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LT3684

3684f

Hot Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LT3684 circuits. However, these capaci-

tors can cause problems if the LT3684 is plugged into a

live supply (see Linear Technology Application Note 88 for

a complete discussion). The low loss ceramic capacitor,

combined with stray inductance in series with the power

source, forms an under damped tank circuit, and the

voltage at the VIN pin of the LT3684 can ring to twice the

nominal input voltage, possibly exceeding the LT3684’s

rating and damaging the part. If the input supply is poorly

controlled or the user will be plugging the LT3684 into an

energized supply, the input network should be designed

to prevent this overshoot. Figure 9 shows the waveforms

that result when an LT3684 circuit is connected to a 24V

supply through six feet of 24-gauge twisted pair. The

ﬁrst plot is the response with a 4.7µF ceramic capacitor

at the input. The input voltage rings as high as 50V and

the input current peaks at 26A. A good solution is shown

in Figure 9b. A 0.7

Ω resistor is added in series with the

APPLICATIONS INFORMATION

Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation

VIAS TO LOCAL GROUND PLANE

VIAS TO VOUT

VIAS TO RUN/SS

VIAS TO PG

VIAS TO VIN

OUTLINE OF LOCAL

GROUND PLANE

3684 F08

RRT

RPG

VOUT

GND

input to eliminate the voltage overshoot (it also reduces

the peak input current). A 0.1µF capacitor improves high

frequency ﬁltering. For high input voltages its impact on

efﬁciency is minor, reducing efﬁciency by 1.5 percent for

a 5V output at full load operating from 24V.

High Temperature Considerations

The PCB must provide heat sinking to keep the LT3684

cool. The Exposed Pad on the bottom of the package must

be soldered to a ground plane. This ground should be tied

to large copper layers below with thermal vias; these lay-

ers will spread the heat dissipated by the LT3684. Place

additional vias can reduce thermal resistance further. With

these steps, the thermal resistance from die (or junction)

to ambient can be reduced to

θJA = 35°C/W or less. With

100 LFPM airﬂow, this resistance can fall by another 25%.

Further increases in airﬂow will lead to lower thermal re-

sistance. Because of the large output current capability of

the LT3684, it is possible to dissipate enough heat to raise

the junction temperature beyond the absolute maximum of

125°C. When operating at high ambient temperatures, the

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