BD9B500MUV Datasheet, PDF (21/34 Pages) Rohm – Integrated MOSFET Single Synchronous Buck DC/DC Converter

BD9B500MUV
Selection of Components Externally Connected
1. Output LC Filter Constant
In order to supply a continuous current to the load, the DC/DC converter requires an LC filter for smoothing the
output voltage. It is recommended to use inductors of values 0.47µH to 1.0µH when FREQ=L or 1.0µH to 1.5µH at
FREQ=H.
PVIN
IL
Inductor saturation current ＞ IOUTMAX + ΔIL / 2
IOUTMAX
ΔIL
Driver
L
VOUT
Average inductor current
COUT
t
Figure 49. Waveform of current through inductor
Figure 50. Output LC filter circuit
Inductor ripple current ΔIL
ΔI _L
= V_OUT × (V _IN
-V_OUT ) × _VIN
×
1
f _SW
×L
= 912
mA 
Where:
VIN= 5V
VOUT= 1.2V
L=1.0µH
fSW=1MHz (switching frequency)
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 ∆IL.
The output capacitor COUT affects the output ripple voltage characteristics. The output capacitor COUT must satisfy the
required ripple voltage characteristics.
The output ripple voltage can be represented by the following equation.
ΔV_RPL =
ΔI
L
×
(R_ESR
+
8
×
1
C_OUT
×
f
SW
)V
where R is the Equivalent Series Resistance (ESR) of the output capacitor.
* The capacitor rating must allow a sufficient margin with respect to the output voltage.
The output ripple voltage can be decreased with a smaller ESR.
A ceramic capacitor of about 22µF to 47µF is recommended.
*Be careful of total capacitance value, when additional capacitor CLOAD is connected in addition to output capacitor COUT.
Use maximum additional capacitor CLOAD (Max) condition which satisfies the following condition.
Maximumstarting inductor bottom ripple current IL_START
< Low side FET Over Current limit 5.5A(min)
Maximum starting inductor ripple bottom current ILSTART can be expressed using the following equation.
IL_START
=
Maximum
starting
output
current(I _OMAX
) + Charge current
to
output
capacitor( I _CAP
) － ^{ΔI L}
2
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