BD9123MUV_11 Datasheet, PDF (12/18 Pages) Rohm – Output 1.5A or Less High-efficiency Step-down Switching Regulator with Built-in Power MOSFET

BD9123MUV
Technical Note
●Selection of components externally connected
1. Selection of inductor (L)
IL
VCC
IL
L
ΔIL
VOUT
Co
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=0.3×IOUTmax. [A]・・・(2)
(VCC-VOUT)×VOUT
L=
ΔIL×VCC×f
[H]・・・(3)
(ΔIL: Output ripple current, and f: Switching frequency)
Fig.34 Output ripple current
※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=0.3×1.2A=0.36A, for example,
(5-1.2)×1.2
L= _{0.6×5×1M} =2.53µ → 4.7 [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
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.35 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. A 10µF to 100µF ceramic capacitor is
recommended.
Less ESR allows reduction in output ripple voltage.
3. Selection of input capacitor (Cin)
VCC
Cin
VOUT
L
Co
Fig.36 Input capacitor
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(VCC-VOUT)}
IRMS=IOUT×
VCC
[A]・・・(5)
< Worst case > IRMS(max.)
When Vcc=2×VOUT, IRMS=
IOUT
2
If VCC=5V, VOUT=1.2V, and IOUTmax.=1.2A,
√_1.2(5-1.2)
IRMS=1.2× ₅
=0.51 [ARMS]
A low ESR 10µF/10V ceramic capacitor is recommended to reduce ESR dissipation of input capacitor for better efficiency.
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12/17
2011.01 - Rev.A