BD9329AEFJ_15 Datasheet, PDF (10/23 Pages) Rohm – Synchronous Buck Converter with Integrated FET

BD9329AEFJ
3. 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 inductance value at this filter results
in less inductor ripple current (∆IL) and less output ripple voltage. However, inductors with large values tend to have
slower load transient-response, a larger physical size, a lower saturation current, and a higher series resistance. A
smaller value of inductance has almost opposite characteristics as 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.
ILI_L
IOUTMAX + ∆∆IILL /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]
L = V_OUT
× (V_IN
− V_OUT ) × _VIN
×
1
f_OSC
× ∆I _L
= 10µ
[H ]
Where:
V_IN= 12V, V_OUT= 3.3V, f_OSC= 380 kHz,
f_OSC is the switching frequency
Also the inductor should have higher saturation current than IOUTMAX + ∆IL / 2.
The output capacitor COUT affects the output ripple-voltage. Choose a high-value capacitor to achieve a smaller
ripple-voltage that is enough to meet the application requirement.
Output ripple voltage ∆VRPL is calculated using the following equation:
[ ] ∆V_RPL
= ∆I _L

× _{} R_ESR
+
1
8× C_OUT ×
f _OSC

V
Where:
R_ESR is the parasitic series resistance of the output capacitor.
Setting COUT = 20µF, RESR = 10mΩ
∆V_RPL = 0.6 × (10m + 1/(8 × 20µ × 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 appropriate compensation resistor and
capacitor is important to achieve good load-transient response and good stability.
An example of a DC/DC converter application bode plot is shown in Figure 22.
The compensation resistor, RCMP, determines the cross-over frequency fCRS (the frequency where the total DC-DC
loop-gain falls to 0dB).
Setting a higher cross-over frequency achieves good response speed, but less stability. On the other hand, setting the
cross-over frequency to a lower value may result to better stability, but poorer response speed.
Setting the cross-over frequency to 1/10 of the switching frequency shows good performance at most applications.
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TSZ22111・15・001
10/19
TSZ02201-0323AAJ00010-1-2
16.Feb.2015 Rev.003