11
FN6921.0
June 18, 2009
Current Limiting
These devices have no internal current-limiting circuitry. If
the output is shorted, it is possible to exceed the Absolute
Maximum Rating for output current or power dissipation,
potentially resulting in the destruction of the device.
Results of Overdriving the Output
Caution should be used when overdriving the output for long
periods of time. Overdriving the output can occur in two ways.
1. The input voltage times the gain of the amplifier exceeds the
supply voltage by a large value or,
2. The output current required is higher than the output stage
can deliver.
These conditions can result in a shift in the Input Offset Voltage
(VOS) (as much as 1V/hr. of exposure under these
conditions).
IN+ and IN- Input Protection
All input terminals have internal ESD protection diodes to both
positive and negative supply rails, limiting the input voltage to
within one diode beyond the supply rails. They also contain
back-to-back diodes across the input terminals (see
“Pinthe input differential voltage is expected to exceed 0.5V, an
external series resistor must be used to ensure the input
currents never exceed 5mA (Figure
36).
Limitations of the Differential Input Protection
If the input differential voltage is expected to exceed 0.5V, an
external current limiting resistor must be used to ensure the
input current never exceeds 5mA. For non-inverting unity gain
applications, the current limiting can be via a series IN+ resistor,
or via a feedback resistor of appropriate value. For other gain
configurations, the series IN+ resistor is the best choice, unless
the feedback (RF) and gain setting (RG) resistors are both
sufficiently large to limit the input current to 5mA.
Large differential input voltages can arise from several
sources:
1. During open loop (comparator) operation. Used this way,
the IN+ and IN- voltages don’t track, so differentials arise.
2. When the amplifier is disabled but an input signal is still
present. An RL or RG to GND keeps the IN- at GND, while
the varying IN+ signal creates a differential voltage. Mux
Amp applications are similar, except that the active
channel VOUT determines the voltage on the IN- terminal.
3. When the slew rate of the input pulse is considerably
faster than the op amp’s slew rate. If the VOUT can’t keep
up with the IN+ signal, a differential voltage results, and
visible distortion occurs on the input and output signals.
To avoid this issue, keep the input slew rate below
1.9V/s, or use appropriate current limiting resistors.
Large (>2V) differential input voltages can also cause an
increase in disabled ICC.
Using Only One Channel
If the application only requires one channel, the user must
configure the unused channel to prevent it from oscillating.
The unused channel will oscillate if the input and output pins
are floating. This will result in higher than expected supply
currents and possible noise injection into the channel being
used. The proper way to prevent this oscillation is to short
the output to the negative input and ground the positive input
Power Dissipation
It is possible to exceed the +125°C maximum junction
temperatures under certain load and power supply
conditions. It is therefore important to calculate the
maximum junction temperature (TJMAX) for all applications
to determine if power supply voltages, load conditions, or
package type need to be modified to remain in the safe
operating area. These parameters are related in Equation
1:where:
PDMAXTOTAL is the sum of the maximum power
dissipation of each amplifier in the package (PDMAX)
PDMAX for each amplifier can be calculated using
where:
TMAX = Maximum ambient temperature
θ
JA = Thermal resistance of the package
PDMAX = Maximum power dissipation of 1 amplifier
VS = Total supply voltage
IMAX = Maximum supply current of 1 amplifier
VOUTMAX = Maximum output voltage swing of the
application
RL = Load resistance
FIGURE 36. LOCAL POWER SUPPLY DECOUPLING AND
INPUT CURRENT LIMITING
-
+
RIN
RL
VIN
VOUT
V+
V-
0.01F
DECOUPLING
CAPACITORS
FIGURE 37. PREVENTING OSCILLATIONS IN UNUSED
CHANNELS
-
+
T
JMAX
T
MAX
θ
JAxPDMAXTOTAL
()
+
=
(EQ. 1)
PD
MAX
V
S
I
SMAX
V
S
(
- V
OUTMAX )
V
OUTMAX
R
L
----------------------------
×
+
×
=
(EQ. 2)
ISL28236