Datasheet

LT8612
13
8612fa
For more information www.linear.com/LT8612
APPLICATIONS INFORMATION
as necessary to maintain control of inductor current to
assure safe operation.
The LT8612 is capable of a maximum duty cycle of greater
than 99%, and the V
IN
-to-V
OUT
dropout is limited by the
R
DS(ON)
of the top switch. In this mode the LT8612 skips
switch cycles, resulting in a lower switching frequency
than programmed by RT.
For applications that cannot allow deviation from the pro-
grammed switching frequency at low V
IN
/V
OUT
ratios use
the following formula to set switching frequency:
V
IN(MIN)
=
V
OUT
+ V
SW(BOT)
1– f
SW
• t
OFF(MIN)
– V
SW(BOT)
+ V
SW(TOP)
(5)
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 (~0.4V, ~0.18V,
respectively at maximum load), f
SW
is the switching
frequency (set by RT), and t
OFF(MIN)
is the minimum
switch 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 LT8612 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 LT8612 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 =
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
(~0.18V) 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
∆I
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 15mΩ, and the core material
should be intended for high frequency applications.
The LT8612 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 9.5A 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
–
∆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 LT8612, and
L is the value of the inductor. Therefore, the maximum
output current that the LT8612 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.
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
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