BD9C301FJ-LB Datasheet, PDF (12/22 Pages) Rohm – 4.5V to 18V Input, 3.0A Integrated MOSFET Single Synchronous Buck DC/DC Converter

BD9C301FJ-LB
Selection of Components Externally Connected
1. Output LC Filter Constant
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 ∆I_L that flows into the inductor to be small.
However, decreasing the ripple voltage generated in the output is not advantageous in terms of the load transient
response characteristic. 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. It is recommended to select an inductance such that the size of the ripple current component of the coil will
be 20% to 40% of the average output current (average inductor current).
VIN
IL
Inductor saturation current > I_OUTMAX +⊿I_L /2
IOUTMAX
⊿I_L
Driver
Average inductor current
t
L
VOUT
C_OUT
Figure 23. Waveform of current through inductor
Figure 24. Output LC filter circuit
With V_IN = 12 V, V_OUT = 3.3 V and the switching frequency F_OSC = 500 kHz, the calculation is shown in the following
equation.
Coil ripple current ⊿IL = 30% x Average output current (3 A) = 0.9 [A]
L
=
V_OUT
×
(　V _IN
-V_OUT
)×
1
V_IN × F_OSC ×
⊿IL
=
5.31μ
≒4.7μ
[H]
where :
F_OSC is a switchingfrequency
The saturation current of the inductor must be larger than the sum of the maximum output current and 1/2 of the inductor
ripple current ∆I_L.
The output capacitor C_OUT affects the output ripple voltage characteristics. The output capacitor C_OUT must satisfy the
required ripple voltage characteristics.
The output ripple voltage can be represented by the following equation.
⊿V _RPL
=
⊿I_L × (　R_ESR +
8
1
× C_OUT
× F_OSC
)　
[V]
where :
R_ESR is the Equivalent Series Resistance(ESR) of the output capacitor.
Also this IC provides 1msec[Typ] soft start function to reduce sudden current which flows in output capacitor when
startup. But when capacity value of output capacitor COUT becomes bigger than the following method, correct soft start
waveform may not appear in some cases. ( ex. V_OUT over shoot at soft start .)
Select output capacitor COUT fulfilling the following condition including scattering and margin.
C_OUT <
I_OCP(= 3.5A[min]) × T_SS (= 0.5ms[min] )
V_OUT
　
[F]
where :
I_OCP 　 is switchcurrent restrictedvalue
T_SS 　 is soft start time
Caution) Concerning COUT total the capacity value of every part connected to Output line.
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TSZ22111・15・001
12/19
TSZ02201-0J3J0AJ00540-1-2
21.Feb.2014 Rev.002