Datasheet

LT3686
21
3686fc
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
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.
Figure 15. A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation
When the LT3686 Is Connected to a Live Supply
+
+
LT3686
2.2µF
V
IN
20V/DIV
I
IN
5A/DIV
20µs/DIV
V
IN
CLOSING SWITCH
SIMULATES HOT PLUG
I
IN
(15a)
(15b)
(15c)
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
+
+
LT3686
2.2µF
10µF
35V
AI.EI.
LT3686
2.2µF0.1µF
1Ω
3686 F15
V
IN
20V/DIV
I
IN
5A/DIV
20µs/DIV
V
IN
20V/DIV
I
IN
5A/DIV
20µs/DIV
DANGER!
RINGING V
IN
MAY EXCEED
ABSOLUTE MAXIMUM
RATING OF THE LT3686