LT3693EMSE-PBF LINER [Linear Technology], LT3693EMSE-PBF Datasheet - Page 17

• Manufacturer Part Number: LT3693EMSE-PBF
• Description: 36V, 3.5A, 2.4MHz Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

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 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
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
IN
pin of the LT3693 can ring to twice the
Figure 9. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation when the LT3693 is Connected to a Live Supply
24V SUPPLY
IMPEDANCE
ENERGIZED
LOW
+
+
+
SIMULATES HOT PLUG
CLOSING SWITCH
0.1 F
22 F
AI.EI.
35V
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
+
I
0.7
IN
V
IN
4.7 F
4.7 F
4.7 F
LT3693
LT3693
LT3693
(9a)
(9b)
(9c)
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.
20V/DIV
10A/DIV
20V/DIV
10A/DIV
20V/DIV
10A/DIV
V
V
V
I
I
I
IN
IN
IN
IN
IN
IN
DANGER
RINGING V
ABSOLUTE MAXIMUM RATING
20 s/DIV
20 s/DIV
20 s/DIV
IN
MAY EXCEED
3693 F10
LT3693
17
3693f