BD9E300EFJ-LB Datasheet, PDF (20/33 Pages) Rohm – 7.0V to 36V Input, 2.5A Integrated MOSFET Single Synchronous Buck DC/DC Converter

BD9E300EFJ-LB
Selection of Components Externally Connected
1. 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 a large inductance causes the ripple current ∆IL that flows into the inductor to be small,
decreasing the ripple voltage generated in the output voltage, but it is not advantageous in terms of the load transient
response characteristic. Selecting an inductor with a small inductance improves the transient response characteristic but
causes the inductor ripple current to be large, which increases the ripple voltage in the output voltage, showing a trade-off
relationship. Here, select an inductance so that the size of the ripple current component of the inductor will be 20% to 50%
of the average output current (average inductor current).
I_L
VIN
Inductor saturation current > I_OUTMAX +⊿I_L /2
IOUTMAX
⊿I_L
Average inductor current
Driver
L
VOUT
COUT
Figure 45. Waveform of current through inductor
Figure 46. Output LC filter circuit
Computation with VIN = 24V, VOUT = 5V, L = 4.7μH, switching frequency FOSC = 1MHz, the method is as below.
Inductor ripple current
ΔIL
= V_OUT
× (V _IN
- V _{O UT}
)×
V _IN
1
× F_OSC
×L
=
842
[mA]
where :
ΔIL is the inductor ripple current
F_OSC is the swithing frequency
L is the inductor
V _IN is the input voltage
V_OUT is the output voltage
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
+
8
×
C
1
OUT
×
F_OSC
)
[V]
where :
ΔV_RPL is the output ripple voltage
R_ESR is the serial equivalent series resistance
C_OUT is the output capacitor
With COUT = 44µF, RESR = 10mΩ the output ripple voltage is calculated as
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21.Feb.2014 Rev.002