LT3481 LINER [Linear Technology], LT3481 Datasheet - Page 17

• Manufacturer Part Number: LT3481
• Description: 36V, 2A, 2.8MHz Step-Down Switching Regulator with 50?A Quiescent Current
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
VIAS TO LOCAL GROUND PLANE
VIAS TO V
D1
OUT
L1
C1
V
OUT
Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation when the LT3481 is Connected to a Live Supply
GND
IMPEDANCE
24V SUPPLY
ENERGIZED
VIAS TO RUN/SS
VIAS TO PG
LOW
+
+
+
C2
R
R
RT
PG
SIMULATES HOT PLUG
CLOSING SWITCH
AI.EI.
0.1μF
22μF
35V
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
VIAS TO V
OUTLINE OF LOCAL
GROUND PLANE
+
C
R1
C
R
I
0.7Ω
IN
C
R2
IN
V
3481 F09
IN
4.7μF
4.7μF
4.7μF
LT3481
LT3481
LT3481
(10a)
(10b)
(10c)
20V/DIV
10A/DIV
20V/DIV
10A/DIV
20V/DIV
10A/DIV
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
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
V
V
V
I
I
I
IN
IN
IN
IN
IN
IN
DANGER
RINGING V
ABSOLUTE MAXIMUM RATING
20μs/DIV
20μs/DIV
20μs/DIV
IN
pin of the LT3481 can ring to twice the
IN
MAY EXCEED
3481 F10
LT3481
17
3481fb