TA0313

TA0313 Technical Article STMicroelectronics Solutions for ADSL Line Interfaces New IGBT Driver This paper describes STMicroelectronics ADSL analog line interface solutions. After a short overview of the ADSL application environment, this article focuses on the implementation of line drivers. Magnetic circuits such as hybrid or line transformer circuits will not be described in detail. The ADSL concept Asymmetric Digital Subscriber Line (ADSL) is a modem technology, which converts existing twisted-pair telephone lines into access paths for multimedia and high speed data communications. An ADSL modem is connected to a twisted-pair telephone line, creating three information channels: a high-speed downstream channel (up to 1.1MHz and 2.2MHz for ADSL2+) depending on the implementation of the ADSL architecture, a medium-speed upstream channel (up to 135kHz or 230kHz) and a POTS (Plain Old Telephone Service), split off from the modem by filters. Figure 1: Typical spectral representation of a DMT ADSL signal (subscriber side) p.s.d. Power Spectral Density Upstream Downstream DMT sub-channel (Discrete Multi-Tone) POTS 4.5kHz 30kHz 135kHz 160kHz 1.1MHz ADSL allows the wide-band access necessary to transmit media such as movies, television, remote CDROMs via LANs and the Internet into individual workplaces and homes. TA0313/1104 1/9 TA0313 STMicroelectronics Solutions for ADSL Line Interfaces The line interface - ADSL remote terminal (RT) Figure 2 shows a typical analog line interface used for ADSL. The upstream and downstream signals are separated from the telephone line by using an hybrid circuit and a line transformer. On this note, emphasis will be placed on the emission path. Figure 2: Typical ADSL Line Interface 12.5Ω TX TS613 TS612 12.5Ω TRANSMISSION DMT AFE 25Ω HYBRID POTS SPLITTER line RECEPTION RX The emission path The features of the TS613 and TS612 drivers are shown in Table 1 below. Table 1: Features of drivers GBP (MHz) BW Gain=4 MHz) 34 34 SR (V/µs) Iout typ. (mA) Icc per HD2/HD31 op. (nV/√Hz ) (dBc) (mA) Noise 3 3 11 14 74/79 74/79 Vout diff. (Vpp min) 18 18 Packages TS613 TS612 VFA VFA 130 130 40 40 320 320 SO8 SO8 Exposed Pad SO20 Batwing 1) Single ended 4Vpp/100kHz on 25Ω//15pF In order to shrink the line interface size, the TS613 comes in the classic SO8 plastic package as well as the SO8 ExposedPAD plastic package, capable of dissipating 1.7W at room temperature. While this circuit does not feature a power-down function, it does have the advantage of featuring a standard pinout. 2/9 STMicroelectronics Solutions for ADSL Line Interfaces TA0313 Figure 3: Thermal considerations: power dissipation of the drivers vs. room temperature 5 TS612ID, Rthja=45°C/W Power dissipation (Watt) 4 3 TS613IPW, Rthja=70°C/W 2 TS613ID, Rthja=175°C/W 1 0 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 Room Temperature (°C) The TS612 comes in SO20 plastic batwing package which increases its power dissipation capability to 2.7W at room temperature. It features a power-down or stand-by function in order to minimize the consumption when the modem is not in communication. Power Supply Remote ADSL modem terminals must be designed to be easily connected to a PC. For such applications, the driver should use a +12V single power supply, which is available via standard PCI connectors. Note that the TS613 and TS612 can also be powered by a dual power supply at +/-6V. Figure 4 shows a single +12V supply circuit with the TS613 as a remote terminal transmitter in differential mode. Note that one could also use the TS612 in exactly the same schema. Figure 4: Implementation of the TS613 as a differential line driver with a +12V single supply 1µ 100n 3 +12V 1k 2 8 + _ +12V 1 12.5Ω 10n R2 3k9 ½ R1 Vi 1:2 Vo 25Ω ½ R1 Hybrid & Transformer 100Ω Vi 1k 10µ 3k9 100n 6 5 _ Vo R4 7 + GND 12.5Ω 100n 4 GND The driver is biased with a mid-supply (nominally +6V) in order to maintain the DC component of the signal at +6V. This allows a maximum dynamic range between 0 and +12 V. Several options are possible in order to provide this bias supply—such as for example, a virtual ground using an operational amplifier, or, the cheapest solution, a two-resistor divider. A high resistance value is required to limit the current consumption. On the other hand, the current must be high enough to bias the inverting input of the driver. If we consider the positive input’s bias current (15µA max) as 1% of the current through the resistance divider (1.5mA), two 3.9kΩ resistors are sufficient to keep a stable mid-supply . 