Datasheet

Technical Note
BD9111NV
9/13
www.rohm.com
2010.04 - Rev.B
© 2010 ROHM Co., Ltd. All rights reserved.
●Selection of components externally connected
1. Selection of inductor (L)
* Current exceeding the current rating of the inductor results in magnetic saturation of the inductor, which decreases efficiency.
The inductor must be selected allowing sufficient margin with which the peak current may not exceed its current rating.
If V
CC=5V, VOUT=3.3V, f=1MHz, ΔIL=0.3×2A=0.6A, for example,(BD9111NV)
* Select the inductor of low resistance component (such as DCR and ACR) to minimize dissipation in the inductor for better efficiency.
2. Selection of output capacitor (C
O)
3. Selection of input capacitor (Cin)
A low ESR 22μF/10V ceramic capacitor is recommended to reduce ESR dissipation of input capacitor for better efficiency.
The inductance significantly depends on output ripple current.
A
s seen in the equation (1), the ripple current decreases as the
inductor and/or switching frequency increases.
Δ
IL=
(V
CC-VOUT)×VOUT
L×VCC×f
[A]・・・
(
1
)
A
ppropriate ripple current at output should be 20% more or less of the
maximum out
p
ut current.
ΔI
L=0.3×IOUTmax. [A]・・・(2)
L=
(V
CC-VOUT)×VOUT
ΔIL×VCC×f
[H]・・・
(
3
)
(ΔI
L: Output ripple current, and f: Switching frequency)
Output capacitor should be selected with the consideration on the stability region
and the equivalent series resistance required to smooth ripple voltage.
Output ripple voltage is determined by the equation (4)：
ΔV
OUT=ΔIL×ESR [V]・・・(4)
(ΔI
L: Output ripple current, ESR: Equivalent series resistance of output capacitor)
*Rating of the capacitor should be determined allowing sufficient margin
against output voltage. Less ESR allows reduction in output ripple voltage.
22μF to 100μF ceramic capacitor is recommended.
Input capacitor to select must be a low ESR capacitor of the capacitance
sufficient to cope with high ripple current to prevent high transient voltage.
The ripple current IRMS is given by the equation (5):
Fig.28 Output capacitor
(
5.0-3.3
)
×3.3
0.6×5.0×1M
L=
=1.87μ → 2.2[μH]
Fig.29 Input capacitor
ΔIL
VCC
IL
L
Co
V
OUT
Fig.27 Output ripple current
IL
VCC
L
Co
V
OUT
ESR
VCC
L
Co
V
OUT
Cin
IRMS=IOUT×
VOUT
(
VCC-VOUT
)
VCC
[
A
]
・・・
(
5
)
√
When Vcc is twice the V
OUT
, IRMS=
I
OUT
2
< Worst case > IRMS(max.)
I
RMS=2×
3.3
(
5.0-3.3
)
5.0
=0.947
[
ARMS
]
√
If VCC=5.0V, VOUT=3.3V, and IOUTmax.=2A, (BD9111NV)