AN1305: ADSL CO Line Driver Design Method

ADSL CO Line Driver Design Method
®
Application Note
July 11, 2007
AN1305.0
By Tamara Ahrens, Kent Chon and Mike Wong
The key specifications of ADSL CO driver are as follows:
peak output line power is 20dBm, POTS line impedance is
100Ω, and the crest factor for ADSL DMT CO system is
14.5dB.
In the last few years, the demand for high speed Internet
access, LAN, and short distance data communication have
been growing exponentially and growth is expected to
continue through the year 2005. Twisted pair, most
commonly found in telephone lines, has been used in local
area networks to transmit data between data terminals and
computers, and for video on demand (VOD) applications.
Video designers who want to replace coaxial cable for the
more cost effective twisted pair may utilize similar
techniques. This transition requires more speed and more
integration.
From output line power and line impedance, we can
calculate average output line voltage swing and current:
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( V OUT ( RMS ) )
P OUT = ----------------------------------------R LINE
V OUT – RMS = 3.16V
(EQ. 1)
I OUT – RMS = 31.6mA
The ever-present problem with pure digital transmission is
that it consumes bandwidth far beyond data rates. In
response, modem technology has blossomed into a family of
digital subscriber loop (DSL) types, collectively referred to as
XDSL. Within these DSL systems, analog technology allows
a hundred-fold increase of data rate over standard phone
line transmissions.
The line peak to peak differential output swing can be
calculated with peak to average ratio (PAR), related to the
specified crest factor (CF).
CF = 20log (PAR) = 14.5dB,
PAR = 5.3
V OUT ( P – P ) – DIFF = V OUT ( RMS )∗ PAR∗ 2 = 33.52V
Digital Subscriber Line (DSL) is a blossoming technology
with a deployment increasing rapidly among cable model,
fiber optics, and satellite Internet access. This discussion will
focus mainly on the ADSL CO (central office) driver portion
of the system. The analysis and design techniques
discussed are applicable to each DSL standard and all
driver designs.
(EQ. 2)
For an ADSL CO output, the peak to peak voltage swing
requirement is 33.52V. Peak output current can be
calculated by peak output voltage divided by 100Ω line
impedance. It is 167.6mA.
The conventional differential pair driver consists of two
amplifiers to transmit a differential signal, then the receiver
converts the signal back to a single-ended signal. Figure 1
shows a typical differential driver circuit configuration. As
calculated previously, the peak to peak output voltage is
33.52V and peak output current is 167.6mA on the line. The
output swing on the driver needed to produce this line
voltage and current are determined by the transformer turns
ratio, N, and back termination resistor value (RTERM). Other
key specifications of the differential pair driver and receiver
are symmetrical dynamic response, wide bandwidth, flexible
After discussing the ADSL DMT CO line driver requirements,
we will show typical line driver circuits. From the transformer
turns ratio and termination resistor value, we will calculate
the driver output current and voltage swing. Then, we will
examine the driver linearity and power dissipation. From
power dissipation we will calculate the thermal resistance
required. Lastly, we will present the power requirements of
different ADSL systems and HDSL II system.
+
12V
VCC
50/N2
U1
4
V+
V-
RTERM
167.6mA*N
TX1
-
T1
VEE
33.52V/N
32.52
RLINE
100
+
12V
1
U2
4
2
17
N
RTERM
B
FIGURE 1. DRIVER CIRCUITRY
1
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Application Note 1305
acceptable for voice but harmful to DSL running at higher
frequencies. The resulting SNR is frequency dependent
because attenuation and crosstalk are frequency dependent.
The worst case, SNR determines the channel capacity of the
system.
operation, high output current, and low total harmonic
distortion.
The transformer ratio, N, determines the driver output
current, voltage swing, and power dissipation. As an
example, let's assume N = 1.41, a typical value. The
transformer then requires that IOUT - peak for the driver is:
Keeping power constant, the transformer reduces the
voltage by the same factor. VOUT(P-P) differential voltage at
the driver side of the transformer is:
With 1.41 turns ratio and 100% termination resistor value
selected, power dissipation can be calculated for an
example driver, the EL1503. Given supply voltages
Vs = ±12V, full power mode selected [C0, C1 are low], and
RSET chosen to be 1.5kΩ, the positive supply current is
12mA. The negative supply current is 11mA. We can now
calculate the quiescent power dissipation:
33.5V
---------------- = 23.7V
1.41
P D ( quiescent ) = V S +∗ ( I S + - 7mA ) + V S -∗ ( I S - - 7mA )
167.6mA∗ 1.41 = 236.3mA
(EQ. 3)
(EQ. 4)
(EQ. 5)
With a two-amplifier, differential drive, the VOUT(P-P) for
each amplifier is half of that, or 11.85V.
(where: 7mA is the quiescent current going to the output
while driving a heavy load.)
The output swing at the driver depends on the size of the
termination resistor. The termination resistor serves to match
the impedance of the line and to dissipate any power
reflected back to the source.
From this equation, the quiescent power dissipation is
108mW. The power dissipation of the output stage can be
calculated from supply voltage (minus the RMS output
voltage) multiplied by the output current. The total power
dissipation is 984mW. With 984mW of driver power
dissipation and 100% termination back-matching resistor,
the efficiency is 9.5%. Maximum operating die temperature
is set at 150°. The maximum ambient temperature is
assumed to be 85°. The maximum junction to ambient
thermal resistance required is 66.3° per watt.
For 100% termination, the resistor value is 25Ω. The output
swing is 23.7V, with half of the power being dissipated
through the termination resistor and the other half reaching
the line through the transformer.
The signal strength needed on the line is quantified in the
Signal to Noise Ratio (SNR). Many factors contribute,
including the resolution of the receiver, the attenuation of the
line, and the additional contributors of noise to the system.
The first source is telephone loop and plant background
noise and the second source is crosstalk from multiple
twisted pair in same sheath. Crosstalk disturbances are
Table 1 lists key power requirements for the current, and
most popular, DSL systems. The design method that has
been discussed is equally applicable for these, and any
other, DSL systems.
TABLE 1. DSL SPECIFICATIONS
ADSL C0
FULL RATE
ADSL C0
G. LITE
ADSL CP
HDSL II
Line Power [dBm]
20
16.3
13
16.5
RMS Line Power [mW]
100
43
20
44.7
Line Impedance [Ω]
100
100
100
135
RMS Line Voltage [V]
3.1
2
1.4
2.46
RMS Line Current [mA]
31
21
15
18.2
Crest Factor
5.3
5.3
5.3
3
Peak-Peak Line Voltage [V]
33
22
15.2
14.8
Peak Line Current [mA]
170
110
76
54.7
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verify that the Application Note or Technical Brief is current before proceeding.
For information regarding Intersil Corporation and its products, see www.intersil.com
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AN1305.0
July 11, 2007
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