FAN5350(2007) データシートの表示（PDF） - Fairchild Semiconductor

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FAN5350
(Rev.:2007)

3MHz, 600mA Step-Down DC-DC Converter in Chip-Scale and MLP Packaging

Fairchild Semiconductor 

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:

ΔI ≈

VOUT

VIN

•

⎜⎜⎝⎛

VIN

L

− VOUT

• FSW

⎟⎟⎠⎞

EQ. 1

The maximum average load current, IMAX(LOAD) is related

to the peak current limit, ILIM(PK) (see figure 17) by the

ripple current:

IMAX(LOAD)

= ILIM(PK)

−

ΔI

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 current crosses zero, IDCM, is:

IDCM

=

ΔI

2

EQ. 3

The FAN5350 is optimized for operation with L=1μH, but

is stable with inductances ranging from 700nH to 3.0μH.

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

its value at ILIM(PK).

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.

IRMS =

IOUT(DC) 2

+

ΔI2

12

EQ. 4

The increased RMS current produces higher losses

through the RDS(ON) of the IC MOSFETs as well as the

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 1μH 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.

Increasing COUT has no effect on loop stability and can

therefore be increased to reduce output voltage ripple or

to improve transient response. Output voltage ripple,

∆VOUT, is:

ΔVOUT

=

ΔI

•

⎜⎜⎝⎛

8

•

1

COUT

•

FSW

+ ESR⎟⎟⎠⎞

EQ. 5

Input Capacitor

The 4.7μF 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 tantalum) should be placed between CIN

and the power source lead to reduce ringing that can

occur between the inductance of the power source leads

and CIN.

Inductor Value IMAX(LOAD) EQ. 2

ILIM(PK)

∆VOUT EQ. 5

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

© 2007 Fairchild Semiconductor Corporation

FAN5350 Rev. 1.0.1

7

www.fairchildsemi.com

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