Datasheet

LT3686A
21
3686afa
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 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 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
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 one 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 LT3686A Is Connected to a Live Supply
+
+
LT3686A
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
+
+
LT3686A
2.2µF
10µF
35V
AI.EI.
LT3686A
2.2µF0.1µF
1Ω
3686A 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 LT3686A