FUJITSU MB39A105 - Manual (Page 21)

MB39A105
21
Example:
Inductance values derived from the above formulas are values that provide sufficient margin for continuous
operation at maximum load current, but at which continuous operation is not possible at light loads. It is therefore
necessary to determine the load level at which continuous operation becomes possible. In this application, the
Sumida CMD5D13-6R8 is used. At 6.8
µ
H, the load current value under continuous operating conditions is
determined by the following formula.
Load current value under continuous operating conditions : I
O
To determine whether the current through the inductor is within rated values, it is necessary to determine the
peak value of the ripple current as well as the peak-to-peak values of the ripple current that affect the output
ripple voltage. The peak value and peak-to-peak value of the ripple current can be determined by the following
formulas.
Peak value : I
L
Peak-to-peak value :
∆
I
L
Example: Using the CMD5D13-6R8
6.8
µ
H (allowable tolerance
±
20
%
) , rated current
=
1.4 A
Peak value:
L
≥
V
IN (Max)
2
ton
2I
O
V
O
≥
4 2
Ч
9
−
4
Ч
1
2
Ч
0.25
Ч
9 9 500
Ч
10 3
≥
3.95
µ
H
I
O
≥
V
IN (Max)
2
ton
2LV
O
≥
4 2
Ч
9
−
4
Ч
1
2
Ч
6.8
Ч
10 − 6
Ч
9 9 500
Ч
10 3
≥
145.2 mA
I
L
≥
V
O
×
I
O +
V
IN
ton ton
=
V
O
−
V
IN
t
V
IN
2L V
O
∆
I
L
=
V
IN
ton
L
I
L
≥
V
O
×
I
O +
V
IN
ton ton
=
V
O
−
V
IN
t
V
IN
2L V
O
≥
9
Ч
0.25
+
2.4
Ч
(9
−
2.4)
Ч
1
2.4 2
Ч
6.8
Ч
10 − 6
Ч
9 500
Ч
10 3
≥
1.20 A