LT3682
12
3682f
APPLICATIONS INFORMATION
Frequency Tradeoffs section). The minimum input voltage
due to duty cycle is:
V
VV
ft
VV
IN MIN
OUT D
SW OFF MIN
DSW()
()
=
+
−
−+
1
where V
IN(MIN)
is the minimum input voltage, and t
OFF(MIN)
is the minimum switch off time (210ns). 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.
The maximum input voltage for LT3682 applications
depends on switching frequency, the Absolute Maximum
Ratings of the V
IN
and BOOST pins, and the operating
mode. The LT3682 can operate from continuous input
voltages up to 36V. Input voltage transients of up to
60V are also safely withstood. However, note that while
V
IN
>V
OVLO
(39V typical), the LT3682 will stop switching,
allowing the output to fall out of regulation.
For a given application where the switching frequency
and the output voltage are already fi xed, the maximum
input voltage that guarantees optimum output voltage
ripple for that application can be found by applying the
following expression:
V
VV
ft
VV
IN MAX
D OUT
SW ON MIN
DSW()
()
=
+
−+
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 reduce the maximum operating
input voltage. Conversely, a lower switching frequency
will be necessary to achieve optimum operation at high
input voltages.
Special attention must be paid when the output is in start-
up, short-circuit, or other overload conditions. In these
cases, the LT3682 tries to bring the output in regulation by
driving lots of current into the output load. During these
events, the inductor peak current might easily reach and
even exceed the maximum current limit of the LT3682,
especially in those cases where the switch already operates
at minimum on time. The circuitry monitoring the current
through the catch diode via the DA pin prevents the switch
from turning on again if the inductor valley current is above
1.6A nominal. In these cases, the inductor peak current is
therefore the maximum current limit of the LT3682 plus
the additional current overshoot during the turn off delay
due to minimum on time:
IA
VV
L
t
LPEAK
IN MAX OUTOL
ON MIN()
()
()
•=+
−
2
where I
L(PEAK)
is the peak inductor current, V
IN(MAX)
is
the maximum expected input voltage, L is the inductor
value, t
ON(MIN)
is the minimum on time and V
OUTOL
is the
output voltage under the overload condition. The part is
robust enough to survive prolonged operation under these
conditions as long as the peak inductor current does not
exceed 3.5A. Inductor current saturation and excessive
junction temperature may further limit performance.
If the output is in regulation and no short-circuit, startup,
or overload events are expected, then input voltage tran-
sients of up to V
OVLO
are acceptable regardless of the
switching frequency. In this case, the LT3682 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.
Input voltage transients above V
OVLO
and up to 60V can
be tolerated. However, since the part will stop switching
during these transients, the output will fall out of regulation
and the output capacitor may eventually be completely
discharged. This case must be treated then as a start-up
condition as soon as V
IN
returns to values below V
OVLO
and the part starts switching again.
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is:
LV V
f
OUT D
SW
=+()•
.1 8
