BD9329AEFJ Datasheet, PDF (8/16 Pages) Rohm – Simple Step-down Switching Regulator with Built-in Power MOSFET

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