11
FN7365.6
September 14, 2010
supplies, the EL5166 and EL5167 have an input range
which spans ±3.2V. The output range of the EL5166 and
EL5167 is also quite large, extending to within 1V of the
supply rail. On a ±5V supply, the output is therefore capable
of swinging from -4V to +4V.
Video Performance
For good video performance, an amplifier is required to
maintain the same output impedance and the same
frequency response as DC levels are changed at the
output. This is especially difficult when driving a standard
video load of 150
Ω because of the change in output current
with DC level. Previously, good differential gain could only
be achieved by running high idle currents through the
output transistors (to reduce variations in output
impedance.) These currents were typically comparable to
the entire 8.5mA supply current of each EL5166 and
EL5167 amplifier. Special circuitry has been incorporated in
the EL5166 and EL5167 to reduce the variation of output
impedance with the current output. This results in dG and
dP specifications of 0.01% and 0.03°, while driving 150
Ω at
a gain of 2.
Output Drive Capability
In spite of their low 8.5mA of supply current, the EL5166 and
EL5167 are capable of providing a minimum of ±110mA of
output current. With so much output drive, the EL5166 and
EL5167 are capable of driving 50
Ω loads to both rails,
making them an excellent choice for driving isolation
transformers in telecommunications applications.
Driving Cables and Capacitive Loads
When used as a cable driver, double termination is always
recommended for reflection-free performance. For those
applications, the back-termination series resistor will
decouple the EL5166 and EL5167 from the cable and allow
extensive capacitive drive. However, other applications may
have high capacitive loads without a back-termination
resistor. In these applications, a small series resistor (usually
between 5
Ω and 50Ω) can be placed in series with the
output to eliminate most peaking. The gain resistor (RG) can
then be chosen to make up for any gain loss which may be
created by this additional resistor at the output. In many
cases it is also possible to simply increase the value of the
feedback resistor (RF) to reduce the peaking.
Current Limiting
The EL5166 and EL5167 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.
Power Dissipation
With the high output drive capability of the EL5166 and
EL5167, it is possible to exceed the +125°C Absolute
Maximum junction temperature under certain very high load
current conditions. Generally speaking, when RL falls below
about 25
Ω, it is important to calculate the maximum junction
temperature (TJMAX) for the application to determine if
power supply voltages, load conditions, or package type
need to be modified for the EL5166 and EL5167 to remain in
the safe operating area. These parameters are calculated as
follows:
where:
TMAX = Maximum ambient temperature
θJA = Thermal resistance of the package
n = Number of amplifiers in the package
PDMAX = Maximum power dissipation of each amplifier in
the package
PDMAX for each amplifier can be calculated as follows:
where:
VS = Supply voltage
ISMAX = Maximum supply current of 1A
VOUTMAX = Maximum output voltage (required)
RL = Load resistance
TJMAX
TMAX θJA nPDMAX
×
()
+
=
(EQ. 1)
PDMAX
2
(
VS ISMAX)
VS
(
VOUTMAX)
VOUTMAX
RL
----------------------------
×
–
+
×
=
(EQ. 2)
EL5166, EL5167