Datasheet

LT8641
17
Rev B
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APPLICATIONS INFORMATION
The LT8641 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 8.2A at low duty cycles and
decreases linearly to 6.4A 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 LT8641, and
L is the value of the inductor. Therefore, the maximum
output current that the LT8641 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 LT8641 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
curve 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 LT8641 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.
For duty cycles greater than 50% (V
OUT
/V
IN
> 0.5), a
minimum inductance is required to avoid subharmonic
oscillation (See Equation 10). See Application Note 19
for more details.
L
MIN
=
V
IN
•
(2
•
DC – 1)
2.5 • f
SW
(10)
where DC is the duty cycle ratio (V
OUT
/V
IN
) and f
SW
is the
switching frequency.
Input Capacitors
The V
IN
of the LT8641 should be bypassed with at least
three ceramic capacitors for best performance. Tw o 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 capaci-
tors, two small 0402 or 0603 may be placed at each side
of the LT8641 near the V
IN1
/GND1 and V
IN2
/GND2 pins.
A third, larger ceramic capacitor of 2.2µF or larger should
be placed close to V
IN1
or V
IN2
. See Low EMI PCB Layout
section for more detail. X7R or X5R capacitors are rec-
ommended for best performance across temperature and
input voltage variations.
Note that larger input capacitance is required when a lower
switching frequency is used. If the input power source has
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