FAN53701, FAN53702, FAN53703

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10

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 average current limit,

output voltage ripple, and efficiency.

The ripple current (

DI) of the regulator is:

DI [

V

OUT

V

IN

@

V

L

@ f

SW

(eq. 1)

, by the ripple

current, given by:

I

DI

2

(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 inductor

I

DI

2

(eq. 3)

The FAN5370X is optimized for operation with L =

1.0

mH, but is stable with inductances up to 1.3 mH

(nominal). The inductor should be rated to maintain at least

.

Efficiency is affected by the inductor DCR and inductance

value. Decreasing the inductor value for a given physical

size typically decreases the DCR; but because

DI increases,

the RMS current increases, as do the core and skin effect

losses.

I

I

OUT(DC)

2

) DI

2

12

(eq. 4)

The increased RMS current produces higher losses

, as well as the

inductor DCR

.

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 and higher DCR.

Table 9 shows the effects of inductance higher or lower

than the recommended 1.0

mH on regulator performance.

Output Capacitor

therefore be increased to reduce output voltage ripple or to

used but with a compromise of load transient response.

Output voltage ripple,

DV

f

2

2

@ D @ (1 * D)

)

1

8

@ f

(eq. 5)

Input Capacitor

The 2.2

mF 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 (electrolytic or

source lead to reduce the ringing that can occur between the

.

increases due to DC bias effects.

PCB Layout Guidelines

close as possible to the VIN and GND pins.

Connect to VIN and GND using only top metal.

2. Place the inductor (L) as close as possible to the

IC. Use short wide traces for the main current

paths.

close as possible to the IC. Connection to GND

should only be on top metal. Feedback signal

connection to VOUT should be routed away from

noisy components and traces (e.g. SW line)

.

Table 9. EFFECTS OF CHANGES in Inductor Value (from 1.0

mH Recommended Value) on Regulator Performance

Inductor Value

Transient Response

Increase

Increase

Decrease

Degraded

Decrease

Decrease

Increase

Improved