LT3686



3686fa

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

ripplefilteringandcanslightlyimprovetheefficiencyofthe

circuit, though it is likely to be the largest component in the

circuit.AnalternativesolutionisshowninFigure15c.A1Ω

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

20V/DIV

5A/DIV

20µs/DIV

CLOSING SWITCH

SIMULATES HOT PLUG

(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

2.2µF

0.1µF

1

3686 F15

20V/DIV

5A/DIV

20µs/DIV

20V/DIV

5A/DIV

20µs/DIV

DANGER!

ABSOLUTE MAXIMUM

RATING OF THE LT3686