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LT1794CSW Datasheet(PDF) 10 Page - Linear Technology |
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LT1794CSW Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  10 LT1794 Power Dissipation and Heat Management xDSL applications require the line driver to dissipate a significant amount of power and heat compared to other components in the system. The large peak to RMS varia- tions of DMT and CAP ADSL signals require high supply voltages to prevent clipping, and the use of a step-up transformer to couple the signal to the telephone line can require high peak current levels. These requirements result in the driver package having to dissipate on the order of 1W. Several multiport cards inserted into a rack in an enclosed central office box can add up to many, many watts of power dissipation in an elevated ambient temperature environment. The LT1794 has built-in ther- mal shutdown circuitry that will protect the amplifiers if operated at excessive temperatures, however data trans- missions will be seriously impaired. It is important in the design of the PCB and card enclosure to take measures to spread the heat developed in the driver away to the ambient environment to prevent thermal shutdown (which occurs when the junction temperature of the LT1794 exceeds 165 °C). Estimating Line Driver Power Dissipation Figure 6 is a typical ADSL application shown for the purpose of estimating the power dissipation in the line driver. Due to the complex nature of the DMT signal, which looks very much like noise, it is easiest to use the RMS values of voltages and currents for estimating the driver power dissipation. The voltage and current levels shown for this example are for a full-rate ADSL signal driving 20dBm or 100mWRMS of power on to the 100Ω telephone line and assuming a 0.5dBm insertion loss in the transformer. The quiescent current for the LT1794 is set to 10mA per amplifier. The power dissipated in the LT1794 is a combination of the quiescent power and the output stage power when driving a signal. The two amplifiers are configured to place a differential signal on to the line. The Class AB output stage in each amplifier will simultaneously dissipate power in the upper power transistor of one amplifier, while sourc- ing current, and the lower power transistor of the other amplifier, while sinking current. The total device power dissipation is then: PD = PQUIESCENT + PQ(UPPER) + PQ(LOWER) PD = (V+ – V–) • IQ + (V+ – VOUTARMS) • ILOAD + (V – – VOUTBRMS) • ILOAD APPLICATIO S I FOR ATIO Figure 6. Estimating Line Driver Power Dissipation 1794 F06 + – B –IN – + A +IN 12V 20mA DC SHDN –12V –2VRMS 17.4 Ω 24.9k – SETS IQ PER AMPLIFIER = 10mA 1:1.7 110 Ω 1000pF 110 Ω 1k 1k 17.4 Ω SHDNREF 100 Ω 3.16VRMS ILOAD = 57mARMS 2VRMS |
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