LTC1622
APPLICATIONS INFORMATION
The basic LTC1622 application circuit is shown in Figure
1. External component selection is driven by the load
requirement and begins with the selection of L and R
SENSE
.
Next, the Power MOSFET and the output diode D1 are
selected followed by C
IN
and C
OUT
.
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current.
With the current comparator monitoring the voltage devel-
oped across R
SENSE
, the threshold of the comparator
determines the inductor’s peak current. The output cur-
rent the LTC1622 can provide is given by:
I
OUT
=
0
.
08
I
−
RIPPLE
R
SENSE
2
V
OUT
. The inductor’s peak-to-peak ripple current is given
by:
V
V
−
V
+
V
I
RIPPLE
=
IN OUT
OUT D
V
IN
+
V
D
fL
where I
RIPPLE
is the inductor peak-to-peak ripple current
(see Inductor Value Calculation section).
A reasonable starting point for setting ripple current is
I
RIPPLE
= (0.4)(I
OUT
). Rearranging the above equation, it
becomes:
1
R
SENSE
=
for Duty Cycle < 40%
15 I
OUT
( )( )
However, for operation that is above 40% duty cycle, slope
compensation has to be taken into consideration to select
the appropriate value to provide the required amount of
current. Using Figure 2, the value of R
SENSE
is:
R
SENSE
=
(
15
)(
I
OUT
)(
100
)
SF
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies permit the use
of a smaller inductor for the same amount of inductor
ripple current. However, this is at the expense of efficiency
due to an increase in MOSFET gate charge losses.
The inductance value also has a direct effect on ripple
current. The ripple current, I
RIPPLE
, decreases with higher
inductance or frequency and increases with higher V
IN
or
U
W
U
U
()
where f is the operating frequency. Accepting larger values
of I
RIPPLE
allows the use of low inductances, but results in
higher output voltage ripple and greater core losses. A
reasonable starting point for setting ripple current is
I
RIPPLE
= 0.4(I
OUT(MAX)
). Remember, the maximum I
RIPPLE
occurs at the maximum input voltage.
With Burst Mode operation selected on the LTC1622, the
ripple current is normally set such that the inductor
current is continuous during the burst periods. Therefore,
the peak-to-peak ripple current should not exceed:
I
RIPPLE
≤
0
.
036
R
SENSE
This implies a minimum inductance of:
V
V
−
V
+
V
L
MIN
=
IN OUT
OUT D
0
.
036
V
IN
+
V
D
f
R
SENSE
(Use V
IN(MAX)
= V
IN
)
A smaller value than L
MIN
could be used in the circuit;
however, the inductor current will not be continuous
during burst periods.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mu
®
cores. Actual core loss is independent of core
size for a fixed inductor value, but it is very dependent on
inductance selected. As inductance increases, core losses
go down. Unfortunately, increased inductance requires
more turns of wire and therefore copper losses will
increase. Ferrite designs have very low core losses and are
Kool Mu is a registered trademark of Magnetics, Inc.
7
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