Datasheet

LT8616
14
8616fa
For more information www.linear.com/LT8616
APPLICATIONS INFORMATION
The highest switching frequency (f
SW(MAX)
) for a given
application can be calculated as follows:
f
SW(MAX)
=
V
OUT
+ V
SW(BOT)
t
ON(MIN)
V
IN
– V
SW(TOP)
+ V
SW(BOT)
( )
(4)
where V
IN
is the typical input voltage, V
OUT
is the output
voltage, V
SW(TOP)
and V
SW(BOT)
are the internal switch
drops (~0.53V, ~0.38V, respectively at maximum load
for channel 1 and ~0.78V, ~0.48V for channel 2) and
t
ON(MIN)
is the minimum top switch on-time of 55ns (see
the Electrical Characteristics). This equation shows that a
lower switching frequency is necessary to accommodate a
high V
IN
/V
OUT
ratio. Choose the lower frequency between
channel 1 and 2.
For transient operation, V
IN
may go as high as the absolute
maximum rating of 42V regardless of the R
T
value, how-
ever the LT8616 will reduce switching frequency on each
channel
independently as necessary to maintain control
of inductor current to assure safe operation.
The LT8616 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 channel that
enters dropout skips switch cycles, resulting in a lower
than programmed switching frequency.
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 (~0.53V, ~0.38V,
respectively at maximum load for channel 1 and ~0.78V,
~0.48V for channel 2), f
SW
is the switching frequency (set
by R
T
), 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.
Note there is no minimum V
IN2
voltage requirement as it
does not supply the internal common bias circuits, mak-
ing channel
2
uniquely capable of operating from very
low input voltages.
Inductor Selection and Maximum Output Current
The LT8616 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 LT8616 safely tolerates opera
-
tion with a saturated inductor through the use of a high
speed peak-current mode ar
chitecture.
A good first choice for the inductor value is:
L1=
V
OUT1
+ V
SW1(BOT )
f
SW
•1.6
L2 =
V
OUT2
+ V
SW2(BOT)
f
SW
(6a)
(6b)
where f
SW
is the switching frequency in MHz, V
OUT
is
the output voltage, V
SW(BOT)
is the bottom switch drop
(~0.38V, ~0.48V) 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
) rat-
ing 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 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
rating 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.
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