BD9111NV_14 Datasheet, PDF (13/25 Pages) Rohm – Synchronous Buck Converter with Integrated FET

BD9111NV
Datasheet
6. Selection of Components Externally Connected
(1) Selection of Inductor (L)
IL
V_CC
IL
L
ΔIL
VOUT
Co
Figure 28. Output Ripple Current
The inductance significantly depends on output ripple current.
As seen in equation (1), the ripple current decreases as the
inductor and/or switching frequency increases.
I _L

V_CC V_OUT V_OUT
L V_CC  f
A ・・・(1)
Appropriate ripple current at output should be 20% to 30% of the maximum
output current.
I _L  0.3 I_OUTMax [A]_{・・・(2)}
L  ^{VCC VOUT VOUT}
I_L V_CC  f
Where:
H  _{・・・(3)}
ΔIL is the Output ripple current, and
f is the switching frequency
Note: 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=5V, VOUT=3.3V, f=1MHz, ΔIL=0.3x2A=0.6A, for example, (BD9111NV)
L  ^{5.0  3.33.3}  1.87  2.2 H 
0.6  5.0 1M
Note: 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)
Output capacitor should be selected with the consideration of the stability region
VCC
and the equivalent series resistance required to minimize ripple voltage.
VOUT
L
ESR
Co
Figure 29 Output Capacitor
Output ripple voltage is determined by the equation (4)：
V_OUT  I_L  ESR V  _{・・・(4)}
Where:
ΔIL is the Output ripple current, and
ESR is the Equivalent series resistance of output capacitor
Note: 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)
Input capacitor must be a low ESR capacitor with a capacitance sufficient to
VCC
CIN
cope with high ripple current to prevent high transient voltage.
The ripple current IRMS is given by the equation (5):
VOUT
L
Co
  _{IRMS IOUT }
V_OUT V_CC  V_OUT
V_CC
 W orstcase I_RMSMax
A ・・・(5)
Figure 30. Input Capacitor
When VCC is twice the VOUT ,
I_RMS
 ^IOUT
2
If VCC=5.0V, VOUT=3.3V, and IOUTMax=2A, (BD9111NV)
3.35.0  3.3
I_RMS  2 
5.0
 0.947
A_RMS 
A low ESR 22μF/10V ceramic capacitor is recommended to reduce ESR dissipation of input capacitor for better
efficiency.
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