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:
T
J
= P
DISS
× θ
JA
+ T
A
where:
T
J
is the AD8614/AD8644 junction temperature.
P
DISS
is the AD8614/AD8644 power dissipation.
θ
JA
is the AD8614/AD8644 junction-to-ambient package thermal
resistance.
T
A
is the ambient temperature of the circuit.
The power dissipated by the device can be calculated as:
P
DISS
= I
LOAD
× (V
S
– V
OUT
)
where:
I
LOAD
is the AD8614/AD8644 output load current.
V
S
is the AD8614/AD8644 supply voltage.
V
OUT
is the AD8614/AD8644 output voltage.
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
is a simple matter to see if P
DISS
exceeds the device’s power derating
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
MAXIMUM POWER DISSIPATION (W)
AMBIENT TEMPERATURE (°C)
–35–155 2545658
5
14-LEAD SOIC PACKAGE
θ
JA
= 120°C/W
14-LEAD TSSOP PACKAGE
θ
JA
= 180°C/W
5-LEAD SOT-23 PACKAGE
θ
JA
= 230°C/W
06485-027
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.
