LT8611
14
8611fa
For more information www.linear.com/LT8611
APPLICATIONS INFORMATION
The LT8611 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 LT8611 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)
=
OUT
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 (~0.3V, ~0.15V,
respectively at maximum load), f
SW
is the switching fre-
quency (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 LT8611 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 LT8611 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 =
OUT
SW(BOT)
f
(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)
+
∆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 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 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 LT8611 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 3.5A 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
–
L
(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 LT8611, and
L is the value of the inductor. Therefore, the maximum
output current that the LT8611 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
reduces the output voltage ripple. For applications requir
-
ing smaller load currents, the value of the inductor may
be lower and the LT8611
may operate with higher ripple
