Datasheet LT1794 (Analog Devices) - 10

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