EL5171 INTERSIL [Intersil Corporation], EL5171 Datasheet - Page 11

• Manufacturer Part Number: EL5171
• Description: 250MHz Differential Twisted-Pair Drivers
• Manufacturer: INTERSIL [Intersil Corporation]

Choice of Feedback Resistor and Gain Bandwidth
Product
For applications that require a gain of +1, no feedback resistor
is required. Just short the OUT+ pin to the FBP pin and the
OUT- pin to the FBN pin. For gains greater than +1, the
feedback resistor forms a pole with the parasitic capacitance
at the inverting input. As this pole becomes smaller, the
amplifier's phase margin is reduced. This causes ringing in
the time domain and peaking in the frequency domain.
Therefore, R
exceeded for optimum performance. If a large value of R
must be used, a small capacitor in the few Pico farad range in
parallel with R
the expense of reducing the bandwidth.
The bandwidth of the EL5171 and EL5371 depends on the
load and the feedback network. R
with the load for gains other than +1. As this combination gets
smaller, the bandwidth falls off. Consequently, R
minimum value that should not be exceeded for optimum
bandwidth performance. For gain of +1, R
For the gains other than +1, optimum response is obtained
with R
The EL5171 and EL5371 have a gain bandwidth product of
100MHz for R
predicted by Equation 3:
Gain
Driving Capacitive Loads and Cables
The EL5171 and EL5371 can drive 50pF differential
capacitor in parallel with 1kΩ differential load with less than
5dB of peaking at gain of +1. If less peaking is desired in
applications, a small series resistor (usually between 5Ω to
50Ω) can be placed in series with each output to eliminate
most peaking. However, this will reduce the gain slightly. If
the gain setting is greater than 1, the gain resistor R
then be chosen to make up for any gain loss, which may be
created by the additional series resistor at the output.
When used as a cable driver, double termination is always
recommended for reflection-free performance. For those
applications, a back-termination series resistor at the
amplifier's output will isolate the amplifier from the cable and
allow extensive capacitive drive. However, other applications
×
F
BW
between 500Ω to 1kΩ.
V
V
V
REF
IN
IN
=
+
-
F
100MHz
F
LD
has some maximum value that should not be
R
can help to reduce the ringing and peaking at
G
= 1kΩ. For gains ≥5, their bandwidth can be
FBP
IN+
IN-
REF
FBN
FIGURE 26.
R
R
11
F1
F2
F
and R
G
F
= 0 is optimum.
appear in parallel
V
V
F
O
O
+
-
also has a
G
(EQ. 3)
EL5171, EL5371
can
F
may have high capacitive loads without a back-termination
resistor. Again, a small series resistor at the output can help
to reduce peaking.
Disable/Power-Down (for EL5371 only)
The EL5371 can be disabled and its outputs placed in a high
impedance state. The turn-off time is about 0.95µs and the
turn-on time is about 215ns. When disabled, the amplifier's
supply current is reduced to 1.7µA for I
typically, thereby effectively eliminating the power
consumption. The amplifier's power-down can be controlled
by standard CMOS signal levels at the ENABLE pin. The
applied logic signal is relative to the V
pin float or applying a signal that is less than 1.5V below V
will enable the amplifier. The amplifier will be disabled when
the signal at the EN pin is above V
Output Drive Capability
The EL5171 and EL5371 have internal short circuit protection.
Its typical short circuit current is ±90mA for EL5171 and
±70mA for EL5371. If the output is shorted indefinitely, the
power dissipation could easily increase such that the part will
be destroyed. Maximum reliability is maintained if the output
current never exceeds ±60mA. This limit is set by the design
of the internal metal interconnections.
Power Dissipation
With the high output drive capability of the EL5171 and
EL5371, it is possible to exceed the +135°C absolute
maximum junction temperature under certain load current
conditions. Therefore, it is important to calculate the maximum
junction temperature for the application to determine if the
load conditions or package types need to be modified for the
amplifier to remain in the safe operating area.
The maximum power dissipation allowed in a package is
determined according to Equation 4:
Where:
• T
• T
• θ
The maximum power dissipation actually produced by an IC
is the total quiescent supply current times the total power
supply voltage, plus the power in the IC due to the load, or
as represented in Equation 5:
PD
Where:
• V
• I
PD
SMAX
MAX
JA
AMAX
JMAX
S
=
= Total supply voltage
i
= Thermal resistance of the package
×
=
⎛
⎜
⎝
= Maximum quiescent supply current per channel
V
= Maximum junction temperature
= Maximum ambient temperature
T
-------------------------------------------- -
S
JMAX
×
I
SMAX
Θ
–
JA
T
AMAX
+
V
S
×
ΔV
----------- -
R
LD
O
⎞
⎟
⎠
S
+ - 0.5V.
S
S
+ pin. Letting the EN
+ and 120µA for I
August 10, 2010
(EQ. 4)
(EQ. 5)
FN7307.7
S
S
+
-