ISL1539IRZ Datasheet, PDF (9/11 Pages) Intersil Corporation – Dual Channel Differential VDSL2 Line Driver

ISL1539
Application Information
The ISL1539 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 30MHz and produce low distortion levels. A typical interface
circuit is shown in Figure 20.
DRIVER
INPUT
+
-
R_F
R_G
R_F
-
+
R_F
R
R_OUT LINE +
Z_LINE
R_OUT
LINE -
RECEVIE
OUT+
-
+
R_IN
RECEVIE
AMPLIFIERS
+
R
-
RECEVIE
OUT-
R_F
R_IN
FIGURE 20. TYPICAL LINE INTERFACE CONNECTION
The amplifiers are wired with one in positive gain and the other in
a negative gain configuration to generate a differential output for
a single-ended input. They will exhibit very similar frequency
responses for gains of three or greater and thus generate very
small common-mode outputs over frequency, but for low gains
the two drivers RF's need to be adjusted to give similar frequency
responses. The positive-gain driver will generally exhibit more
bandwidth and peaking than the negative-gain driver.
If a differential signal is available to the drive amplifiers, they
may be wired so:
+
-
R_F
2R_G
R_F
-
+
FIGURE 21. DRIVERS WIRED FOR DIFFERENTIAL INPUT
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.
Power Supplies and Dissipation
Due to the high power drive capability of the ISL1539, much
attention needs to be paid to power dissipation. The power that
needs to be dissipated in the ISL1539 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 ISL1539 is not constant with varying
outputs. In reality, 7mA of the 15mA needed to power the drivers
is converted in to output current. Therefore, in the equation below
we should subtract the average output current, I_O, or 7mA,
whichever is the lowest. We’ll call this term I_X.
Therefore, we can determine a quiescent current with
Equation 1:
P_Dquiescent = V_S × (I_S – 2I_X)
(EQ. 1)
where:
• V_S is the supply voltage (V_S+ to V_S-)
• I_S is the maximum quiescent supply current (I_S+ + I_S-)
• I_X is the lesser of I_O or 7mA (generally I_X = 7mA)
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 ISL1539 is
close to the maximum available output swing. There is a trade
off, however, with the selection of transformer ratio. As the ratio
is increased, the receive signal available to the receivers is
reduced.
Once the user has selected the transformer ratio, the dissipation
in the output stages can be selected with Equation 2:
P_Dtransistors
=
2
×
I_O
×
⎛
⎝
V_-----S--
2
– V_{O⎠⎞}
(EQ. 2)
where:
• V_S is the supply voltage (V_S+ to V_S-)
• V_O is the average output voltage per channel
• I_O is the average output current per channel
The overall power dissipation (P_DISS) is obtained by adding
P_Dquiescent and P_Dtransistor.
Then, the θ_JA requirement needs to be calculated. This is done
using Equation 3:
θ_JA
=
-⁽--^T----^J---^U----^N----^C----^T-----–-----^T----^A---^M-----^B-----⁾
P_DISS
(EQ. 3)
9
FN7516.4
June 21, 2013