BD8151EFV, BD8157EFV
Technical Note
9/17
www.rohm.com
2009.07 - Rev.B
© 2009 ROHM Co., Ltd. All rights reserved.
●Selecting Application Components
(1) Setting the output L constant
The coil L to use for output is decided by the rating current I
LR
and input current maximum value I
INMAX
of the coil.
Adjust so that I
INMAX
+ ∆I
L
does not reach the rating current value I
LR
. At this time, ∆I
L
can be obtained by the following
equation.
∆I
L =
1
Vcc
Vo-Vcc
1
[A] Where, f is the switching frequency.
L Vcc f
Set with sufficient margin because the coil L value may have the dispersion of approx. 30%. If the coil current exceeds
the rating current I
LR
of the coil, it may damage the IC internal element.
BD8157EFV uses the current mode DC/DC converter control and has the optimized design at the coil value. The following
coil values are recommended from the aspects of power efficiency, response and safety. When the coil out of this range is
selected, the stable continual operation is not guaranteed such as the switching waveform becomes irregular. Please pay
attention to it.
Switching frequency: L = 10 µH to 22 µH at 600 kHz
Switching frequency: L = 4.7 µH to 15 µH at 1,200 kHz
(2) Setting the output capacitor
For the capacitor C to use for the output, select the capacitor which has the larger value in the ripple voltage V
PP allowance
value and the drop voltage allowance value at the time of sudden load change. Output ripple voltage is decided by the
following equation.
∆V
PP = ILMAX×RESR +
1
Vcc
(ILMAX-
∆IL
) [V] Where, f is the switching frequency.
fCo Vo 2
Perform setting so that the voltage is within the allowable ripple voltage range.
For the drop voltage during sudden load change; V
DR
, please perform the rough calculation by the following equation.
VDR =
∆I
10 µ sec [V]
Co
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 this reason, the low ESR capacitor is recommended as an input capacitor which has the value more than
10 µF and less than 100 m. 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.
IL
t
I
INMAX+ ∆IL should not reach
the rating value level
I
average
current
Fig. 29 Coil Current Waveform
Fig. 30 Output Application Circuit Diagram
L
Vcc
IL
Vo
Co
