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LT1739CUE Datasheet(PDF) 8 Page - Linear Technology |
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LT1739CUE Datasheet(HTML) 8 Page - Linear Technology |
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8 / 20 page  8 LT1739 1739fas, sn1739 Two Control Inputs RESISTOR VALUES (k Ω) RSHDN TO VCC (12V) RSHDN TO VLOGIC VLOGIC RSHDN RC1 RCO 3V 40.2 11.5 19.1 3.3V 43.2 13.0 22.1 5V 60.4 21.5 36.5 3V 4.99 8.66 14.3 3.3V 6.81 10.7 17.8 5V 19.6 20.5 34.0 VC0 H L H L VC1 H H L L 10 7 5 2 10 7 5 2 10 7 5 2 10 7 5 2 10 7 5 2 10 7 5 2 SUPPLY CURRENT PER AMPLIFIER (mA) One Control Input RESISTOR VALUES (k Ω) RSHDN TO VCC (12V) RSHDN TO VLOGIC VLOGIC RSHDN RC 3V 40.2 7.32 3.3V 43.2 8.25 5V 60.4 13.7 3V 4.99 5.49 3.3V 6.81 6.65 5V 19.6 12.7 VC H L 10 2 10 2 10 2 10 2 10 2 10 2 SUPPLY CURRENT PER AMPLIFIER (mA) RSHDN RC1 VC1 VLOGIC 12V OR VLOGIC 0V VC0 RC0 SHDN SHDNREF 2k RSHDN RC VC VLOGIC 12V OR VLOGIC 0V SHDN SHDNREF 1739 F04 2k APPLICATIO S I FOR ATIO Logic Controlled Operating Current The DSP controller in a typical xDSL application can have I/O pins assigned to provide logic control of the LT1739 line driver operating current. As shown in Figure 4 one or two logic control inputs can set two or four different operating modes. The logic inputs add or subtract current to the SHDN input to set the operating current. The one logic input example selects the supply current to be either full power, 10mA per amplifier or just 2mA per amplifier, which significantly reduces the driver power consumption while maintaining less than 2 Ω output impedance to frequencies less than 1MHz. This low power mode retains termination impedance at the amplifier outputs and the line driving back termination resistors. With this termina- tion, while a DSL port is not transmitting data, it can still sense a received signal from the line across the back- termination resistors and respond accordingly. The two logic input control provides two intermediate (approximately 7mA per amplifier and 5mA per amplifier) operating levels between full power and termination modes. For proper operation of the current control cir- cuitry, it is necessary that the SHDNREF pin be biased at least 2V more positive than V–. In single supply applica- tions where V– is at ground potential, special attention to the DC bias of the SHDNREF pin is required. Contact Linear Technology for assistance in implementing a single supply design with operating current control. These modes can be useful for overall system power manage- ment when full power transmissions are not necessary. Shutdown and Recovery The ultimate power saving action on a completely idle port is to fully shut down the line driver by pulling the SHDN pin to within 0.4V of the SHDNREF potential. As shown in Figure 5 complete shutdown occurs in less than 10 µs and, more importantly, complete recovery from the shut down state to full operation occurs in less than 2 µs. The biasing circuitry in the LT1739 reacts very quickly to bring the amplifiers back to normal operation. Figure 4. Providing Logic Input Control of Operating Current VSHDN SHDNREF = 0V AMPLIFIER OUTPUT 1794 F05 Figure 5. Shutdown and Recovery Timing 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 significant amounts of power. 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 LT1739 has built- in thermal shutdown circuitry that will protect the ampli- fiers if operated at excessive temperatures, however data transmissions will be seriously impaired. It is important in |
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