Datasheet

10
LTC3701
3701fa
APPLICATIO S I FOR ATIO
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The basic LTC3701 application circuit is shown in Fig-
ure␣ 1. External component selection is driven by the load
requirement and begins with the selection of L and R
SENSE
.
Next, the power MOSFET M1 and the output diode D1 are
selected. Finally C
IN
(C1) and C
OUT
(C2) are chosen.
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current.
Since the current comparator monitors the voltage devel-
oped across R
SENSE
, the threshold of the comparator
determines the inductor’s peak current. The output cur-
rent that the LTC3701 can provide is given by:
I
R
I
OUT
SENSE
RIPPLE
=
0 095
2
.
–
where I
RIPPLE
is the inductor peak-to-peak ripple current
(see Inductor Value Calculation).
A reasonable starting point for setting ripple current is
I
RIPPLE
= (0.4)(I
OUT
). Rearranging the above equation
yields:
R
I
SENSE
OUT
=
1
12 7.•
for Duty Cycle < 20%
However, for operation above 20% duty cycle, slope
compensation has to be taken into consideration to select
the appropriate value of R
SENSE
to provide the required
amount of current. Using Figure 2, the value of R
SENSE
is:
R
SF
I
SENSE
OUT
=
()()()
12 7 100.
For noise sensitive applications, a 1nF capacitor placed
between the SENSE
+
and SENSE
–
pins very close to the
chip is suggested.
Inductor Value Calculation
The inductor selection will depend on the operating fre-
quency of the LTC3701. The internal nominal frequency is
550kHz, but can be externally synchronized or set from
approximately 300kHz to 750kHz.
The operating frequency and inductor selection are inter-
related in that higher frequencies permit the use of a
Kool Mµ is a registered trademark of Magnetics, Inc.
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 and switching
losses.
The inductance value also has a direct effect on ripple
current. The ripple current, I
RIPPLE
, decreases with higher
inductance or frequency. The inductor’s peak-to-peak
ripple current is:
I
VV
fL
VV
VV
RIPPLE
IN OUT OUT D
IN D
=
+
+






–
•
where f is the operating frequency and V
D
is the forward
voltage drop of the external Schottky diode. 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 cur-
rent is I
RIPPLE
= 0.4(I
OUT(MAX)
). The maximum I
RIPPLE
occurs at the maximum input voltage.
With Burst Mode operation selected on the LTC3701, the
ripple current is normally set such that the inductor
current is continuous during the burst periods. Therefore,
the peak-to-peak ripple current must not exceed:
I
RIPPLE
≤ (0.03)/R
SENSE
This implies a minimum inductance of:
L
VV
f
R
VV
VV
V
MIN
IN OUT
SENSE
OUT D
IN D
IN MAX
=






+
+






=
()
–
.
()
003
Use 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 of L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mµ
®
cores. Actual core loss is independent of core