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ISL1557 Datasheet(Arkusz danych) 8 Page - Renesas Technology Corp

Numer części ISL1557
Szczegółowy opis  xDSL Differential Line Driver
Pobierz  12 Pages
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Producent  RENESAS [Renesas Technology Corp]
Strona internetowa  http://www.renesas.com
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ISL1557
FN7522 Rev 3.00
Page 8 of 12
Apr 6, 2018
Applications Information
Product Description
The ISL1557 is a dual operational amplifier designed for line
driving in DMT ADSL2+ and VDSL solutions. It is a dual current
mode feedback amplifier with low distortion while drawing
moderately low supply current. It is built using the Renesas
proprietary complimentary bipolar process and is offered in
industry standard pinouts. Due to the current feedback
architecture, the ISL1557 closed-loop 3dB bandwidth is
dependent on the value of the feedback resistor. First, select
the desired bandwidth by choosing the feedback resistor, RF,
then set the gain by choosing the gain resistor, RG. The curves
at the beginning of the “Typical Performance Curves” section,
on page 4, show the effect of varying both RF and RG. The 3dB
bandwidth is somewhat dependent on the power supply
voltage.
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
within 1V/µs. If the power supply rising time is undetermined,
a series 10Ω resistor on the power supply line 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
resistors are acceptable, but avoid using wire-wound resistors
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 effect the AC and
transient performance of the ISL1557 when operating in the
non-inverting configuration.
In the inverting gain mode, added capacitance at the inverting
input has little effect because this point is at a virtual ground
and stray capacitance is therefore not “detected” by the
amplifier.
Feedback Resistor Values
The ISL1557 has been designed and specified with RF = 750Ω
for AV = +5. This value of feedback resistor yields extremely
flat frequency response with 1dB peaking out to 250MHz. As
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
feedback resistor will cause rolloff to occur at a lower
frequency. See the curves in the “Typical Performance Curves”
section, beginning on page 4, which shows 3dB bandwidth and
peaking vs frequency for various feedback resistors and
various supply voltages.
Bandwidth vs Temperature
Whereas many amplifier's supply current and consequently
3dB bandwidth drop off at high temperature, the ISL1557 is
designed to have little supply current variations with
temperature. An immediate benefit is the 3dB bandwidth does
not drop off drastically with temperature.
Supply Voltage Range
The ISL1557IRZ is designed to operate with supply voltages
from ±2.25V to ±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 +4.5V to +12V
nominal can be used as long as the input common mode
range is not exceeded. When using a single supply, be sure to
either:
• DC bias the inputs at an appropriate common mode voltage
and AC couple the signal, or:
• Ensure the driving signal is within the common mode range
of the ISL1557. The ISL1557IUEZ must be used in single
supply applications.
ADSL CPE Applications
The ISL1557 is designed as a line driver for ADSL CPE
modems. It is capable of outputting 450mA of output current
with a typical supply voltage headroom of 1.3V. It can achieve
-85dBc of distortion at low 7.1mA of supply current per
amplifier.
The average line power requirement for the ADSL CPE
application is 14.5dBm (28mW) into a 100Ω
 line. The
average line voltage is 1.67VRMS. The ADSL DMT peak to
average ratio (crest factor) of 5.3 implies peak voltage of 7.5V
into the line. Using a differential drive configuration and
transformer coupling with standard back termination, a
transformer ratio of 1:2 is selected. The circuit configuration is
shown in Figure 24.
-
+
-
+
TX1
1:2
12.5
12.5
750
750
250Ω
AFE
100
FIGURE 24. CIRCUIT CONFIGURATION




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