Datasheet

L >
V
IN
x D x (1-D)
2 x f
S
x I
OUT(AVG)
>
V
IN
x D
2 x f
S
x L
I
OUT(AVG)
1-D
=
V
L
(t)
L
di
L
(t)
dt
t (s)
I
L(AVG)
D*T
s
T
s
D*T
s
T
s
I
L
(A)
I
D
(A)
(a)
(b)
'i
L
t (s)
I
D(AVG)
=
I
OUT(AVG)
V
IN
- V
OUT
L
V
IN
- V
OUT
L
V
IN
L
LM2698
www.ti.com SNVS153E –MAY 2001–REVISED APRIL 2013
Inductor
Figure 15. (a) Inductor Current (b) Diode Current
The inductor is one of the two energy storage elements in a boost converter. Figure 15 shows how the inductor
current varies during a switching cycle. The current through an inductor is quantified as:
(2)
If V
L(t)
is constant, di
L
/ dt must be constant, thus the current in the inductor changes at a constant rate. This is
the case in DC/DC converters since the voltages at the input and output can be approximated as a constant. The
current through the inductor of the LM2698 boost converter is shown in Figure 15(a). The important quantities in
determining a proper inductance value are I
L(AVG)
(the average inductor current) and Δi
L
(the inductor current
ripple). If Δi
L
is larger than I
L(AVG)
, the inductor current will drop to zero for a portion of the cycle and the converter
will operate in discontinuous conduction mode. If Δi
L
is smaller than I
L(AVG)
, the inductor current will stay above
zero and the converter will operate in continuous conduction mode (CCM). All the analysis in this datasheet
assumes operation in continuous conduction mode. To operate in CCM:
I
L(AVG)
> Δi
L
(3)
(4)
(5)
Choose the minimum I
OUT
to determine the minimum L for CCM operation. A common choice is to set Δi
L
to 30%
of I
L(AVG)
.
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