AD8614/AD8644

–6–

REV. 0

AD86x4

261

18V

Figure 23. Output Short-Circuit Protection

Input Overvoltage Protection

As with any semiconductor device, whenever the condition exists for

the input to exceed either supply voltage, attention needs to be paid

to the input overvoltage characteristic. As an overvoltage occurs, the

amplifier could be damaged, depending on the voltage level and the

magnitude of the fault current. When the input voltage exceeds

either supply by more than 0.6 V, internal pin junctions energize,

allowing current to flow from the input to the supplies. Observing

Figure 22, the AD8614/AD8644 has 1.5 k

Ω resistors in series with

each input, which helps limit the current. This input current is not

inherently damaging to the device as long as it is limited to 5 mA or

less. If the voltage is large enough to cause more than 5 mA of cur-

rent to flow, an external series resistor should be added. The size of

this resistor is calculated by dividing the maximum overvoltage by

5 mA and subtracting the internal 1.5 k

Ω resistor. For example, if

the input voltage could reach 100 V, the external resistor should be

(100 V/5 mA) – 1.5 k

Ω = 18.5 kΩ. This resistance should be placed

in series with either or both inputs if they are subjected to the over-

voltages. For more information on general overvoltage characteristics

of amplifiers refer to the 1993 System Applications Guide, available

from the Analog Devices Literature Center.

Output Phase Reversal

The AD8614/AD8644 is immune to phase reversal as long as the

input voltage is limited to within the supply rails. Although the

device’s output will not change phase, large currents due to

input overvoltage could result, damaging the device. In applica-

tions where the possibility of an input voltage exceeding the

supply voltage exists, overvoltage protection should be used, as

described in the previous section.

Power Dissipation

The maximum power that can be safely dissipated by the

AD8614/AD8644 is limited by the associated rise in junction

temperature. The maximum safe junction temperature is 150

°C,

and should not be exceeded or device performance could suffer.

If this maximum is momentarily exceeded, proper circuit opera-

tion will be restored as soon as the die temperature is reduced.

Leaving the device in an “overheated” condition for an extended

period can result in permanent damage to the device.

To calculate the internal junction temperature of the AD86x4,

the following formula can be used:

× θ

θ

ambient; and

The power dissipated by the device can be calculated as:

× (V

Figure 24 provides a convenient way to see if the device is being

overheated. The maximum safe power dissipation can be found

graphically, based on the package type and the ambient tem-

perature around the package. By using the previous equation, it

derating curve. To ensure proper operation, it is important to

observe the recommended derating curves shown in Figure 24.

AMBIENT TEMPERATURE – C

1.5

0

–35

–15

5

25

45

65

85

1.0

0.5

14-LEAD SOIC PACKAGE

14-LEAD TSSOP PACKAGE

5-LEAD SOT-23 PACKAGE

Figure 24. Maximum Power Dissipation vs. Temperature

for 5-Lead and 14-Lead Package Types

Unused Amplifiers

It is recommended that any unused amplifiers in the quad pack-

age be configured as a unity gain follower with a 1 k

Ω feedback

resistor connected from the inverting input to the output, and

the noninverting input tied to the ground plane.

Capacitive Load Drive

The AD8614/AD8644 exhibits excellent capacitive load driving

capabilities. Although the device is stable with large capacitive

loads, there is a decrease in amplifier bandwidth as the capacitive

load increases.

When driving heavy capacitive loads directly from the AD8614/

AD8644 output, a snubber network can be used to improve the

transient response. This network consists of a series R-C connected

from the amplifier’s output to ground, placing it in parallel with the

capacitive load. The configuration is shown in Figure 25. Although

this network will not increase the bandwidth of the amplifier, it will

significantly reduce the amount of overshoot.

AD86x4

5V

Figure 25. Snubber Network Compensation for Capacitive

Loads