BD9329AEFJ-LB Datasheet, PDF (9/21 Pages) Rohm – 4.2V to 18V, 3A 1ch Synchronous Buck Converter integrated FET

BD9329AEFJ-LB
Datasheet
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 + ΔIL /2
should not reach
VIN
the rated value level
ILR
VOUT
Inductor averaged current
L
COUT
t
Figure 19
Figure 20
Setting ∆IL = 30% x Averaged Inductor current (2A) = 0.6 [A]
1
L=
VOUT × (VIN - VOUT) x
= 10µ [H]
VIN x FOSC x ∆IL
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
(2) Loop Compensation
Choosing compensation capacitor CCMP and resistor RCMP
The current-mode buck converter has 2-poles and 1-zero system. Choosing the compensation resistor and capacitor is
important for a good load-transient response and good stability.
The example of DC/DC converter application bode plot is shown below.
The compensation resistor RCMP will decides the cross over frequency FCRS (the frequency that the total DC-DC
loop-gain falls to 0dB).
Setting the higher cross over frequency achieves good response speed, however less stability. While setting the lower
cross over frequency shows good stability but worse response speed.
The 1/10 of switching frequency for the cross over frequency shows a good performance at most applications.
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TSZ02201-0323AAJ00320-1-2
27.Feb.2014 Rev.002