Datasheet

LT8614
16
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APPLICATIONS INFORMATION
To avoid overheating and poor efficiency, an inductor must
be chosen with an RMS current rating that is greater than
the maximum expected output load of the application.
In addition, the saturation current (typically labeled I
SAT
)
rating of the inductor must be higher than the load current
plus 1/2 of in inductor ripple current:
I
L(PEAK)
= I
LOAD(MAX)
+
1
2
ΔI
L
(7)
where ∆I
L
is the inductor ripple current as calculated in
Equation 9 and I
LOAD(MAX)
is the maximum output load
for a given application.
As a quick example, an application requiring 1A output
should use an inductor with an RMS rating of greater than
1A and an I
SAT
of greater than 1.3A. During long duration
overload or short-circuit conditions, the inductor RMS
rating requirement is greater to avoid overheating of the
inductor. To keep the efficiency high, the series resistance
(DCR) should be less than 0.04Ω, and the core material
should be intended for high frequency applications.
The LT8614 limits the peak switch current in order to
protect the switches and the system from overload faults.
The top switch current limit (I
LIM
) is at least 8.5A at low
duty cycles and decreases linearly to 7.2A at DC = 0.8.
The inductor value must then be sufficient to supply the
desired maximum output current (I
OUT(MAX)
), which is a
function of the switch current limit (I
LIM
) and the ripple
current.
I
OUT(MAX)
= I
LIM
–
ΔI
L
2
(8)
The peak-to-peak ripple current in the inductor can be
calculated as follows:
ΔI
L
=
V
OUT
L • f
SW
• 1–
V
OUT
V
IN(MAX)
⎛
⎝
⎜
⎞
⎠
⎟
(9)
where f
SW
is the switching frequency of the LT8614, and
L is the value of the inductor. Therefore, the maximum
output current that the LT8614 will deliver depends on
the switch current limit, the inductor value, and the input
and output voltages. The inductor value may have to be
increased if the inductor ripple current does not allow
sufficient maximum output current (I
OUT(MAX)
) given the
switching frequency, and maximum input voltage used in
the desired application.
In order to achieve higher light load efficiency, more energy
must be delivered to the output during the single small
pulses in Burst Mode operation such that the LT8614 can
stay in sleep mode longer between each pulse. This can be
achieved by using a larger value inductor (i.e., 4.7µH), and
should be considered independent of switching frequency
when choosing an inductor. For example, while a lower
inductor value would typically be used for a high switch-
ing frequency application, if high light load efficiency is
desired, a higher inductor value should be chosen. See
cur
ve in Typical Performance Characteristics.
The optimum inductor for a given application may differ
from the one indicated by this design guide. A larger value
inductor provides a higher maximum load current and
reduces the output voltage ripple. For applications requir-
ing smaller load currents, the value of the inductor may
be lower and the LT8614 may operate with higher ripple
current. This allows use of a physically smaller inductor,
or one with a lower DCR resulting in higher efficiency. Be
aware that low inductance may result in discontinuous
mode operation, which further reduces maximum load
current.
For more information about maximum output current
and discontinuous operation, see Linear Technology’s
Application Note 44.
Finally, for duty cycles greater than 50% (V
OUT
/V
IN
> 0.5),
a minimum inductance is required to avoid sub-harmonic
oscillation. See Application Note 19.
Input Capacitors
The V
IN
of the LT8614 should be bypassed with at least
three ceramic capacitors for best performance. Two small
ceramic capacitors of 1µF should be placed close to the
part; one at the V
IN1
/GND1 pins and a second at V
IN2
/
GND2 pins. These capacitors should be 0402 or 0603 in
size. For automotive applications requiring 2 series input
capacitors, two small 0402 or 0603 may be placed at
each side of the LT8614 near the V
IN1
/GND1 and V
IN2
/
GND2 pins.
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