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LT3470A

3470afa

APPLICATIONS INFORMATION

Hot-Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LT3470A. However, these capacitors

can cause problems if the LT3470A 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 volt-

age at the VIN pin of the LT3470A can ring to twice the

nominal input voltage, possibly exceeding the LT3470A’s

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

controlled or the user will be plugging the LT3470A into an

energized supply, the input network should be designed

to prevent this overshoot. Figure 6 shows the waveforms

that result when an LT3470A 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 6b 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 6c. 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.

High Temperature Considerations

The die junction temperature of the LT3470A must be

lower than the maximum rating of 125°C. This is generally

not a concern unless the ambient temperature is above

85°C. For higher temperatures, care should be taken in

the layout of the circuit to ensure good heat sinking of the

LT3470A. The maximum load current should be derated

as the ambient temperature approaches the maximum

junction rating. The die temperature is calculated by

multiplying the LT3470A power dissipation by the thermal

resistance from junction to ambient. Power dissipation

within the LT3470A can be estimated by calculating the

total power loss from an efﬁciency measurement. Thermal

resistance depends on the layout of the circuit board and

choice of package. The DFN package with the exposed

pad has a thermal resistance of approximately 80°C/W.

Finally, be aware that at high ambient temperatures the

internal Schottky diode will have signiﬁcant leakage current

(see Typical Performance Characteristics) increasing the

quiescent current of the LT3470A converter.

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