10
LTC3700
3700f
The basic LTC3700 application circuit is shown inFigure1.
External component selection for the buck is driven by the
load requirement and begins with the selection of L1 and
R
SENSE
(= R1). Next, the power MOSFET, M1 and the
output diode D1 are selected followed by C
IN
(= C1) and
C
OUT
(= C2).
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 inductors peak current. The output cur-
rent the buck can provide is given by:
I
R
I
OUT
SENSE
RIPPLE
=
012
2
.
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:
R
I
SENSE
OUT
=
<
1
10
( )( )
for Duty Cycle 40%
However, for operation that is above 40% duty cycle, slope
compensation effect 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:
APPLICATIONS INFORMATION
U
U
U
SF
I
SENSE
=
10 100
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
V
OUT
. The inductors peak-to-peak ripple current is given
by:
I
V V
fL
V V
V V
RIPPLE
IN OUT OUT D
IN D
=
+
+
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
()
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.
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