LT1936 Linear Technology, LT1936 Datasheet - Page 15

• Manufacturer Part Number: LT1936
• Description: 1.4A 500kHz Step-Down Switching Regulator
• Manufacturer: 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 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
IMPEDANCE
24V SUPPLY
ENERGIZED
LOW
+
+
+
Figure 8. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT1936 is Connected to a Live Supply
SIMULATES HOT PLUG
CLOSING SWITCH
AI.EI.
0.1μF
22μF
35V
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
+
I
0.7Ω
IN
V
IN
4.7μF
4.7μF
4.7μF
LT1936
LT1936
LT1936
(8a)
(8b)
(8c)
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.
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
20μs/DIV
20μs/DIV
20μs/DIV
RINGING V
ABSOLUTE MAXIMUM
RATING OF THE LT1936
IN
MAY EXCEED
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1936 F08
LT1936
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