BD91361MUV_14 Datasheet, PDF (15/27 Pages) Rohm – Synchronous Buck Converter with Integrated FET

BD91361MUV
Datasheet
7. Selection of Components Externally Connected
(1) Selection of Inductor (L)
IL
ΔIL
The inductance significantly affects the output ripple current. As seen
in the equation (1), the ripple current decreases as the inductor
and/or switching frequency increases.
VCC
I _L

V_CC  V_OUT
L V_CC
  V_OUT
f
A ・・・(1)
IL
VOUT
L
CO
Figure 31. Output Ripple Current
Appropriate ripple current at output should be ±20% of the maximum
output current.
  I _L  0.2 I_OUTMax
A _{・・・(2)}
L  ^{VCC VOUT VOUT}
where:
I_L V_CC  f
ΔIL is the Output ripple current.
f is the Switching frequency.
H  _{・・・(3)}
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 peak current may
not exceed its current rating.
If VCC =5.0V, VOUT=1.2V, f=1MHz, ΔIL=0.2x3A=0.6A, for example, (BD91361MUV)
L  ^{5.0 1.21.2}  1.52  2.0
0.6  51M
H 
Note: Select an inductor with low resistance (such as DCR and ACR) to minimize inductor dissipation 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 minimize ripple voltage.
VOUT
L
ESR
CO
Figure 32. 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.
ESR is the Equivalent series resistance of output capacitor.
Note: Rating of the capacitor should be determined by 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 must be a low ESR capacitor with 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
Figure 33. Input Capacitor
  _{IRMS  IOUT }
V_OUT V_CC  V_OUT
V_CC
< Worst case > IRMSMax
V_CC
 2  V_OUT , I_RMS
 ^IOUT
2
A ・・・(5)
If VCC=3.3V, VOUT=1.8V, and IOUTMax=3A, (BD91361MUV)
1.83.3 1.8
I _RMS  3 
3.3
 1.49
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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