BD9A300MUV
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. BD9A300MUV is returned to the IC and IL ripple current flowing through the inductor for SLLMTM control. This
feedback current, Inductance value is the behavior of the best when the 1.5µH. Therefore, the inductor to use is
recommended 1.5µH.
PVIN
IL
Inductor saturation current > IOUTMAX +ΔIL /2
L
IOUTMAX
VOUT
ΔIL
Driver
Average inductor current
t
COUT
Figure 49. Waveform of Current Through Inductor
Figure 50. Output LC Filter Circuit
Computation with VIN= 5V, VOUT= 1.8V, L=1.5µH, and the switching frequency FOSC= 1MHz, the method is as below.
Inductor ripple current ΔIL
1
[ ]
= 768 mA
ΔIL =VOUT × (VIN -VOUT ) ×
VIN × FOSC × L
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 ΔIL.
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.
1
[ ]
) V
ΔVRPL = ΔIL × (RESR
+
8 × COUT × FOSC
RESR is the Equivalent Series Resistance (ESR) of the output capacitor.
With COUT= 44µF, RESR= 10mΩ the output ripple voltage is calculated as
1
[ ]
) = 9.8 mV
ΔVRPL = 0.768 × (10m+
(8 × 44μ × 1MHz)
*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 method.
.
Maximumstarting inductor ripple current ILSTART < Over Current limit 3.8A(min)
Maximum starting inductor ripple current ILSTART can be expressed in the following method.
ΔIL
ILSTART = Maximum starting output current(IOMAX ) + Charge current to output capacitor(ICAP ) +
2
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