DIO6905

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© 2023 DIOO MICROCIRCUITS CO., LTD

DIO6905 • Rev. 1.8

- 11 -

For most designs, the inductance value can be estimated with Equation (3):

OSC

L

IN

OUT

IN

OUT

1

f

*

ΔI

*

V

)

V

-

(V

*

V

=

L

(3)

Where ΔIL is the inductor ripple current.

Choose an inductor current to be approximately 30% of the maximum load current. The maximum inductor

peak current can be calculated with Equation (4):

2

ΔI

+

I

=

I

L

LOAD

L(MAX)

(4)

Selecting the input capacitor

The input current to the step-down converter is discontinuous, and therefore requires a capacitor to supply the

AC current to the step-down converter while maintaining the DC input voltage. Use low-ESR capacitors for the

best performance. Ceramic capacitors with X5R or X7R dielectrics are highly recommended because of their low

ESR values and small temperature coefficients. For most applications, a 10 µF capacitor is sufficient. Higher

output voltages may require a 22 μF capacitor to increase system stability.

The input capacitor requires an adequate ripple current rating since it absorbs the input switching current.

Estimate the RMS current in the input capacitor with Equation (5):

)

V

V

(1

*

V

V

*

I

=

I

IN

OUT

IN

OUT

LOAD

C1

-

(5)

The worst-case scenario occurs at VIN = 2 VOUT, shown in Equation (6):

2

I

=

I

LOAD

C1

(6)

For simplification, choose an input capacitor with 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, 0.1 μF ceramic capacitor as close to the IC as possible. When using ceramic capacitors,

ensure that they have enough capacitance to provide sufficient charge to prevent excessive voltage ripple at the

input. The input voltage ripple caused by capacitance can be estimated with Equation (7):

)

V

V

-

(1

*

V

V

*

C

*

f

I

=

V

IN

OUT

IN

OUT

1

S

LOAD

IN

△

(7)

Selecting the output capacitor

The output capacitor (C2) stabilizes the DC output voltage. Ceramic capacitors are recommended. Use low ESR

capacitors to limit the output voltage ripple. Estimate the output voltage ripple with Equation (8):

)

C

*

f

*

8

1

+

(R

*

)

V

V

-

(1

*

L

*

f

V

=

V

2

S

ESR

IN

OUT

1

S

OUT

OUT

△

(8)

Where L1 is the inductor value and RESR is the equivalent series resistance (ESR) value of the output capacitor.