Datasheet

LTC3614
14
3614fa
APPLICATIONS INFORMATION
Frequency Synchronization
The LTC3614’s internal oscillator can be synchronized to
an external frequency by applying a square wave clock
signal to the RT/SYNC pin. During synchronization, the top
switch turn-on is locked to the falling edge of the external
frequency source. The synchronization frequency range
is 300kHz to 4MHz. During synchronization all operation
modes can be selected.
It is recommended that the regulator is powered down
(RUN pin to ground) before removing the clock signal
on the RT/SYNC pin in order to reduce inductor current
ripple.
AC coupling should be used if the external clock genera-
tor cannot provide a continuous clock signal throughout
start-up, operation and shutdown of the LTC3614. The
size of capacitor C
SYNC
depends on parasitic capacitance
on the RT/SYNC pin and is typically in the range of 10pF
to 22pF.
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ΔI
L
increases with higher V
IN
and decreases
with higher inductance:
ΔI
L
=
V
OUT
f
SW
•L
⎛
⎝
⎜
⎞
⎠
⎟
•1–
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
Having a lower ripple current reduces the core losses
in the inductor, the ESR losses in the output capacitors
and the output voltage ripple. A reasonable starting point
for selecting the ripple current is ΔI
L
= 0.3 • I
OUT(MAX)
.
The largest ripple current occurs at the highest V
IN
. 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
SW
• ΔI
L(MAX)
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
•1–
V
OUT
V
IN(MAX)
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
The inductor value will also have an effect on Burst Mode
operation. The transition to 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 effi ciency in the upper range of low cur-
rent operation. In Burst Mode operation, lower inductance
values will cause the burst frequency to increase.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. Actual core loss is independent of core size for
fi xed inductor value, but it is very dependent on the induc-
tance selected. As the inductance increases, core losses de-
crease. Unfortunately, increased inductance requires more
turns of wire and therefore, copper losses will increase.
LTC3614
SV
IN
V
IN
RT/SYNC
LTC3614
SV
IN
V
IN
0.4V
RT/SYNC
R
T
R
T
SGND
LTC3614
SV
IN
f
OSC
2.25MHz
f
OSC
1/T
P
f
OSC
t1/R
T
V
IN
RT/SYNC
SGND
T
P
1.2V
0.3V
LTC3614
SV
IN
f
OSC
1/T
P
V
IN
C
SYNC
RT/SYNC
SGND
3614 F02
Figure 2. Setting the Switching Frequency