Datasheet
LTC3417A-2
9
3417a2fa
A general LTC3417A-2 application circuit is shown in
Figure 4. External component selection is driven by the
load requirement, and begins with the selection of the
inductors L1 and L2. Once L1 and L2 are chosen, C
IN
,
C
OUT1
and C
OUT2
can be selected.
Operating Frequency
Selection of the operating frequency is a tradeoff between
effi ciency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves effi ciency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output ripple voltage.
The operating frequency, f
O
, of the LTC3417A-2 is deter-
mined by pulling the FREQ pin to V
IN
for 1.5MHz opera-
tion or by connecting an external resistor from FREQ to
ground. The value of the resistor sets the ramp current
that is used to charge and discharge an internal timing
capacitor within the oscillator and can be calculated by
using the following equation:
for 0.6MHz ≤ f
O
≤ 4MHz. Alternatively, use Figure 1 to
select the value for R
T
.
The maximum operating frequency is also constrained
by the minimum on-time and duty cycle. This can be
calculated as:
The minimum frequency is limited by leakage and noise
coupling due to the large resistance of R
T
.
Inductor Selection
Although the inductor does not infl uence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current, ΔI
L
, decreases with
higher inductance and increases with higher V
IN
or V
OUT
.
Accepting larger values of ΔI
L
allows the use of low induc-
tances, but results in higher output voltage ripple, greater
core losses and lower output current capability.
A reasonable starting point for setting ripple current is
ΔI
L
= 0.35I
LOAD(MAX)
, where I
LOAD(MAX)
is the maximum
current output. The largest ripple, ΔI
L
, occurs at the maxi-
mum input voltage. To guarantee that the ripple current
stays below a specifi ed maximum, the inductor value
should be chosen according to the following equation:
L =
V
OUT
f
O
• I
L
1–
V
OUT
V
IN(MAX)
_
Figure 1. Frequency vs R
T
APPLICATIONS INFORMATION
FREQUENCY (MHz)
0
R
T
(kΩ)
60
80
100
1.5 2.5 3.53.0 4.0
3417A-2 F01
40
20
0
0.5 1.0 2.0
120
140
160
4.5
I
L
=
V
OUT
f
O
•L
1–
V
OUT
V
IN
R
T
≈
1.61• 10
11
f
O
Ω
()
– 16.586kΩ
f
O(MAX)
6.67
V
OUT
V
IN(MAX)
MHz
()