BD9137MUV Datasheet, PDF (9/17 Pages) Rohm – High Efficiency Step-down Switching Regulator

●Selection of components externally connected
1. Selection of inductor (L)
IL
ΔIL
VCC
IL
V_OUT
L
Co
Fig.27 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=2.5V, f=1MHz, ΔIL=0.2×3A=0.6A, for example,(BD9137MUV)
(5-2.5)×2.5
L= _{0.6×5×1M} =2.08μ → 2.2[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.28 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.
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