Datasheet

LT8620
14
8620fa
For more information www.linear.com/LT8620
APPLICATIONS INFORMATION
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
routing 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 LT8620 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 3.8A at low
duty cycles and decreases linearly to 2.8A 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
–
∆I
L
2
(8)
The peak-to-peak ripple current in the inductor can be
calculated as follows:
∆I
L
=
V
OUT
L • f
SW
• 1–
V
OUT
V
IN(MAX)








(9)
where f
SW
is the switching frequency of the LT8620, and
L is the value of the inductor. Therefore, the maximum
output current that the LT8620 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 LT8620 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.
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 LT8620 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 Capacitor
Bypass the input of the LT8620 circuit with a ceramic ca-
pacitor of X7R or X5R type placed as close as possible to
the V
IN
and PGND pins. Y5V types have poor performance
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypass the LT8620 and will easily handle the ripple current.
Note that larger input capacitance is required when a lower
switching frequency is used. If the input power source has
high impedance, or there is significant inductance due
to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT8620 and to force this very high frequency
switching current into a tight
local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT8620 (see the PCB Layout section).
A second precaution regarding the ceramic input capaci-
tor concerns the maximum input voltage rating of the
LT8620. A ceramic input capacitor combined with trace
or cable inductance forms a high quality (under damped)
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