3/9 TA0313 STMicroelectronics Solutions for ADSL Line Interfaces The input provides two high-pass filters with a break frequency of about 1.6kHz which is necessary to remove the DC component of the input signal. To avoid DC current flowing into the primary side of the transformer, an output capacitor is used. The 1µF capacitance provides a path for low frequencies, the 10nF capacitance provides a path for high end of the spectrum. Filtering As the hybrid circuit cannot perfectly separate the upstream signal and the downstream signal, any distortion from the upstream signal could affect the downstream signal. For the upstream path, a lowpass filter becomes absolutely necessary in order to cut off the higher frequencies from the DAC analog output and the driver distortions. In this simple non-inverting amplification configuration, it is easy to implement a Sallen-Key low-pass filter by using the TS613 or TS612. Figure 5: Transmission path filtering p.s.d. Power Spectral Density D is to r tio n U p s tre a m D o w n s t re am POTS 30 kH z 1 3 0 k H z (o v e r P O T S ) 2 7 6 k H z (o v e r IS D N ) lo w p a s s filte r (tra n s m is s io n p a th ) A first solution is to use a LC cell before the driver to provide the low-pass filtering. Nevertheless, as shown in Figure 6, using 2nd order active filtering is a good solution especially as regards cost and space-saving considerations. Figure 6: TS613 line driver with 2nd order active filtering C2 µ 8 +12V 1 100n +12V 1k R4 R5 3 2 + _ 12.5Ω 10n R2 C1 Vi 3k9 ½ R1 Vo 25Ω ½ R1 Vi 1k 10µ 3k9 100n C3 Vo 6 _ GND R6 R7 R3 7 12.5Ω 5 + 100n C4 4 GND In the configuration shown in Figure 6, we assume R4=R6, R5=R7, C1=C3 and C2=C4. 4/9 STMicroelectronics Solutions for ADSL Line Interfaces The resistances R1, R2 and R3 allow us to calculate the gain of the structure as follows: 2R2 2R3 Gain = 1 + ---------- = 1 + ---------R1 R1 TA0313 The damping factor can be derived from these resistances and the capacitances C1, C2, C3 and C4: 2C3 – αC4 2 C1 – αC2 ζ = ---------------------------- = ---------------------------2 C3C4 2 C1C2 with: 2R3 2R2 α = ---------- = ---------R1 R1 The higher the gain, the more sensitive the damping factor is. When the gain is higher than 1 it is preferable to use very stable resistance and capacitance values. The value of the gain does not affect the cut-off frequency, fc, which is derived as follows: 1 1 fc = ------------------------------------------- = ------------------------------------------2π R4R5C1C2 2π R6R7C3C4 Moreover this expression shows that it is possible to shift the cut-off frequency by simply changing the values of the resistances R4, R5 or R6, R7 — with neither a change of capacitance nor of the damping factor. The following table shows a calculations of components for a cut-off frequency around 130kHz for the ADSL over POTS and 270kHz for the ADSL over ISDN. The final, accurate settings are made by compromising between the attenuation of the highest frequencies of the upstream signal and the impact of the distortion on the downstream signal. This is best done directly in the application. Nevertheless we can start with the following initial values: R1 (Ω ) 180 180 R2 R3 (Ω ) 536 536 R4 R6 (Ω ) 261 127 R5 R7 (Ω ) 261 127 C1 C3 (nF) 10 10 C2 C4 (nF) 2.2 2.2 Gain (dB) 16.8 16.8 fc (kHz) 130 270 ζ 0.73 0.73 Transformation ratio In differential mode, the TS613 and TS612 are able to deliver a typical amplitude signal of 18V peak to peak. The dynamic line impedance is 100Ω. The typical value of the amplitude signal required on the line is up to 12.4V peak to peak. By using a 1:2 transformer ratio the reflected impedance back to the primary will be a quarter (25Ω) and therefore the amplitude of the signal required with this impedance will be halved (6.2 V peak to peak). Assuming a 25Ω series resistance (12.5Ω for both outputs) is necessary for impedance matching, the output signal amplitude required is 12.4V peak to peak. This value is acceptable for both TS613 and TS612. In this case, the load impedance is 25Ω for each driver in single ended. 