ISL1539A_14 Datasheet, PDF (16/23 Pages) Intersil Corporation – Dual Port VDSL2 Line Driver

ISL1539A
Applications Information
Applying Wideband Current Feedback Op
Amps as Differential Drivers
A current feedback amplifier (CFA) like the ISL1539A is
particularly suited to the requirements of high output
power, high full power bandwidth, differential drivers.
This topology offers a very high slew rate on low
quiescent power and the ability to hold relatively
constant AC characteristics over a wide range of gains.
The AC characteristics are principally set by the feedback
resistor value in simple differential gain circuits as shown
in Figure 44.
+12V
Rb ^{+ ¼}
Ro
ISL1539A
- _Rf
3.2k
1:n
VI
SOURCE
Rg 711
VO
LOAD
Rf
- ^3.2k
¼
ISL1539A
Ro
+
Rb
AV-DIFF = VO/VI = 10V/V
-12V
TYPICAL DIFFERENTIAL I/O LINE DRIVER
(1 OF 2 PORTS)
FIGURE 44. PASSIVE TERMINATION CIRCUIT
In this differential gain of 10 V/V circuit, the 3.2k
feedback resistors are setting the bandwidth while the
711 gain resistor controls the gain. The Vo/Vi gain for
this circuit is set by Equation 1:
V_{o = 1+ 2} ^R_{f = 1+ 2} 3.2kΩ _{= 10}
V_i
R_g
711Ω
(EQ. 1)
The effect of increasing or decreasing the feedback
resistor value is shown in Figures 19, 24 and 29 (at the 3
power settings). Increasing R_F will tend to roll off the
response while decreasing it will peak the frequency
response up extending the bandwidth. R_G was adjusted
in each of these plots to hold a constant gain of 10 (or
20dB). This shows the flexibility offered by the CFA
topology - the frequency response can be controlled with
the value of the feedback resistor with the R_G resistor
then setting the desired gain.
The ISL1539A provides 4 very power efficient, high
output current, CFA's. These are intended to be
connected as two pairs of differential drivers. The pinout
diagrams of page two show that Channels A and B are
intended to operate as a pair while Channels C and D
comprise the other pair. Power control is also provided
through two pairs of control pins which separately set the
power for Channels A and B together and then the other
pair controls Channels C and D together.
Very low output distortion at low power can be provided
by the differential configuration. The high slew rate
intrinsic to the CFA topology also contributes to the
exceptional performance shown in Figures 22, 27 and 32.
These swept frequency distortion plots show extremely
low distortion at 200kHz holding to very low levels up
through 20MHz. At the lowest operating power
(Figure 32, which is at low power, or 6.75mA per
amplifier or 13.5mA/port) we still see < -70dBc through
5MHz for a 5V_P-P differential output swing.
Advanced Configurations - Active
Termination
Where the best power efficiency is required in a full
duplex DSL line interface application, it is common to
apply the circuit shown below to reduce the power loss in
the matching element while retaining a higher
impedance for the upstream signal coming into this
output stage. This circuit acts to provide a higher
apparent output impedance (through its cross-coupled
positive feedback through the Rp resistors) while
physically taking a smaller IR drop through the Rm
resistors for the output signal..
1 PORT OF 2
DRAWN
+
+12V
I = 13.5mA
50Ω
¼
ISL1539A _0V
C0
-
3.3V
C1
Vdiff
Vi ^POWER
SPLITTER
Rg
Rf
Rp
Rp
Rf
Rm
RL 82.6Ω Vo
Rm
RL = 100Ω/(1.1)² = 82.6
-
Vo/Vdiff = 9.77 V/V (19.8dB)
¼
ISL1539A
+
50Ω
Radj
0Ω
-12V
FIGURE 45. ACTIVE TERMINATION TEST CIRCUIT
This circuit is showing one of two ports configured in an
active termination circuit used for some of the
specification and characterization tests. This is showing
the device operating in the low power mode, but data
has been shown at the other power settings as well.
The 82.6Ω differential load is intended to emulate a 100Ω
line load reflected through a 1:1.1 turns ratio
transformer (100Ω/(1.1²) = 82.6Ω load). The gain and
output impedance for this circuit can be described by the
following equations.
The ideal transfer function is set by the open circuit gain
(RL = infinite) and an equivalent output impedance Z_O.
16
FN6916.0
September 23, 2009