LT3682 LINER [Linear Technology], LT3682 Datasheet - Page 12

• Manufacturer Part Number: LT3682
• Description: 1A Micropower Step-Down
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3682
Frequency Tradeoffs section). The minimum input voltage
due to duty cycle is:
where V
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
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
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:
where V
V
(~0.5V), V
f
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
12
SW
IN
OUT
V
V
>V
is the switching frequency (set by R
IN MIN
IN MAX
OVLO
is the output voltage, V
(
(
IN(MIN)
IN(MAX)
SW
)
)
(39V typical), the LT3682 will stop switching,
=
=
is the internal switch drop (~0.5V at max load),
1
f
−
is the minimum input voltage, and t
SW ON MIN
V
is the maximum operating input voltage,
V
D
f
IN
SW OFF MIN
OUT
+
t
and BOOST pins, and the operating
V
t
(
OUT
+
V
(
D
)
−
)
V
−
D
D
V
+
is the catch diode drop
D
V
+
SW
V
SW
T
), and t
ON(MIN)
OFF(MIN)
is
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:
where I
the maximum expected input voltage, L is the inductor
value, t
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
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
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
and the part starts switching again.
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is:
I
L
L PEAK
(
=
(
ON(MIN)
V
L(PEAK)
OUT
)
=
+
2
A
V
is the minimum on time and V
is the peak inductor current, V
D
+
) •
OVLO
V
IN MAX
f
. 1 8
SW
(
are acceptable regardless of the
IN
returns to values below V
)
L
−
V
OUTOL
OVLO
•
and up to 60V can
t
ON MIN
(
OUTOL
)
IN(MAX)
is the
OVLO
3682f
is