BD9104FVM datasheet(12/18 Pages) ROHM

12 / 18 page

BD9102FVM, BD9104FVM, BD9106FVM

Technical Note

12/17

www.rohm.com

2009.05 - Rev.A

●

Selection of components externally connected

1. Selection of inductor (L)

* Current exceeding the current rating of the inductor results in magnetic saturation of the inductor, which decreases efficiency.

The inductor must be selected allowing sufficient margin with which the peak current may not exceed its current rating.

If VCC=5V, VOUT=3.3V, f=1MHz, ΔIL=0.3×0.8A=0.24A, for example,

*Select the inductor of low resistance component (such as DCR and ACR) to minimize dissipation in the inductor for better efficiency.

2. Selection of output capacitor (CO)

As the output rise time must be designed to fall within the soft-start time, the capacitance of output capacitor should be

determined with consideration on the requirements of equation (5):

In case of BD9104FVM, for instance, and if VOUT=3.3V, IOUT=0.8A, and TSS=1ms,

Inappropriate capacitance may cause problem in startup. A 10 μF to 100 μF ceramic capacitor is recommended.

3. Selection of input capacitor (Cin)

A low ESR 10μF/10V ceramic capacitor is recommended to reduce ESR dissipation of input capacitor for better efficiency.

The inductance significantly depends on output ripple current.

As seen in the equation (1), the ripple current decreases as the

inductor and/or switching frequency increases.

ΔIL=

(VCC-VOUT)×VOUT

L×VCC×f

[A]・・・(1)

Appropriate ripple current at output should be 30% more or less of the

maximum output current.

ΔIL=0.3×IOUTmax. [A]・・・(2)

(VCC-VOUT)×VOUT

ΔIL×VCC×f

[H]・・・(3)

(ΔIL: Output ripple current, and f: Switching frequency)

Output capacitor should be selected with the consideration on the stability region

and the equivalent series resistance required to smooth ripple voltage.

Output ripple voltage is determined by the equation (4)：

ΔVOUT=ΔIL×ESR [V]・・・(4)

(ΔIL: Output ripple current, ESR: Equivalent series resistance of output capacitor)

*Rating of the capacitor should be determined allowing sufficient margin against

output voltage. Less ESR allows reduction in output ripple voltage.

Input capacitor to select must be a low ESR capacitor of the capacitance

sufficient to cope with high ripple current to prevent high transient voltage. The

ripple current IRMS is given by the equation (6):

IRMS=IOUT×

VCC(VCC-VOUT)

VCC

[A]・・・(6)

√

When VCC is twice the Vout, IRMS=

IOUT

Fig.51 Output capacitor

(5-3.3)×3.3

0.24×5×1M

=4.675μ → 4.7[μH]

< Worst case > IRMS(max.)

If VCC=5V, VOUT=3.3V, and IOUTmax.=0.8A,

IRMS=0.8×

5(5-3.3)

=0.46[ARMS]

√

Co≦

TSS×(Ilimit-IOUT)

VOUT

・・・

(5)

Tss: Soft-start time

Ilimit: Over current detection level, 2A(Typ)

Fig.52 Input capacitor

Fig.50 Output ripple current

VCC

VOUT

VCC

VOUT

ESR

Co≦

1m×(2-0.8)

3.3

≒

364 [μF]

VCC

VOUT

Cin

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