ISL1532IRZ Datasheet, PDF (13/19 Pages) Intersil Corporation – Dual Channel Differential DSL Line Driver

ISL1532
Applications Information
The ISL1532 consists of two sets of high-power line driver
amplifiers that can be connected for full duplex differential
line transmission. The amplifiers are designed to be used
with signals up to 10MHz and produce low distortion levels.
The ISL1532 has been optimized as a line driver for ADSL2+
CO application. The driver output stage has been sized to
provide full ADSL2+ CO power level of 20dBM onto the
telephone lines. Realizing that the actual peak output
voltages and currents vary with the line transformer turns
ratio, the ISL1532 is designed to support 450mA of output
current which exceeds the level required for 1:2 transformer
ratio. A typical ADSL2+ interface circuit is shown in
Figure 45. Each amplifier has identical positive gain
connections, and optimum common-mode rejection occurs.
Further, DC input errors are duplicated and create
common-mode rather than differential line errors.
DRIVER
INPUT +
+
-
R_F
2R_G
R_F
DRIVER
INPUT -
-
+
R_F
R
RECEIVE
R_IN
-
OUT +
+
RECEIVE
AMPLIFIERS ₊
R
RECEIVE
-
R_F
R_IN
OUT -
R_OUT
LINE +
R_LINE
R_OUT
LINE -
FIGURE 45. TYPICAL LINE INTERFACE CONNECTION
Input Connections
The ISL1532 amplifiers are somewhat sensitive to source
impedance. In particular, they do not like being driven by
inductive sources. More than 100nH of source impedance
can cause ringing or even oscillations. This inductance is
equivalent to about 4” of unshielded wiring, or 6” of
unterminated transmission line. Normal high-frequency
construction obviates any such problem.
Power Control Function
The ISL1532 contains two forms of power control operation.
Two digital inputs, C₀ and C₁, can be used to control the
supply current of the ISL1532 drive amplifiers. C₀ and C₁
inputs are designed to pull high initially. Floating these inputs
will put the device in disable mode.
As the supply current is reduced, the ISL1532 will start to
exhibit slightly higher levels of distortion and the frequency
response will be limited. The four power modes of the
ISL1532 are set up as shown in Table 1.
TABLE 1. POWER MODES OF THE ISL1532
C₁
C₀
OPERATION
0
0
I_S Full Power Mode
0
1
3/4 I_S Power Mode
1
0
1/2 I_S Power Mode
1
1
Power-down
Another method for controlling the power consumption of the
ISL1532 is to connect a resistor from the I_ADJ pin to ground.
When the I_ADJ pin is grounded (the normal state), the supply
current per channel is as per the Electrical Specifications
table on page 3. When a resistor is inserted, the supply
current is scaled according to the “R_SET vs I_S” graphs in the
Performance Curves section.
Both methods of power control can be used simultaneously.
In this case, positive and negative supply currents (per amp)
are given by the equations below:
I_S+ = 0.34mA + ₁-----+-----₍---_R-⁵---_S-^.-⁰--_E--⁶--_T--^m---_/--^A--₁----₃---₀---₀----₎ x
(3/4C₁ + 1/2C₀ - C₁ × C₀ × 1/4)
(EQ. 1)
I_S-
=
-------------^----⁵---^.--⁰---⁶----^m-----^A---------------
1 + (R_SET / 1300)
x
(3/4C₁ + 1/2C₀ - C₁ × C₀ × 1/4)
Power Supplies and Dissipation
Due to the high power drive capability of the ISL1532, much
attention needs to be paid to power dissipation. The power
that needs to be dissipated in the ISL1532 has two main
contributors. The first is the quiescent current dissipation.
The second is the dissipation of the output stage.
The quiescent power in the ISL1532 is not constant with
varying outputs. In reality, 50% of the total quiescent supply
current needed to power each driver is converted in to output
current. Therefore, in the equation below we should subtract
the average output current, I_O, or 1/2 I_Q, whichever is the
lowest. We’ll call this term I_X.
P_Dquiescent = V_S × (I_S - 2I_X )
(EQ. 2)
where:
• V_S is the supply voltage (V_S+ to V_S-)
• I_S is the operating supply current (I_S+ - I_S-) / 2
• I_X is the lesser of I_O or 1/2 I_Q
The dissipation in the output stage has two main contributors.
Firstly, we have the average voltage drop across the output
transistor and secondly, the average output current. For
minimal power dissipation, the user should select the supply
voltage and the line transformer ratio accordingly. The supply
voltage should be kept as low as possible, while the
transformer ratio should be selected so that the peak voltage
required from the ISL1532 is close to the maximum available
13
FN6173.3
December 4, 2006