LT1933 Linear Technology, LT1933 Datasheet - Page 12

• Manufacturer Part Number: LT1933
• Description: 600mA 500kHz Step-Down Switching Regulator in SOT-23
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT1933
pull large currents from the output through the SW pin and
the V
the input voltage is present and that protects against a
shorted or reversed input.
V
12
Figure 5. Diode D4 Prevents a Shorted Input from Discharging
a Backup Battery Tied to the Output; It Also Protects the Circuit
from a Reversed Input. The LT1933 Runs Only When the Input
is Present
IN
IN
D4: MBR0540
pin. Figure 5 shows a circuit that will run only when
D4
5
4
V
SHDN
IN
GND
2
LT1933
24V SUPPLY
IMPEDANCE
U
ENERGIZED
BOOST
LOW
FB
+
+
SW
3
+
Figure 6. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT1933 is Connected to a Live Supply
U
1
6
SIMULATES HOT PLUG
CLOSING SWITCH
AI.EI.
0.1µF
10µF
35V
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
W
+
I
IN
1Ω
1933 F05
V
IN
U
2.2µF
2.2µF
2.2µF
V
BACKUP
OUT
LT1933
LT1933
LT1933
(6a)
(6b)
(6c)
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT1933 circuits. However, these ca-
pacitors can cause problems if the LT1933 is plugged into
a live supply (see Linear Technology Application Note 88
for a complete discussion). The low loss ceramic capaci-
tor 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 LT1933’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT1933 into an
energized supply, the input network should be designed to
prevent this overshoot.
Figure 6 shows the waveforms that result when an LT1933
circuit is connected to a 24V supply through six feet of 24-
gauge twisted pair. The first plot is the response with a
20V/DIV
20V/DIV
20V/DIV
5A/DIV
5A/DIV
5A/DIV
V
V
V
I
I
I
IN
IN
IN
IN
IN
IN
IN
pin of the LT1933 can ring to twice the
DANGER!
RINGING V
ABSOLUTE MAXIMUM
RATING OF THE LT1933
20µs/DIV
20µs/DIV
20µs/DIV
IN
MAY EXCEED
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1933 F06
1933f