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LT1739 Datasheet(PDF) 9 Page - Linear Technology |
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LT1739 Datasheet(HTML) 9 Page - Linear Technology |
9 / 20 page 9 LT1739 1739fas, sn1739 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 LT1739 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 LT1739 is set to 10mA per amplifier. The power dissipated in the LT1739 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 With no signal being placed on the line and the amplifier biased for 10mA per amplifier supply current, the quies- cent driver power dissipation is: PDQ = 24V • 20mA = 480mW This can be reduced in many applications by operating with a lower quiescent current value. When driving a load, a large percentage of the amplifier quiescent current is diverted to the output stage and becomes part of the load current. Figure 7 illustrates the total amount of biasing current flowing between the + and – power supplies through the amplifiers as a function of load current. As much as 60% of the quiescent no load operating current is diverted to the load. APPLICATIO S I FOR ATIO Figure 6. Estimating Line Driver Power Dissipation 1739 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 |
Similar Part No. - LT1739_15 |
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Similar Description - LT1739_15 |
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