larger value inductor will have a larger

physical footprint, a higher series resistance,

and/or a 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 in the range of 30% to 40% of the

maximum output current. Make sure that the

peak inductor current is below the maximum

switch current limit. The inductance value can

be calculated by:

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 by:

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 the AC while maintaining

the DC input voltage. Ceramic capacitors are

recommended for 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 as follows:

OUT

OUT

CIN

OUT

IN

IN

VV

II

(1

)

VV



 

(16)

where:

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 the input capacitor that meets

the specification.

The input-voltage ripple can be estimated as

follows:

OUT

OUT

OUT

IN

SW

IN

IN

IN

IV

V

V(1

)

FC

V

V





 



(18)

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 as:

OUT

OUT

OUT

ESR

SW

IN

SW

OUT

VV

1

V(1

) (R

)

FL

V

8 F

C



 









(20)

If using 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 as follows:

OUT

OUT

OUT

2

SW

OUT

IN

VV

V(1

)

8F

L C

V



 

 

(21)

The output-voltage ripple caused by ESR is

very small. Therefore, an external ramp is

needed to stabilize the system. The external

ramp can be generated through the capacitor

Cr.

When using POSCAP capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated as: