Datasheet

LT3480
10
3480fe
For more information www.linear.com/LT3480
APPLICATIONS INFORMATION
A good choice of switching frequency should allow ad-
equate input voltage range (see next section) and keep
the inductor and capacitor values small.
Input Voltage Range
The maximum input voltage for LT3480 applications depends
on switching frequency, the Absolute Maximum Ratings of
the V
IN
and BOOST pins, and the operating mode.
The LT3480 can operate from input voltages up to 38V,
and safely withstand input voltages up 60V. Note that while
V
IN
>38V (typical), the LT3480 will stop switching, allowing
the output to fall out of regulation.
While the output is in start-up, short-circuit, or other
overload conditions, the switching frequency should be
chosen according to the following discussion.
For safe operation at inputs up to 60V the switching fre-
quency must be set low enough to satisfy V
IN(MAX)
≥ 40V
according to the following equation. If lower V
IN(MAX)
is
desired, this equation can be used directly.
V
IN(MAX)
=
V
OUT
+ V
D
f
SW
t
ON(MIN)
– V
D
+ V
SW
where V
IN(MAX)
is the maximum operating input voltage,
V
OUT
is the output voltage, V
D
is the catch diode drop
(~0.5V), V
SW
is the internal switch drop (~0.5V at max
load), f
SW
is the switching frequency (set by R
T
), and
t
ON(MIN)
is the minimum switch on time (~150ns). Note that
a higher switching frequency will depress the maximum
operating input voltage. Conversely, a lower switching
frequency will be necessary to achieve safe operation at
high input voltages.
If the output is in regulation and no short-circuit, start-
up, or overload events are expected, then input voltage
transients of up to 60V are acceptable regardless of the
switching frequency. In this mode, the LT3480 may enter
pulse skipping operation where some switching pulses
are skipped to maintain output regulation. In this mode
the output voltage ripple and inductor current ripple will
be higher than in normal operation. Above 38V switching
will stop.
The minimum input voltage is determined by either the
LT3480’s minimum operating voltage of ~3.6V or by its
maximum duty cycle (see equation in previous section).
The minimum input voltage due to duty cycle is:
V
IN(MIN)
=
V
OUT
+ V
D
1– f
SW
t
OFF(MIN)
– V
D
+ V
SW
where V
IN(MIN)
is the minimum input voltage, and t
OFF(MIN)
is the minimum switch off time (150ns). Note that higher
switching frequency will increase the minimum input
voltage. If a lower dropout voltage is desired, a lower
switching frequency should be used.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current ΔI
L
increases with higher V
IN
or V
OUT
and decreases with higher inductance and faster switching
frequency. A reasonable starting point for selecting the
ripple current is:
ΔI
L
= 0.4(I
OUT(MAX)
)
where I
OUT(MAX)
is the maximum output load current. To
guarantee sufficient output current, peak inductor current
must be lower than the LT3480’s switch current limit (I
LIM
).
The peak inductor current is:
I
L(PEAK)
= I
OUT(MAX)
+ ΔI
L
/2
where I
L(PEAK)
is the peak inductor current, I
OUT(MAX)
is
the maximum output load current, and ΔI
L
is the inductor
ripple current. The LT3480’s switch current limit (I
LIM
) is
at least 3.5A at low duty cycles and decreases linearly to
2.5A at DC = 0.8. The maximum output current is a func-
tion of the inductor ripple current:
I
OUT(MAX)
= I
LIM
– ΔI
L
/2
Be sure to pick an inductor ripple current that provides
sufficient maximum output current (I
OUT(MAX)
).
The largest inductor ripple current occurs at the highest
V
IN
. To guarantee that the ripple current stays below the
specified maximum, the inductor value should be chosen
according to the following equation:
L =
V
OUT
+ V
D
f
SW
∆I
L






1–
V
OUT
+ V
D
V
IN(MAX)





