Datasheet

LT3481
17
3481fc
VIAS TO LOCAL GROUND PLANE
VIAS TO V
OUT
VIAS TO RUN/SS
VIAS TO PG
VIAS TO V
IN
OUTLINE OF LOCAL
GROUND PLANE
3481 F09
L1
C2
R
RT
R
PG
R
C
R2
R1
C
C
V
OUT
D1
C1
GND
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3481 circuits. However, these capaci-
tors can cause problems if the LT3481 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 LT3481 can ring to twice the
nominal input voltage, possibly exceeding the LT3481’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3481 into an
energized supply, the input network should be designed to
prevent this overshoot. Figure 10 shows the waveforms
that result when an LT3481 circuit is connected to a 24V
supply through six feet of 24-gauge twisted pair. The
Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation when the LT3481 is Connected to a Live Supply
+
LT3481
4.7μF
V
IN
20V/DIV
I
IN
10A/DIV
20μs/DIV
V
IN
CLOSING SWITCH
SIMULATES HOT PLUG
I
IN
(10a)
(10b)
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
+
LT 3 4 8 1
4.7μF0.1μF
0.7Ω
V
IN
20V/DIV
I
IN
10A/DIV
20μs/DIV
DANGER
RINGING V
IN
MAY EXCEED
ABSOLUTE MAXIMUM RATING
(10c)
+
LT 3 4 8 1
4.7μF
22μF
35V
AI.EI.
3481 F10
V
IN
20V/DIV
I
IN
10A/DIV
20μs/DIV
+
APPLICATIONS INFORMATION