Datasheet

LT1936
15
1936fd
APPLICATIONS INFORMATION
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT1936 circuits. However, these capaci-
tors can cause problems if the LT1936 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 under damped tank circuit, and the voltage
at the V
IN
pin of the LT1936 can ring to twice the nominal
input voltage, possibly exceeding the LT1936’s rating and
damaging the part. If the input supply is poorly controlled
or the user will be plugging the LT1936 into an energized
supply, the input network should be designed to prevent
this overshoot.
Figure 8 shows the waveforms that result when an LT1936
circuit is connected to a 24V supply through six feet of
24-gauge twisted pair. The fi rst plot is the response with
a 4.7μF ceramic capacitor at the input. The input voltage
rings as high as 50V and the input current peaks at 26A.
Figure 8. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT1936 is Connected to a Live Supply
+
+
LT1936
4.7μF
V
IN
20V/DIV
I
IN
10A/DIV
20μs/DIV
V
IN
CLOSING SWITCH
SIMULATES HOT PLUG
I
IN
(8a)
(8b)
(8c)
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
+
+
LT1936
4.7μF
22μF
35V
AI.EI.
LT1936
4.7μF0.1μF
0.7Ω
1936 F08
V
IN
20V/DIV
I
IN
10A/DIV
20μs/DIV
V
IN
20V/DIV
I
IN
10A/DIV
20μs/DIV
DANGER
RINGING V
IN
MAY EXCEED
ABSOLUTE MAXIMUM
RATING OF THE LT1936