BD8179MUV-E2 查看數據表（PDF） - ROHM Semiconductor
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BD8179MUV-E2 Datasheet PDF : 13 Pages
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BD8179MUV
Technical Note
●Selecting Application Components
(1) Setting the Output L Constant
The coil to use for output is decided by the rating current ILR and input current maximum value IINMAX of the coil.
IL
IINMAX + ∆IL should not reach
the rating value level
VCC
L
IL
ILR
IINMAX
Vo
average current
Co
Fig.14 Coil Current Waveform
Fig. 15 Output Application Circuit Diagram
Adjust so that IINMAX +∆IL does not reach the rating current value ILR. At this time, ∆IL can be obtained by the following
equation.
ΔIL =
1 Vcc
L
Vo-Vcc
Vcc
1
f
[A]
Here, f is the switching frequency.
Set with sufficient margin because the coil value may have the dispersion of 30%. If the coil current exceeds the rating
current ILR of the coil, it may damage the IC internal element.
BD8179MUV uses the current mode DC/DC converter control and has the optimized design at the coil value. A coil
inductance (L) of 4.7 µH to 15 µH is recommended from viewpoints of electric power efficiency, response, and stability.
(2) Output Capacity Settings
For the capacitor to use for the output, select the capacitor which has the larger value in the ripple voltage VPP allowance
value and the drop voltage allowance value at the time of sudden load change. Output ripple voltage is decided by the
following equation.
⊿VPP
= ILMAX RESR +
1
fCo
Vcc
Vo
(ILMAX -
⊿IL
2
) [V]
Here, f is the switching frequency.
Perform setting so that the voltage is within the allowable ripple voltage range. For the drop voltage during sudden load
change; VDR, please perform the rough calculation by the following equation.
VDR =
⊿I
Co
10 us
[V]
However, 10 µs is the rough calculation value of the DC/DC response speed. Please set the capacitance considering the
sufficient margin so that these two values are within the standard value range.
(3) Selecting the Input Capacitor
Since the peak current flows between the input and output at the DC/DC converter, a capacitor is required to install at the
input side. For the reason, the low ESR capacitor is recommended as an input capacitor which has the value more than
10 µF and less than 100 mΩ. If a capacitor out of this range is selected, the excessive ripple voltage is superposed on the
input voltage, accordingly it may cause the malfunction of IC.
However these conditions may vary according to the load current, input voltage, output voltage, inductance and switching
frequency. Be sure to perform the margin check using the actual product.
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8/12
2009.07 - Rev.B
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