Datasheet

LT3693
17
3693f
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3693 circuits. However, these capaci-
tors can cause problems if the LT3693 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 LT3693 can ring to twice the
nominal input voltage, possibly exceeding the LT3693’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3693 into an
Figure 9. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation when the LT3693 is Connected to a Live Supply
+
LT3693
4.7MF
V
IN
20V/DIV
I
IN
10A/DIV
20Ms/DIV
V
IN
CLOSING SWITCH
SIMULATES HOT PLUG
I
IN
(9a)
(9b)
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
+
LT3693
4.7MF0.1MF
0.77
V
IN
20V/DIV
I
IN
10A/DIV
20Ms/DIV
DANGER
RINGING V
IN
MAY EXCEED
ABSOLUTE MAXIMUM RATING
(9c)
+
LT3693
4.7MF
22MF
35V
AI.EI.
3693 F10
V
IN
20V/DIV
I
IN
10A/DIV
20Ms/DIV
+
APPLICATIONS INFORMATION
energized supply, the input network should be designed
to prevent this overshoot. Figure 9 shows the waveforms
that result when an LT3693 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. A good solution is shown
in Figure 9b. A 0.7 resistor is added in series with the
input to eliminate the voltage overshoot (it also reduces
the peak input current). A 0.1μF capacitor improves high
frequency fi ltering. For high input voltages its impact on
effi ciency is minor, reducing effi ciency by 1.5 percent for
a 5V output at full load operating from 24V.
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