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LT3685E

36V, 2A, 2.4MHz Step-Down Switching Regulator

Linear Technology 

LT3685

APPLICATIONS INFORMATION

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 LT3685 can ring to twice the

nominal input voltage, possibly exceeding the LT3685’s

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

controlled or the user will be plugging the LT3685 into an

energized supply, the input network should be designed

to prevent this overshoot. Figure 9 shows the waveforms

that result when an LT3685 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 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. 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 LT3685

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 LT3685. Place

additional vias can reduce thermal resistance further. With

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

to ambient can be reduced to JA = 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 LT3685, 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

maximum load current should be derated as the ambient

temperature approaches 125°C.

Power dissipation within the LT3685 can be estimated by

calculating the total power loss from an efficiency measure-

ment and subtracting the catch diode loss and inductor

loss. The die temperature is calculated by multiplying the

LT3685 power dissipation by the thermal resistance from

junction to ambient.

Other Linear Technology Publications

Application Notes 19, 35 and 44 contain more detailed de-

scriptions and design information for buck regulators and

other switching regulators. The LT1376 data sheet has a more

extensive discussion of output ripple, loop compensation and

stability testing. Design Note 100 shows how to generate a

bipolar output supply using a buck regulator.

3685fb

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