lt3686a Linear Technology Corporation, lt3686a Datasheet - Page 21

• Manufacturer Part Number: lt3686a
• Description: 37v/1.2a Step-down Regulator In 3mm
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
the LT3686A into an energized supply, the input network
should be designed to prevent this overshoot. Figure 15
shows the waveforms that result when an LT3686A
circuit is connected to a 24V supply through six feet of
24-gauge twisted pair. The fi rst plot is the response with
a 2.2μF ceramic capacitor at the input. The input voltage
rings as high as 35V and the input current peaks at 20A.
One method of damping the tank circuit is to add another
capacitor with a series resistor to the circuit. In Figure 15b
an aluminum electrolytic capacitor has been added. This
capacitor’s high equivalent series resistance damps the
circuit and eliminates the voltage overshoot. The extra
Figure 15. A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation
When the LT3686A Is Connected to a Live Supply
24V SUPPLY
IMPEDANCE
ENERGIZED
LOW
+
+
+
SIMULATES HOT PLUG
CLOSING SWITCH
AI.EI.
0.1μF
10μF
35V
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
+
I
IN
1Ω
V
IN
2.2μF
2.2μF
2.2μF
LT3686A
LT3686A
LT3686A
(15a)
(15b)
(15c)
capacitor improves low frequency ripple fi ltering and
can slightly improve the effi ciency of the circuit, though
it is likely to be the largest component in the circuit. An
alternative solution is shown in Figure 15c. A 1Ω 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. This solution is
smaller and less expensive than the electrolytic capacitor.
For high input voltages its impact on effi ciency is minor,
reducing effi ciency one percent for a 5V output at full load
operating from 24V.
20V/DIV
20V/DIV
20V/DIV
5A/DIV
5A/DIV
5A/DIV
V
V
V
I
I
I
IN
IN
IN
IN
IN
IN
DANGER!
RINGING V
ABSOLUTE MAXIMUM
RATING OF THE LT3686A
20μs/DIV
20μs/DIV
20μs/DIV
IN
MAY EXCEED
3686A F15
LT3686A
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