BD9E302EFJ Datasheet, PDF (31/44 Pages) Rohm – Integrated MOSFET Single Synchronous Buck DC/DC Converter

BD9E302EFJ
Datasheet
4. Output LC Filter
The DC/DC converter requires an LC filter for smoothing the output voltage in order to supply a continuous current to the
load. Selecting an inductor with large inductance causes the ripple current ∆IL that flows into the inductor to be small,
decreasing the ripple voltage generated in the output voltage, it is a trade-off of size and cost of the inductor. In BD9E302EFJ,
the ripple current feedback to IC, and internal SLLM^TM(Simple Light Load Mode)control it, Since the optimal operation
feedback ripple current designed based on the recommended inductance, please use recommended inductor values.
.
VIN
IL
Inductor saturation current > IOUTMAX +∆IL /2
IOUTMAX
∆IL
Driver
L
VOUT
Average inductor current
t
C_OUT
Figure 75. Waveform of current through inductor
Figure 76. Output LC filter circuit
Here, select an inductance so that the size of the ripple current component of the inductor will be 20% to 50% of the Max
output current (3A).
Now calculating with VIN = 12V, VOUT = 5V, switching frequency FOSC = 550kHz, ∆IL is1.0A, inductance value
That can be used is calculated as follows:
L V_OUT
V _IN ‐ V_OUT
V _IN
1
F_OSC
ΔI _L
5.3 ≒ 4.7 μH
* If the output voltage setting is larger than half of VIN please calculated as follows:
L
4
V _IN
F_OSC ΔI _L
Also for saturation current of inductor, select the one with larger current than maximum output current added by 1/2 of
inductor ripple current ∆IL.
Output capacitor COUT affects output ripple voltage characteristics. Select output capacitor COUT so that necessary ripple
voltage characteristics are satisfied.
Output ripple voltage can be expressed in the following method.
ΔV _RPL ΔI _L
R _ESR
1
8 C _OUT F_OSC
RESR is the equivalent series resistance of the output capacitor
With COUT = 44µF, RESR = 10mΩ the output ripple voltage is calculated as
ΔV _RPL
1.0
10m
8
1
44μ 550k
15.17 [mV]
End the calculation.
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