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LM3524D Datasheet - Page 11

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Typical Applications
(Continued)
d
V
T
i
L
From the relation V
L
I
j
e
L
L
d
L1
t
(V
V
) t
V
b
IN
o
ON
I
a
I
b
e
e
L
L
L1
Neglecting V
V
and settling
I
a
e
SAT
D
L
t
ON
V
j V
V
e
o
IN
IN
t
t
a
OFF
ON
where T
Total Period
e
The above shows the relation between V
IN
cycle
t
ON
I
I
e
IN(DC)
OUT(DC)
t
t
a
ON
OFF
as Q1 only conducts during t
ON
t
ON
P
I
V
(I
)
e
e
IN
IN(DC)
IN
o(DC)
t
t
a
ON
P
I
V
e
o
o
o
The efficiency
of the circuit is
P
I
V
o
o
o
MAX
e
e
P
(t
)
(V
t
IN
ON
SAT
ON
I
V
a
o
IN
T
V
o
for V
V
1V
e
e
e
SAT
D1
V
1
a
o
MAX will be further decreased due to switching losses in
Q1 For this reason Q1 should be selected to have the maxi-
mum possible f
which implies very fast rise and fall times
T
CALCULATING INDUCTOR L1
( I
a
)
L1
( I
b
c
L
L
t
t
j
e
ON
OFF
(V
V
)
V
b
IN
o
( I
a
)
L1
( I
b
c
L
L
t
t
T
a
e
e
a
ON
OFF
(V
V
)
V
b
IN
Q
0 4I
L1
0 4I
L1
o
o
e
a
(V
V
)
V
b
IN
o
o
Since I
a
I
b
0 4I
e
e
L
L
o
FIGURE 14
Solving the above for L1
t
L1
o
OFF
L1
I
b
L
where L1 is in Henrys
f is switching frequency in Hz
t
ON
Also see LM1578 data sheet for graphical methods of in-
T
ductor selection
CALCULATING OUTPUT FILTER CAPACITOR C
V
and duty
o
Figure 14 shows L1’s current with respect to Q1’s t
t
times This curent must flow to the load and C
OFF
current will then be the difference between I
From Figure 14 it can be seen that current will be flowing
into C
for the second half of t
o
V
IN
t
or a time t
OFF
ON
OFF
time is
I
4 The resulting
L
1
I
L
V
e
c
op-p
C
4
I
t
L
ON
V
t
)
a
e
D1
OFF
I
4C
o
T
V
(T
b
o
Since I
e
L
L1
T
V
o
V
e
op-p
4C L1
C
o
)
L1
c
where C is in farads T is
o
)
L1
c
V
is p-p output ripple
o
o
For best regulation the inductor’s current cannot be al-
lowed to fall to zero Some minimum load current I
thus inductor current is required as shown below
(V
IN
I
e
o(MIN)
TL H 8650– 18
11
2 5 V
(V
V
)
b
o
IN
o
e
I
V
f
o
IN
o
and
ON
C
’s
o
o
and I
L
o
Ic
I
I
e
b
o
L
o
through the first half of
ON
2
t
2 The current flowing for this
a
OFF
V
or
V
is described by
c
o
t
t
ON
OFF
a
c
2
2
t
a
OFF
2
t
)
V
T
ON
o
and t
e
ON
V
IN
V
T
o
b
2
V
T
(V
V
) V
T
b
IN
IN
o
o
or
e
2
8V
C
L1
IN
o
2
(V
V
) V
T
b
IN
o
o
e
8 V
V
L1
o
IN
1
switching frequency
and
o
V
) t
(V
V
) V
b
b
o
ON
IN
o
o
e
2L1
2fV
L1
IN
TL H 8650 – 19

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