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LM3524D Datasheet - Page 15

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Typical Applications
(Continued)
V
T
V
t
L
IN
ON
From I
I
a
j
e
L
L
L
L1
(V
V
) t
b
o
IN
OFF
and I
b
j
L
L1
Since
I
a
I
b
V
t
V
t
e
e
b
L
L
IN
ON
o
OFF
and neglecting V
and V
SAT
D1
t
ON
V
j V
1
a
o
IN
t
OFF
The above equation shows the relationship between V
and duty cycle
In calculating input current I
which equals the induc-
IN(DC)
tor’s DC current assume first 100% efficiency
P
I
V
e
IN
IN(DC)
IN
t
ON
P
I
V
I
V
1
e
e
a
OUT
o
o
o
IN
t
OFF
for
100% P
P
e
e
OUT
IN
t
ON
I
V
1
I
V
a
e
o
IN
IN(DC)
t
OFF
t
ON
I
I
1
e
a
IN(DC)
o
t
OFF
This equation shows that the input or inductor current is
larger than the output current by the factor (1
Since this factor is the same as the relation between V
V
I
can also be expressed as
IN
IN(DC)
V
o
I
I
e
IN(DC)
o
V
IN
So far it is assumed
100% where the actual efficiency
e
or
will be somewhat less due to the saturation voltage
MAX
of Q1 and forward on voltage of D1 The internal power loss
due to these voltages is the average I
current flowing or
L
I
through either V
or V
For V
e
IN
SAT
D1
SAT
power loss becomes I
(1V)
is then
IN(DC)
MAX
P
V
I
o
o
o
e
e
e
MAX
P
V
I
I
(1V)
a
IN
o
o
IN
V
I
I
a
o
o
From V
V
1
e
a
o
IN
V
t
IN
OFF
This equation assumes only DC losses however
further decreased because of the switching time of Q1 and
D1
In calculating the output capacitor C
supplies I
during t
o
ON
V
IN
o
time will be some
regulator Calculation of C
V
o
T
From V
V
e
o
IN
t
OFF
where T
t
t
e
a
ON
OFF
V
IN
t
T
e
b
ON
V
I
T
o
C
e
o
t
t
)
a
ON
OFF
and
o
where C
is in farads f is the switching frequency
o
V
is the p-p output ripple
o
Calculation of inductor L1 is as follows
L1
e
V
is applied across L1
IN
I
0 4 I
e
e
Lp-p
L
V
t
V
1V this
e
IN
ON
D1
L1
e
V
0 4 I
o
V
I
V
o
o
t
ON
1
a
o
t
L1
OFF
where L1 is in henrys f is the switching frequency in Hz
15
t
ON
t
OFF
V
IN
e
max
V
1
a
IN
is
MAX
it can be seen that C
o
o
The voltage change on C
during this
o
V
V
or the output ripple of the
e
c
o
is
o
I
t
I
t
o
ON
o
ON
or C
e
e
o
C
V
o
o
V
IN
t
T
e
OFF
V
o
1
e
f
V
V
b
IN
o
IN
T
T
therefore
e
V
o
o
V
V
b
o
IN
V
I
(V
V
)
b
o
o
o
IN
e
V
f V
V
o
o
o
V
t
IN
ON
since during t
ON
I
a
L
V
o
0 41 I
0 4 I
therefore
e
IN
o
V
IN
T (V
V
)
b
o
IN
and since t
e
ON
V
o
o
IN
IN 2
2 5 V
(V
V
)
b
o
IN
e
o 2
f I
V
o

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