AD8614/AD8644

Rev. B | Page 10 of 16

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 can 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 26, the

AD8614/AD8644 have 1.5 kΩ resistors in series with each

input, which helps to 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 current 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 kΩ resistor. For example, if the input voltage could reach 100 V,

the external resistor should be (100 V ÷ 5 mA) – 1.5 kΩ = 18.5 kΩ.

This resistance should be placed in series with either or both

inputs if they are subjected to the overvoltages.

OUTPUT PHASE REVERSAL

The AD8614/AD8644 are immune to phase reversal as long as

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

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

input overvoltage can 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

operation is 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

AD8614/AD8644, the following formula can be used:

where:

resistance.

The power dissipated by the device can be calculated as:

where:

Figure 27 provides a convenient way to determine if the device

is being overheated. The maximum safe power dissipation can

be found graphically, based on the package type and the ambient

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

curve. To ensure proper operation, it is important to observe the

recommended derating curves shown in Figure 27.

1.5

0

1.0

0.5

AMBIENT TEMPERATURE (°C)

–35

–15

5

254565

85

14-LEAD SOIC PACKAGE

14-LEAD TSSOP PACKAGE

5-LEAD SOT-23 PACKAGE

Figure 27. Maximum Power Dissipation vs. Temperature

(5-Lead and 14-Lead Package Types)

UNUSED AMPLIFIERS

It is recommended that any unused amplifiers in the quad

package be configured as a unity-gain follower with a 1 kΩ

feedback resistor connected from the inverting input to the

output, and the noninverting input tied to the ground plane.