© 2007 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN5350 Rev. 1.0.1

7

Applications Information

Selecting the Inductor

The output inductor must meet both the required

inductance and the energy handling capability of the

application.

The inductor value affects the average current limit, the

PWM-to-PFM transition point, the output voltage ripple,

and the efficiency.

The ripple current (∆I) of the regulator is:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

•

−

•

≈

Δ

SW

OUT

IN

IN

OUT

F

L

V

V

V

V

I

EQ. 1

ripple current:

2

I

I

I

)

PK

(

LIM

)

LOAD

(

MAX

Δ

−

=

EQ. 2

The transition between PFM and PWM operation is

determined by the point at which the inductor valley

current crosses zero. The regulator DC current when the

2

I

Δ

=

EQ. 3

The FAN5350 is optimized for operation with L=1

μH, but

is stable with inductances ranging from 700nH to 3.0

μH.

The inductor should be rated to maintain at least 80% of

Efficiency is affected by the inductor DCR and

inductance value. Decreasing the inductor value for a

given physical size typically decreases the DCR; but

since ∆I increases, the RMS current increases, as do

the core and skin effect losses.

12

I

I

I

2

2

)

DC

(

OUT

RMS

Δ

+

=

EQ. 4

The increased RMS current produces higher losses

inductor ESR.

Increasing the inductor value produces lower RMS

currents, but degrades transient response. For a given

physical inductor size, increased inductance usually

results in an inductor with lower saturation current.

Table 1 shows the effects of inductance higher or lower

than the recommended 1

μH on regulator performance.

Output Capacitor

Table 2 suggests 0603 capacitors. 0805 capacitors may

further improve performance in that the effective

capacitance is higher and ESL is lower than 0603. This

improves the transient response and output ripple.

therefore be increased to reduce output voltage ripple or

to improve transient response. Output voltage ripple,

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

•

•

•

Δ

=

Δ

ESR

F

C

8

1

I

V

SW

OUT

OUT

EQ. 5

Input Capacitor

The 4.7

μF ceramic input capacitor should be placed as

close as possible between the VIN pin and GND to

minimize the parasitic inductance. If a long wire is used

to bring power to the IC, additional “bulk” capacitance

and the power source lead to reduce ringing that can

occur between the inductance of the power source leads

Inductor Value

Transient Response

Increase

Increase

Decrease

Decrease

Degraded

Decrease

Decrease

Increase

Increase

Improved

Table 1. Effects of changes in inductor value (from 1µH recommended value) on regulator performance