Datasheet

LT8614
15
Rev. E
For more information www.analog.com
APPLICATIONS INFORMATION
Table 1. SW Frequency vs R
T
Value
f
SW
(MHz) R
T
(kΩ)
0.2 232
0.3 150
0.4 110
0.5 88.7
0.6 71.5
0.7 60.4
0.8 52.3
1.0 41.2
1.2 33.2
1.4 28.0
1.6 23.7
1.8 20.5
2.0 18.2
2.2 15.8
3.0 10.7
Operating Frequency Selection and Trade-Offs
Selection of the operating frequency is a trade-off between
efficiency, component size, and input voltage range. The
advantage of high frequency operation is that smaller
inductor and capacitor values may be used. The disad-
vantages are lower efficiency and a smaller input voltage
range.
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 (~0.45V, ~0.15V, respectively at maximum load)
and t
ON(MIN)
is the minimum top switch on-time (see the
Electrical Characteristics). This equation shows that a
slower switching frequency is necessary to accommodate
a high V
IN
/V
OUT
ratio.
For transient operation, V
IN
may go as high as the abso-
lute maximum rating of 42V regardless of the R
T
value,
however the LT8614 will reduce switching frequency
as necessary to maintain control of inductor current to
assure safe operation.
The LT8614 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 LT8614 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 (~0.45V, ~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 LT8614 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 LT8614 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
(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.
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