Datasheet LT1739 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | Dual 500mA, 200MHz xDSL Line Driver Amplifier |
| Pages / Page | 20 / 9 — APPLICATIO S I FOR ATIO. Estimating Line Driver Power Dissipation. Figure … |
| File Format / Size | PDF / 293 Kb |
| Document Language | English |
APPLICATIO S I FOR ATIO. Estimating Line Driver Power Dissipation. Figure 6. Estimating Line Driver Power Dissipation
Model Line for this Datasheet
Text Version of Document
LT1739
U U W U APPLICATIO S I FOR ATIO
the design of the PCB and card enclosure to take measures the upper power transistor of one amplifier, while sourc- to spread the heat developed in the driver away to the ing current, and the lower power transistor of the other ambient environment to prevent thermal shutdown (which amplifier, while sinking current. The total device power occurs when the junction temperature of the LT1739 dissipation is then: exceeds 165°C). PD = PQUIESCENT + PQ(UPPER) + PQ(LOWER)
Estimating Line Driver Power Dissipation
PD = (V+ – V–) • IQ + (V+ – VOUTARMS) • I Figure 6 is a typical ADSL application shown for the LOAD + (V – – VOUTBRMS) • ILOAD purpose of estimating the power dissipation in the line With no signal being placed on the line and the amplifier driver. Due to the complex nature of the DMT signal, biased for 10mA per amplifier supply current, the quies- which looks very much like noise, it is easiest to use the cent driver power dissipation is: RMS values of voltages and currents for estimating the P driver power dissipation. The voltage and current levels DQ = 24V • 20mA = 480mW shown for this example are for a full-rate ADSL signal This can be reduced in many applications by operating driving 20dBm or 100mW with a lower quiescent current value. RMS of power on to the 100Ω telephone line and assuming a 0.5dBm insertion loss in When driving a load, a large percentage of the amplifier the transformer. The quiescent current for the LT1739 is quiescent current is diverted to the output stage and set to 10mA per amplifier. becomes part of the load current. Figure 7 illustrates the The power dissipated in the LT1739 is a combination of the total amount of biasing current flowing between the + and quiescent power and the output stage power when driving – power supplies through the amplifiers as a function of a signal. The two amplifiers are configured to place a load current. As much as 60% of the quiescent no load differential signal on to the line. The Class AB output stage operating current is diverted to the load. in each amplifier will simultaneously dissipate power in 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 1739 F06 B SHDNREF –IN + –12V –2VRMS
Figure 6. Estimating Line Driver Power Dissipation
1739fas, sn1739 9
U U W U APPLICATIO S I FOR ATIO
the design of the PCB and card enclosure to take measures the upper power transistor of one amplifier, while sourc- to spread the heat developed in the driver away to the ing current, and the lower power transistor of the other ambient environment to prevent thermal shutdown (which amplifier, while sinking current. The total device power occurs when the junction temperature of the LT1739 dissipation is then: exceeds 165°C). PD = PQUIESCENT + PQ(UPPER) + PQ(LOWER)
Estimating Line Driver Power Dissipation
PD = (V+ – V–) • IQ + (V+ – VOUTARMS) • I Figure 6 is a typical ADSL application shown for the LOAD + (V – – VOUTBRMS) • ILOAD purpose of estimating the power dissipation in the line With no signal being placed on the line and the amplifier driver. Due to the complex nature of the DMT signal, biased for 10mA per amplifier supply current, the quies- which looks very much like noise, it is easiest to use the cent driver power dissipation is: RMS values of voltages and currents for estimating the P driver power dissipation. The voltage and current levels DQ = 24V • 20mA = 480mW shown for this example are for a full-rate ADSL signal This can be reduced in many applications by operating driving 20dBm or 100mW with a lower quiescent current value. RMS of power on to the 100Ω telephone line and assuming a 0.5dBm insertion loss in When driving a load, a large percentage of the amplifier the transformer. The quiescent current for the LT1739 is quiescent current is diverted to the output stage and set to 10mA per amplifier. becomes part of the load current. Figure 7 illustrates the The power dissipated in the LT1739 is a combination of the total amount of biasing current flowing between the + and quiescent power and the output stage power when driving – power supplies through the amplifiers as a function of a signal. The two amplifiers are configured to place a load current. As much as 60% of the quiescent no load differential signal on to the line. The Class AB output stage operating current is diverted to the load. in each amplifier will simultaneously dissipate power in 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 1739 F06 B SHDNREF –IN + –12V –2VRMS
Figure 6. Estimating Line Driver Power Dissipation
1739fas, sn1739 9
