BD9120HFN Rohm, BD9120HFN Datasheet - Page 32

• Manufacturer Part Number: BD9120HFN
• Description: (BD9106FVM - BD9120HFN) Synchronous Buck Converter Integrated FET
• Manufacturer: Rohm

BD9106FVM BD9107FVM BD9109FVM BD9110NV BD9120HFN
© 2012 ROHM Co., Ltd. All rights reserved.
TSZ22111･15･001
●Selection of Components Externally Connected
www.rohm.com
1. Selection of inductor (L)
2. Selection of output capacitor (C
3. Selection of input capacitor (Cin)
V
CC
* Current exceeding the current rating of the inductor results in magnetic saturation of the inductor, which decreases efficiency.
* Select the inductor of low resistance component (such as DCR and ACR) to minimize dissipation in the inductor for
As the output rise time must be designed to fall within the soft-start time, the capacitance of output capacitor should be
determined with consideration on the requirements of equation (5):
In case of BD9109FVM, for instance, and if V
Inappropriate capacitance may cause problem in startup. A 10 μF to 100 μF ceramic capacitor is recommended.
A low ESR 10μF/10V ceramic capacitor is recommended to reduce ESR dissipation of input capacitor for better efficiency.
Fig.98 Input capacitor
Fig.97 Output capacitor
The inductor must be selected allowing sufficient margin with which the peak current may not exceed its current rating.
If V
better efficiency.
Co≦
Co≦
I
L
CC
L=
=5V, V
V
Fig.96 Output ripple current
CC
T
1m×(2-0.8)
L
V
L
SS
CC
(5-3.3)×3.3
0.24×5×1M
Cin
×(I
IL
L
3.3
OUT
limit
V
OUT
=3.3V, f=1MHz, ΔI
Co
-I
OUT
Co
VOUT
ESR
Co
)
≒364 [μF]
ΔI
V
L
V
OUT
OUT
=4.675μ → 4.7[μH]
O
)
・・・(5)
L
*Rating of the capacitor should be determined allowing sufficient margin
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 (6):
=0.3×0.8A=0.24A, for example,(BD9109FVM)
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
(ΔI
against output voltage. Less ESR allows reduction in output ripple voltage.
When VCC is twice the Vout,
I
I
< Worst case > I
If V
I
L
RMS
OUT
RMS
RMS
: Output ripple current, ESR: Equivalent series resistance of output capacitor)
CC
=ΔI
=
=I
=0.8×
OUT
=5V, V
OUT
L
×ESR [V]・・・(4)
=3.3V, I
×
The inductance significantly depends on output ripple current.
As seen in the equation (1), the ripple current decreases as the
inductor and/or switching frequency increases.
Appropriate ripple current at output should be 30% more or less of
the maximum output current.
OUT
Tss: Soft-start time
I
limit
√
√
32/40
V
3.3(5-3.3)
=3.3V, and I
OUT
RMS(max.)
: Over current detection level, 2A(Typ)
OUT
5
ΔI
(ΔI
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ΔI
=0.8A, and T
(V
L
V
L
L=
L
=0.3×I
=
CC
: Output ripple current, and f: Switching frequency)
CC
-V
(V
(V
OUT
=0.38[A
OUTmax.=
OUT
CC
CC
ΔI
L×V
)
-V
-V
max. [A]・・・(2)
SS
L
OUT
OUT
×V
[A]・・・(6)
=1ms,
CC
RMS
I
OUT
0.8A, (BD9109FVM)
CC
2
)×V
)×V
×f
]
×f
OUT
OUT
TSZ02201-0J3J0AJ00090-1-2
[H]・・・(3)
[A]・・・(1)
02.MAR.2012 Rev.001
Datasheet
Datasheet pdf - http://www.DataSheet4U.net/