ISL1550 Datasheet, PDF (11/13 Pages) Intersil Corporation – Single Port, VDSL2 Differential Line Driver

ISL1550
Applications Information
Product Description
The ISL1550 is a dual operational amplifier designed for line driving
in DMT VDSL2 8MHz, 12MHz, 17MHz and 30MHz bandplans
solutions. It is a current mode feedback amplifier with low distortion
drawing moderately low supply current. Due to the current feedback
architecture, the ISL1550 closed-loop 3dB bandwidth is dependent
on the value of the feedback resistor. First, the desired bandwidth is
selected by choosing the feedback resistor, R_F, and then the gain is
set by picking the gain resistor, R_G (Figure 3).
VDSL CO Applications
The ISL1550 is designed as a VDSL line driver for CO. At an
output current of ±450mA, the typical supply voltage headroom
is 1.5V on each side of the differential output.
The average line power requirement for the VDSL CO application
is 20dBm (100mW) into a 100Ω line. The average line voltage is
3.16V_RMS. The VDSL DMT peak-to-average ratio (crest factor) of
5.3 implies peak voltage of 16.8V_P into the line. Using a
differential drive configuration and transformer coupling with
standard back termination, a transformer ratio of 1:2.5 is
selected. The active termination technique provides better power
efficiency by reducing the backmatch resistor by a factor of
K = 5. Positive feedback resistors, RP, can be sized to make the
effective backmatch impedance larger. The circuit configuration
is shown in Figure 35.
+
12.5/k
-
750Ω
RP
TX1
AFE
1.5kΩ
750Ω
-
+
RPΩ
12.5/k
100
1:2.5
R_P = R_F(K/(K-1))
FIGURE 35. CIRCUIT CONFIGURATION
Power Supply Bypassing and Printed Circuit
Board Layout
As with any high frequency device, good printed circuit board
layout is necessary for optimum performance. Ground plane
construction is highly recommended. Lead lengths should be as
short as possible (below 0.25”). The power supply pins must be
well bypassed to reduce the risk of oscillation. A 4.7µF tantalum
capacitor in parallel with a 0.1µF ceramic capacitor is adequate
for each supply pin. During power-up, it is necessary to limit the
slew rate of the rising power supply to less than 1V/µs. If the
power supply rising time is undetermined, a series 10Ω resistor
on the power supply line before the decoupling caps can be used
to ensure the proper power supply rise time.
For good AC performance, parasitic capacitances should be kept
to a minimum, especially at the inverting input. This implies
keeping the ground plane away from this pin. Carbon or metal
film resistors are acceptable, while use of wire-wound resistors
should be avoided because of their parasitic inductance.
Similarly, capacitors should be low inductance for best
performance.
Capacitance at the Inverting Input
Due to the topology of the current feedback amplifier, stray
capacitance at the inverting input will affect the AC and transient
performance of the ISL1550 when operating in the non-inverting
configuration.
Feedback Resistor Values
The ISL1550 has been designed and specified with R_F = 1.5kΩ
for A_V = +5 (Figure 3). As is the case with all current feedback
amplifiers, wider bandwidth at the expense of slight peaking, can
be obtained by reducing the value of the feedback resistor.
Inversely, larger values of the feedback resistor will cause rolloff
to occur at a lower frequency.
Quiescent Current vs Temperature
The ISL1550 was designed to slightly increase quiescent current
with temperature to maintain good distortion performance at
high temperatures. Refer to “Typical Performance Curves”
beginning on page 5.
Supply Voltage Range
The ISL1550 has been designed to operate with supply voltages
from ±4.0V to ±6.6V nominal. Optimum bandwidth, slew rate,
and video characteristics are obtained at higher supply voltages.
Single Supply Operation
If a single supply is desired, values from +8.0V to +13.2V
nominal can be used as long as the input common mode range
is not exceeded. When using a single supply, be sure to either,
1. DC bias the inputs at an appropriate common mode voltage
and AC-couple the signal, or
2. Ensure the driving signal is within the common mode range of
the ISL1550.
11
FN6795.0
March 16, 2012