BD9329AEFJ データシートの表示（PDF） - ROHM Semiconductor

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BD9329AEFJ

Simple Step-down Switching Regulator with Built-in Power MOSFET

ROHM Semiconductor 

BD9329AEFJ

Technical Note

●Selecting Application Components

(1) Output LC filter constant selection (Buck Converter)

The Output LC filter is required to supply constant current to the output load. A larger value inductance at this filter

results in less inductor ripple current (∆IL) and less output ripple voltage. However, the larger value inductors tend to

have less fast load transient-response, a larger physical size, a lower saturation current and higher series resistance.

A smaller value inductance has almost opposite characteristics above.So Choosing the Inductor ripple current (∆IL)

between 20 to 40% of the averaged inductor current (equivalent to the output load current) is a good compromise.

IL

IOUTMAX + IL /2

should not reach

VIN

the rated value level

ILR

VOUT

Inductor averaged current

L

COUT

t

Fig.18

Fig.19

Setting ∆IL = 30% x Averaged Inductor current (2A) = 0.6 [A]

1

L=

VOUT  (VIN - VOUT) x

VIN x FOSC x ∆IL

= 10µ [H]

Where VIN= 12V, VOUT= 3.3V, FOSC= 380 kHz,

; FOSC is a switching frequency

Also the inductor should have the higher saturation current than IOUTMAX + ∆IL / 2.

The output capacitor COUT affects the output ripple-voltage. Choose the large capacitor to achieve the small

ripple-voltage enough to meet the application requirement.

Output ripple voltage ∆VRPL is calculated by the following equation.

∆VRPL = ∆IL  ( RESR +

1

8x COUT x FOSC

) [V]

Where RESR is a parasitic series resistance in output capacitor.

Setting COUT = 20µF, RESR = 10mΩ

∆VRPL = 0.6 x (10m + 1 / (8 x 20u x 380k)) = 15.8mV

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8/15

2011.02 - Rev.A

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