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: 2 ( 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 CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2007. All Rights Reserved All other trademarks mentioned are the property of their respective owners 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 Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that the Application Note or Technical Brief is current before proceeding. For information regarding Intersil Corporation and its products, see www.intersil.com 2 AN1305.0 July 11, 2007