Datasheet

LT8641
16
Rev B
For more information www.analog.com
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 (~0.3V, ~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 65V regardless of the R
T
value,
however the LT8641 will reduce switching frequency as
necessary to maintain control of inductor current to as-
sure safe operation.
The LT8641 is capable of a maximum duty cycle of ap-
proximately 99%, and the V
IN
-to-V
OUT
dropout is limited
by the R
DS(ON)
of the top switch. In this mode the LT8641
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 (M IN )
– 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 LT8641 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 LT8641 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.
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(M AX )
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 2A output
should use an inductor with an RMS rating of greater than
2A and an I
SAT
of greater than 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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