BD9123MUV_14 Datasheet, PDF (16/28 Pages) Rohm – Synchronous Buck Converter with Integrated FET

BD9123MUV
Datasheet
7. Selection of Components Externally Connected
(1) Selection of Inductor (L)
The inductance significantly depends on output ripple current. As seen
in equation (1), the ripple current decreases as the inductor and/or
IL
switching frequency increases.
VCC
IL
L
ΔIL
VOUT
Co
I _L

V_CC V_OUT
L V_CC
V_OUT
f
A ・・・(1)
Appropriate ripple current at output should be 30% more or less of the
maximum output current.
I _L  0.3  I_{OUTMax [A] ・・・(2)}
L  ^{VCC VOUT VOUT}
I_L V_CC  f
H  ^{・・・(3)}
Figure 34. Output ripple current
Where:
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=5.0V, VOUT=1.2V, f=1MHz, ΔIL=0.3x1.2A=0.36A, for example,
L  ^{5 1.21.2}  2.53  4.7 H 
0.6  51M
Note: Select the inductor with 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 in consideration with the stability
region and the equivalent series resistance required to smoothen the
ripple voltage.
VOUT
L
ESR
CO
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.
Figure 35. Output Capacitor
Note: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
The input capacitor to be selected must have sufficiently low ESR to cope with
high ripple current to prevent high transient voltage. The ripple current IRMS is
given by the equation (5):
  _{IRMS  IOUT }
< Worst case
>
V_OUT V_CC
V _{IRMSMax CC}

V_OUT
[A] ・・・(5)
When VCC=2xVOUT,
I_RMS
 ^IOUT
2
IIRf ^VMCS^{C==5V, VOUT=1.2V, and IOUTMax=1.2A,}
Figure 36. Input Capacitor
1.25 1.2
I_RMS  1.2 
5
 0.51
A_RMS 
A low ESR 10µF/10V ceramic capacitor is recommended to reduce ESR dissipation of input capacitor for better
efficiency.
www.rohm.com
©2012 ROHM Co., Ltd. All rights reserved.
TSZ22111ï½¥15ï½¥001
16/24
TSZ02201-0J3J0AJ00120-1-2
02.Oct.2014 Rev.002