5/9 TA0313 STMicroelectronics Solutions for ADSL Line Interfaces Increasing the line level by using an active impedance matching With passive matching, the output signal amplitude of the driver must be twice the amplitude on the load. To go beyond this limitation an active matching impedance can be used. With this technique it is possible to maintain good impedance matching with an amplitude on the load higher than half of the output driver amplitude. This concept is shown in Figure 7 for a differential line. Figure 7: TS613 as a differential line driver with an active impedance matching 1µ 100n 3 +12V 1k 2 8 + _ +12V 1 Rs1 Vo° 10n R2 3k9 ½ R1 Vi 1:2 R3 Vo 25Ω Hybrid & Transformer 100Ω ½ R1 R5 Vi 1k 10µ 3k9 100n 6 5 _ Vo R4 7 + GND Vo° Rs2 100n 4 GND Component calculation Let us consider the equivalent circuit for a single-ended configuration, as shown in Figure 8. Figure 8: Single ended equivalent circuit + Rs1 Vi _ R2 Vo° Vo -1 R3 1/2R1 1/2RL For unloaded system, we can assume that currents through R1, R2 and R3 are respectively: 2 Vi ( Vi – Vo ° ) ( V i + Vo ) -------- , -------------------------- and -----------------------R1 R2 R3 6/9 STMicroelectronics Solutions for ADSL Line Interfaces As Vo° equals Vo without loading, the gain in this case becomes: 2R2 R2 1 + ----------- + ------Vo ( noloa d ) R1 R3 G = -------------------------------- = -----------------------------------Vi R2 1 – ------R3 The gain for the loaded system will be: 2R2 R2 1 + ----------- + ------Vo ( w ithload ) 1 R1 R3 GL = ------------------------------------- = -- -----------------------------------Vi 2 R2 1 – ------R3 TA0313 Equation 1 As shown in Figure 9, this system is an ideal generator, with a synthesized impedance equal to the internal impedance of the system. Therefore, the output voltage becomes: Equation 2 Vo = ( ViG ) – ( Ro Io ut ) with Ro the synthesized impedance and Iout the output current. On the other hand Vo can be expressed as: Equation 3 2R2 R 2 Vi  1 + ----------- + -------  R 1 R 3 Rs 1 Io ut Vo = ----------------------------------------------- – ---------------------R2 R2 1 – ------1 – ------R3 R3 Figure 9: Equivalent schematic, where Ro is the synthesized impedance Ro Iout Vi.Gi 1/2RL By identifying of both Equation 2 and Equation 3, the synthesized impedance is, with Rs1=Rs2=Rs: Equation 4 Rs Ro = ---------------R2 1 – -----R3 Unlike the level of Vo° required for passive impedance, Vo° will be smaller than 2Vo in this case. Let us write Vo°=kVo with k the matching factor varying between 1 and 2. Assuming that the current through R3 is negligible, the resistance divider becomes: kVoR L Ro = ---------------------------RL + 2 R s 1 After choosing the k factor, Rs will equal to 1/2RL(k-1). 7/9 TA0313 STMicroelectronics Solutions for ADSL Line Interfaces A good impedance matching assumes: Equation 5 From Equation 3 and Equation 5 we derive: Equation 6 R2 2 Rs ------ = 1 – ---------R3 RL 1 R o = -- RL 2 By fixing an arbitrary value for R2 in Equation 6, we arrive at: R2 R 3 = ------------------2 Rs 1 – ---------RL Finally, the values of R2 and R3 allow us to extract R1 from Equation 1 so that: Equation 7 2R2 R 1 = -------------------------------------------------------R 2 R2 2  1 – ------- G L – 1 – ----- R 3 R3 GL is fixed for the application requirements GL=Vo/Vi=0.5(1+2R2/R1+R2/R3)/(1-R2/R3) 2R2/[2(1-R2/R3)GL-1-R2/R3] Arbitrarily fixed R2/(1-Rs/0.5RL) 0.5RL(k-1) with GL the required gain. GL (gain for the loaded system) R1 R2 (=R4) R3 (=R5) Rs Capabilities The table below shows the calculated components for different values of k for a differential load of 25Ω. In all cases, R2=1000Ω and the gain=16dB. The last column displays the maximum amplitude level on the line regarding the TS613 maximum output capabilities (18Vp-p diff.) and a 1:2 line transformer ratio. Active matching k 1.3 1.4 1.5 1.6 1.7 R1 (Ω ) 953 590 422 316 261 R3 (Ω ) 1400 1620 2000 2490 3300 Passive matching Rs (Ω ) 3.75 (3.9//100) 5 (10//10) 6.36 (6.8//100) 7.57 (8.2//100) 8.71 (10//68) TS613 Output Level to get 12.4Vpp on the line with a turn ratio of 2. (Vp-p diff) 8.06 8.68 9.35 9.95 10.52 12.4 Maximum Line Level (Vp-p diff) 27.5 25.7 25.3 23.7 22.3 18 Measurement of power consumption in application Conditions: l Passive impedance matching l Transformer turns ratio: 2 l Maximum level required on the line: 12.4Vpp l Maximum output level of the driver: 12.4Vpp 8/9 STMicroelectronics Solutions for ADSL Line Interfaces l Crest factor: 5.3 (Vp/Vrms) l Power Supply: 12V TA0313 Power consumption of the driver during emission on 900 and 4550 meter twisted pair telephone lines: l TS613: 3 60mW l TS612: 4 50mW Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics All other names are the property of their respective owners © 2004 STMicroelectronics - All rights reserved STMicroelectronics group of companies Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America www.st.com 9/9