LT3686 LINER [Linear Technology], LT3686 Datasheet - Page 20

• Manufacturer Part Number: LT3686
• Manufacturer: LINER [Linear Technology]

LT3686
ApplicAtions inForMAtion
Short and Reverse Protection
If the inductor is chosen so that it won’t saturate exces-
sively, the LT3686 will tolerate a shorted output. When
operating in short-circuit condition, the LT3686 will reduce
its frequency until the valley current is 1.7A (Figure 13).
There is another situation to consider in systems where
the output will be held high when the input to the LT3686
is absent. This may occur in battery charging applications
or in battery backup systems where a battery or some
other supply is diode ORed with the LT3686’s output. If
the V
high (either by a logic signal or because it is tied to V
then the LT3686’s internal circuitry will pull its quiescent
current through its SW pin. This is fine if your system
can tolerate a few mA in this state. If you ground the
EN/UVLO pin, the SW pin current will drop to essentially
zero. However, if the V
is held high, then parasitic diodes inside the LT3686 can
0
Figure 13. The LT3686 Reduces its Frequency from 2MHz to
160kHz to Protect Against Shorted Output
500mA/DIV
IN
20V/DIV
pin is allowed to float and the EN/UVLO pin is held
V
SW
I
L
V
L = 6.8µH
C
R
V
IN
OUT
OUT
T
= 17.4k
= 35V
= 22µF
= 0V
IN
pin is grounded while the output
2µs/DIV
3686 F13
IN
),
pull large currents from the output through the SW pin
and the V
only when the input voltage is present and that protects
against a shorted or reversed input.
Hot-Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3686 circuits. However, these capaci-
tors can cause problems if the LT3686 are plugged into a
live supply (see Linear Technology Application Note 88 for
a complete discussion). The low loss ceramic capacitor
combined with stray inductance in series with the power
source forms an underdamped tank circuit, and the voltage
at the V
input voltage, possibly exceeding the LT3686’s rating and
damaging the part. If the input supply is poorly controlled
or the user will be plugging the LT3686 into an energized
V
IN
Figure 14. Input Diode Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output; it Also
Protects the Circuit from a Reversed Input. The LT3686
Runs Only When the Input is Present
IN
IN
pin of the LT3686 can ring to twice the nominal
pin. Figure 14 shows a circuit that will run
EN/UVLO
MODE
V
SS
RT
IN
LT3686
GND
BOOST
SW
BD
DA
FB
3686 F14
3686fa
V
OUT