LT3686 LINER [Linear Technology], LT3686 Datasheet - Page 21

• Manufacturer Part Number: LT3686
• Manufacturer: LINER [Linear Technology]

ApplicAtions inForMAtion
supply, the input network should be designed to prevent
this overshoot. Figure 15 shows the waveforms that re-
sult when an LT3686 circuit is connected to a 24V supply
through six feet of 24-gauge twisted pair. The first plot is
the response with a 2.2μF ceramic capacitor at the input.
The input voltage rings as high as 35V and the input cur-
rent 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
Figure 15. A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation
When the LT3686 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
1
V
IN
2.2µF
2.2µF
2.2µF
LT3686
LT3686
LT3686
(15a)
(15b)
(15c)
overshoot. The extra capacitor improves low frequency
ripple filtering and can slightly improve the efficiency 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 filtering. This
solution is smaller and less expensive than the electrolytic
capacitor. For high input voltages its impact on efficiency
is minor, reducing efficiency less than one half 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 LT3686
20µs/DIV
20µs/DIV
20µs/DIV
IN
MAY EXCEED
3686 F15
LT3686

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