BD9161FVM_12 Datasheet, PDF (12/19 Pages) Rohm – 2.5V to 4.5V, 0.6A 1ch Synchronous Buck Converter Integrated FET

BD9161FVM
Datasheet
Selection of Components Externally Connected
1. Selection of inductor (L)
IL
ΔIL
VCC
IL
VOUT
L
Co
Fig.28 Output ripple current
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.
(VCC-VOUT)×VOUT
ΔIL=
L×VCC×f
[A]・・・(1)
Appropriate ripple current at output should be 20 to 30% more or less
of the maximum output current.
ΔIL=0.25×IOUTmax. [A]・・・(2)
(VCC-VOUT)×VOUT
L=
ΔIL×VCC×f
[H]・・・(3)
(ΔIL: Output ripple current, and f: Switching frequency)
* 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 VCC=3.3V, VOUT=2.5V, f=1MHz, ΔIL=0.25×0.6A=0.15A
L≧
(3.3-2.5)×2.5
0.15×3.3×1M
≧4.04μ
* 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 (CO)
VCC
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)：
VOUT
L
ESR
Co
Fig.29 Output capacitor
ΔVOUT=ΔIL×ESR [V]・・・(4)
(ΔIL: 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.
Inappropriate capacitance may cause problem in startup. A 10μF to 100μF ceramic capacitor is recommended.
3. Selection of input capacitor (Cin)
VCC
Cin
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):
VOUT
L
Co
√_{VOUT(VCC-VOUT)}
IRMS=IOUT×
VCC
[A]・・・(5)
< Worst case > IRMS(max.)
Fig.30 Input capacitor
IOUT
When VCC is twice the Vout, ₂
IRMS=
If VCC=3.3V, VOUT=2.5V, and IOUTmax.=0.6A
IRMS=0.6×
√_2.5(3.3-2.5)
5
=0.284[ARMS]
A low ESR 10μF/10V ceramic capacitor is recommended to reduce ESR dissipation of input capacitor for better efficiency.
www.rohm.com
© 2011 ROHM Co., Ltd. All rights reserved.
TSZ22111ï½¥15ï½¥001
12/17
TSZ02201-0J3J0AJ00190-1-2
02.MAR.2012 Rev.001