BD9122GUL_12 Datasheet, PDF (12/19 Pages) Rohm – 2.5V to 5.5V, 0.3A 1ch Synchronous Buck Converter integrated FET

0J3J0AJ00110
Datasheet
Selection of Components Externally Connected
1. Selection of inductor (L)
IL
VCC
IL
L
ΔIL
VOUT
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.
ΔIL=
(VCC-VOUT)×VOUT
L×VCC×f
[A]・・・(1)
Appropriate ripple current at output should be 30% more or less of
the maximum output current.
ΔIL=0.3×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.
* 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
ΔVOUT=ΔIL×ESR [V]・・・(4)
L
ESR
(ΔIL: Output ripple current, ESR: Equivalent series resistance of output capacitor)
Co
* Rating of the capacitor should be determined allowing sufficient margin
against output voltage. Less ESR allows reduction in output ripple voltage.
Fig.29 Output capacitor
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):
Co≦ ^{TSS×(Ilimit-IOUT)}
VOUT
・・・(5)
if VOUT=1.5V, IOUT=0.3A, and TSS=1ms,
Tss: Soft-start time
Ilimit: Over current detection level, 1A(Typ)
Co≦ ^{1m×(1-0.3)} ≒467 [μF]
1.5
Inappropriate capacitance may cause problem in startup.
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 (6):
VOUT
L
Co
√_{VOUT(VCC-VOUT)}
IRMS=IOUT×
VCC
[A]・・・(6)
< Worst case > IRMS(max.)
IOUT
When Vcc is twice the VOUT, IRMS= ₂
Fig.30 Input capacitor
If VCC=3.3V, VOUT=1.5V, and IOUTmax.=0.3A
√_1.5(3.3-1.5)
IRMS=0.3×
33.3
=0.15[ARMS]
A low ESR 10μF/10V ceramic capacitor is recommended to redu^.ce ESR dissipation of input capacitor for better efficiency.
3
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02.MAR.2012 Rev.001