Datasheet

LTC3417A
9
3417afc
applicaTions inForMaTion
Figure 1. Frequency vs R
T
A general LTC3417A 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
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency 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 is determined
by pulling the FREQ pin to V
IN
for 1.5MHz operation 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:
R
T
≈
1.61• 10
11
f
O
Ω
( )
– 16.586kΩ
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:
f
O(MAX)
≈ 6.67
V
OUT
V
IN(MAX)
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
MHz
( )
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 influence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current,
∆ I
L
, decreases with
higher inductance and increases with higher V
IN
or
V
OUT
.
∆I
L
=
V
OUT
f
O
• L
1–
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
Accepting larger values of ∆ I
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
∆I
L
= 0.35I
LOAD(MAX)
, where I
LOAD(MAX)
is the maximum
current output. The largest ripple, ∆I
L
, occurs at the maxi-
mum input voltage. To guarantee that the ripple current
stays below a specified 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)
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
The inductor value will also have an effect on Burst Mode
operation. The transition from low current operation begins
when the peak inductor current falls below a level set by the
burst clamp. Lower inductor values result in higher ripple
current which causes this to occur at lower load currents.
This causes a dip in efficiency in the upper range of low
current operation. In Burst Mode operation, lower inductor
values will cause the burst frequency to increase.
FREQUENCY (MHz)
0
R
T
(kΩ)
60
80
100
1.5 2.5 3.53.0 4.0
3417 F01
40
20
0
0.5 1.0 2.0
120
140
160
4.5