Datasheet

LT8619/LT8619-5
16
Rev A
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For low V
IN
applications that cannot allow deviation from
the programmed oscillator frequency, use the following
formula to set the switching frequency:
V
IN(MIN)
=
V
SW(BOT)
+
V
OUT
1– t
OFF(MIN)
• f
OSC
+ V
SW(TOP)
– V
SW(BOT)
where V
IN(MIN)
is the minimum input voltage without
skipped cycles, V
OUT
is the output voltage, V
SW(TOP)
and
V
SW(BOT)
are the internal switch drops (~0.54V, ~0.264V,
respectively at maximum load), f
OSC
is the oscillating fre-
quency (set by R
T
), and t
OFF(MIN)
is the minimum switch-
ing off-time. Note that higher switching frequency will
increase the minimum input voltage below which cycles
will be dropped to achieve higher duty cycle.
Inductor Selection and Maximum Output Current
The LT8619 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 LT8619 safely tolerates opera-
tion 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 = 2
V
OUT
+
V
SW(BOT)
f
OSC
where f
OSC
is the switching frequency in MHz, V
OUT
is
the output voltage, V
SW(BOT)
is the bottom switch drop
(~0.264V) 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 applica-
tion. In addition, the saturation current (typically labeled
I
SAT
) rating of the inductor must be higher than the load
current plus one-half of inductor ripple current:
I
SAT
> I
LOAD(MAX)
+
ΔI
L(MAX)
2
APPLICATIONS INFORMATION
where I
LOAD(MAX)
is the maximum output load for a given
application and ∆I
L(MAX)
is the inductor ripple current as
calculated in the following equation:
ΔI
L(MAX)
=
1
f
OSC
•L
V
OUT
1–
V
OUT
V
IN(MAX)
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
As a quick example, an application requiring 1A output
current should use an inductor with an RMS rating of
greater than 1A and an I
SAT
of greater than 1.5A. During
long duration overload or short-circuit conditions, the
inductor RMS rating requirement is greater to avoid over-
heating of the inductor. To push for high efficiency, select
an inductor with low series resistance (DCR), preferably
below 0.04Ω, and the core material should be intended
for high frequency application. However, achieving this
requires a large size inductor. An inductor with DCR
around 0.1Ω is generally a good compromise for both
efficiency and board area, at the expense of trimming 1%
to 2% from the efficiency number.
The LT8619 limits the peak switch current in order to pro-
tect the switches and the system from overload faults. The
top switch current limit (I
LIM
) is at least 1.5A. The induc-
tor value must then be sufficient to supply the desired
maximum output current (I
LOAD(MAX)
), which is a function
of the switch current limit (I
LIM
) and the ripple current:
I
LOAD(MAX)
= I
LIM
–
Δ
I
L
2
Therefore, the maximum output current that the LT8619
will deliver depends on the switch current limit, the induc-
tor value, and the input and output voltages. The inductor
value may have to be increased if the inductor ripple cur-
rent does not allow sufficient maximum output current
(I
LOAD(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 single small
pulses in Burst Mode operation such that the LT8619 can
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