Datasheet

LT8640/LT8640-1
19
Rev.C
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APPLICATIONS INFORMATION
Inductor Selection and Maximum Output Current
The LT8640/LT8640-1 is designed to minimize solution size
by allowing the inductor to be chosen based on the output
load requirements of the application. During overload or
short-circuit conditions the LT8640/LT8640-1 safely toler
-
ates operation with a saturated inductor through the use
of a high speed peak-current mode architecture.
A good first choice for the inductor value is:
L =
V
OUT
+
V
SW(BOT)
f
SW
⎛
⎝
⎜
⎞
⎠
⎟
•0.7
(6)
where f
SW
is the switching frequency in MHz, V
OUT
is
the output voltage, V
SW(BOT)
is the bottom switch drop
(~0.15V) and L is the inductor value in µH.
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 3A output
should use an inductor with an RMS rating of greater than
3A and an I
SAT
of greater than 4A. 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 0.02Ω, and the core material
should be intended for high frequency applications.
The LT8640/LT8640-1 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 10A at low
duty cycles and decreases linearly to 7A 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 LT8640/
LT8640-1, and L is the value of the inductor. Therefore,
the maximum output current that the LT8640/LT8640-
1 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 LT8640/
LT8640-1 can stay in sleep mode longer between each
pulse. This can be achieved by using a larger value induc
-
tor (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 switching frequency application, if high
light load efficiency is desired, a higher inductor value
should be chosen. See curve in Typical Performance
Characteristics.
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