Datasheet

15
LT3503
3503f
APPLICATIO S I FOR ATIO
WUU
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Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT3503 is Connected to a Live Supply
a live supply (see Linear Technology Application Note 88
for a complete discussion). The low loss ceramic capaci-
tor 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 LT3503 can ring to twice the
nominal input voltage, possibly exceeding the LT3503’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3503 into an
energized supply, the input network should be designed to
prevent this overshoot.
Figure 10 shows the waveforms that result when an
LT3503 circuit is connected to a 20V supply through six
feet of 24-gauge twisted pair. The first plot is the response
with a 2.2µF ceramic capacitor at the input. The input
voltage rings as high as 35V and the input current peaks
at 20A. One method of damping the tank circuit is to add
another capacitor with a series resistor to the circuit. In
Figure 10b an aluminum electrolytic capacitor has been
added. This capacitor’s high equivalent series resistance
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 LT3503 Runs Only When the Input
is Present
V
IN
BOOST
GND FB
SHDN SW
D4
V
IN
LT3503
3503 F09
V
OUT
BACKUP
+
+
LT3503
2.2µF
V
IN
20V/DIV
I
IN
5A/DIV
20µs/DIV
V
IN
CLOSING SWITCH
SIMULATES HOT PLUG
I
IN
(10a)
(10b)
(10c)
LOW
IMPEDANCE
ENERGIZED
20V SUPPLY
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
+
+
LT3503
2.2µF
10µF
35V
AI.EI.
LT3503
2.2µF0.1µF
1Ω
3503 F10
V
IN
20V/DIV
I
IN
5A/DIV
20µs/DIV
V
IN
20V/DIV
I
IN
5A/DIV
20µs/DIV
DANGER!
RINGING V
IN
MAY EXCEED
ABSOLUTE MAXIMUM
RATING OF THE LT3503
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3503 circuits. However, these ca-
pacitors can cause problems if the LT3503 is plugged into