BD91361MUV_12 Datasheet, PDF (15/23 Pages) Rohm – 2.7V to 5.5V, 4A 1ch Synchronous Buck Converter Integrated FET

BD91361MUV
Datasheet
●Selection of Components Externally Connected
1. Selection of inductor (L)
IL
ΔIL
VCC
IL
VOUT
L
Co
Fig.31 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% more or less of
the maximum output current.
ΔIL=0.2×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 =5.0V, VOUT=1.2V, f=1MHz, ΔIL=0.2×3A=0.6A, for example,(BD91361MUV)
(5-1.2)×1.2
L= _{0.6×5×1M} =1.52µ → 2.0[µH]
※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
VOUT
L
ESR
Co
Fig.32 Output capacitor
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) :
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. A 22µF to 100µF ceramic capacitor is recommended.
Less ESR allows reduction in output ripple voltage.
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
< Worst case > IRMS(max.)
[A]・・・(5)
IOUT
When Vcc=2×VOUT, IRMS= ₂
Fig.33 Input capacitor
If VCC=3.3V, VOUT=1.8V, and IOUTmax.=3A, (BD91361MUV)
IRMS=2×
√_1.8(3.3-1.8)
33.3
=1.49[ARMS]
.
A low ESR 22µF/10V ceramic capacitor is recommended to 3reduce ESR dissipation of input capacitor for better efficiency.
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