Data Sheet

ADD5205

Rev. 0 | Page 11 of 16

EXTERNAL COMPONENT SELECTION GUIDE

Inductor Selection

The inductor is an integral part of the step-up converter. It stores

energy during the switch-on time and transfers that energy to

the output through the output diode during the switch-off time.

An inductor in the 3.3 μH to 6.8 μH range is recommended. In

general, lower inductance values result in higher saturation

current and lower series resistance for a given physical size.

duty cycle (D), which in turn can be used to determine the

inductor ripple current.

OUT

IN

OUT

V

V

V

D

−

=

SW

ON

f

D

t

=

L

t

V

I

ON

IN

L

×

=

Δ

Solve for the inductance value (L).

L

ON

IN

I

t

V

L

Δ

×

=

Ensure that the peak inductor current (that is, the maximum

input current plus half of the inductor ripple current) is less

than the rated saturation current of the inductor. In addition,

ensure that the maximum rated rms current of the inductor is

greater than the maximum dc input current to the regulator.

L

LAVG

LPK

I

I

I

Δ

2

1

+

=

,

IN

OUT

OUT

LAVG

V

η

V

I

I

×

×

=

OUT

SW

IN

OUT

IN

L

V

f

V

V

V

L

I

×

−

=

Δ

1

where:

is the peak inductor current.

is the input average current.

Table 6 shows a list of recommend inductors.

Table 6. Recommended Inductors

Coilcraft Part No.

L (μH)

Size (mm)

XFL4020-332ML

3.3

2.7

4 × 4 × 2

LPS4012-472ML

4.7

1.6

3.9 × 3.9 × 1.1

LPS4018-472ML

4.7

1.8

3.9 × 3.9 × 1.7

LPS4018-682ML

6.8

1.2

3.9 × 3.9 × 1.7

Input and Output Capacitors Selection

The ADD5205 requires input and output bypass capacitors to

supply transient currents while maintaining a constant input

and output voltage. Use a low effective series resistance (ESR)

4.7 μF or greater capacitor for the input capacitor to prevent noise

at the ADD5205 input. Place the input between VIN and GND,

as close as possible to the ADD5205.

The output capacitor maintains the output voltage and supplies

current to the load while the ADD5205 switch is on. The value

and characteristics of the output capacitor greatly affect the

output voltage ripple and stability of the regulator. Use a ceramic

X5R or X7R dielectric capacitor, and for the output capacitor, a

4.7 μF or greater capacitor is preferred.

Place a 100 nF or greater capacitor as close as possible to the

OVP pin of ADD5205.

Diode Selection

The output diode conducts the inductor current to the output

capacitor and loads while the switch is off. For high efficiency,

minimize the forward voltage drop of the diode. Schottky diodes

are recommended.

The output diode for a boost regulator must be chosen depending

on the output voltage and the output current. The diode must

be rated for a reverse voltage greater than the output voltage used.

The average current rating must be greater than the maximum

load current expected, and the peak current rating must be greater

than the peak inductor current.