LT3505
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 VIN pin is allowed to float and the SHDN pin
is held high (either by a logic signal or because it is tied
to VIN), then the LT3505’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 SHDN pin, the SW pin current will drop to essentially
zero. However, if the VIN 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 VIN pin. Figure 9 shows a circuit that will run only
whentheinputvoltageispresentandthatprotectsagainst
a shorted or reversed input.
RUN
SHDN
GND
V
SW
5V/DIV
I
L
1A/DIV
V
OUT
2V/DIV
3505 F08a
10 s/DIV
V
V
= 12V
IN
OUT
= 3.3V
L = 2.5 H
= 10 F
C
D4
OUT
= 20.0k
R
T
V
OUT
V
IN
V
BOOST SW
LT3505
IN
RUN
15k
SHDN
FB
R
GND
V
C
T
BACKUP
SHDN
GND
0.068 F
3505 F09
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
SW
5V/DIV
Hot Plugging Safely
I
L
1A/DIV
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
foracompletediscussion).Thelowlossceramiccapacitor
combined with stray inductance in series with the power
sourceformsanunderdampedtankcircuitandthevoltage
at the VIN pin of the LT3505 can ring to twice the nominal
input voltage, possibly exceeding the LT3505’s rating and
V
OUT
2V/DIV
3505 F08b
10 s/DIV
V
V
= 12V
IN
OUT
= 3.3V
L = 2.5 H
= 10 F
C
OUT
= 20.0k
R
T
Figure 8. To Soft-Start the LT3505, Add a Resistor and Capacitor
to the SHDN pin. VIN = 12V, VOUT = 3.3V, COUT = 10µF, RLOAD
=
5Ω, RT = 20.0k, L = 2.5µH
3505fc
17
Linear [ Linear ]
