Datasheet

LT3505
17
3505fc
APPLICATIONS INFORMATION
plications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT3505’s
output. If the V
IN
pin is allowed to fl oat and the SHDN pin
is held high (either by a logic signal or because it is tied
to V
IN
), then the LT3505’s internal circuitry will pull its
quiescent current through its SW pin. This is fi ne if your
system can tolerate a few mA in this state. If you ground
the SHDN pin, the SW pin current will drop to essentially
zero. However, if the V
IN
pin is grounded while the output
is held high, then parasitic diodes inside the LT3505 can
pull large currents from the output through the SW pin
and the V
IN
pin. Figure 9 shows a circuit that will run only
when the input voltage is present and that protects against
a shorted or reversed input.
Figure 8. To Soft-Start the LT3505, Add a Resistor and Capacitor
to the SHDN pin. V
IN
= 12V, V
OUT
= 3.3V, C
OUT
= 10µF, R
LOAD
=
5Ω, R
T
= 20.0k, L = 2.5µH
Figure 9. Diode D4 Prevents a Shorted Input from Discharging
a Backup Battery Tied to the Output; It Also Protects the Circuit
from a Reversed Input. The LT3505 Runs Only When the Input
is Present
V
IN
BOOST
GND
FB
V
C
SHDN
SW
D4
V
IN
LT3505
3505 F09
V
OUT
BACKUP
R
T
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3505 circuits. However, these ca-
pacitors can cause problems if the LT3505 is 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
IN
pin of the LT3505 can ring to twice the nominal
input voltage, possibly exceeding the LT3505’s rating and
RUN
V
SW
5V/DIV
V
IN
= 12V
V
OUT
= 3.3V
L = 2.5 H
C
OUT
= 10 F
R
T
= 20.0k
V
OUT
2V/DIV
10 s/DIV
I
L
1A/DIV
SHDN
GND
3505 F08a
V
SW
5V/DIV
V
IN
= 12V
V
OUT
= 3.3V
L = 2.5 H
C
OUT
= 10 F
R
T
= 20.0k
V
OUT
2V/DIV
10 s/DIV
I
L
1A/DIV
3505 F08b
RUN
15k
0.068 F
SHDN
GND
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