Datasheet LT1739 (Analog Devices) - 7
| Manufacturer | Analog Devices |
| Description | Dual 500mA, 200MHz xDSL Line Driver Amplifier |
| Pages / Page | 20 / 7 — APPLICATIO S I FOR ATIO. Figure 1. Internal Current Biasing Circuitry. … |
| File Format / Size | PDF / 293 Kb |
| Document Language | English |
APPLICATIO S I FOR ATIO. Figure 1. Internal Current Biasing Circuitry. Setting the Quiescent Operating Current. Figure 2. R
Model Line for this Datasheet
Text Version of Document
LT1739
U U W U APPLICATIO S I FOR ATIO
The LT1739 is a high speed, 200MHz gain bandwidth on supply current per amplifier with RBIAS connected product, dual voltage feedback amplifier with high output between the SHDN pin and the 12V V+ supply of the current drive capability, 500mA source and sink. The LT1739 and the approximate design equations. Figure 3 LT1739 is ideal for use as a line driver in xDSL data illustrates the same control with RBIAS connected between communication applications. The output voltage swing the SHDNREF pin and ground while the SHDN pin is tied has been optimized to provide sufficient headroom when to V+. Either approach is equally effective. operating from ±12V power supplies in full-rate ADSL applications. The LT1739 also allows for an adjustment of SHDN the operating current to minimize power consumption. In addition, the LT1739 is available in small footprint 5I 2k 3mm × 4mm DFN and 20-lead TSSOP surface mount I 2I 2I package to minimize PCB area in multiport central office DSL cards. 1k To minimize signal distortion, the LT1739 amplifiers are TO decompensated to provide very high open-loop gain at START-UP CIRCUITRY IBIAS high frequency. As a result each amplifier is frequency TO AMPLIFIERS BIAS CIRCUITRY stable with a closed-loop gain of 10 or more. If a closed- SHDNREF 1739 F01 loop gain of less than 10 is desired, external frequency 2 IBIAS = I 5 SHDN = ISHDNREF compensating components can be used. ISUPPLY PER AMPLIFIER (mA) = 64 • IBIAS
Figure 1. Internal Current Biasing Circuitry Setting the Quiescent Operating Current
30 Power consumption and dissipation are critical concerns VS = ±12V V+ = 12V in multiport xDSL applications. Two pins, Shutdown 25 RBIAS (SHDN) and Shutdown Reference (SHDNREF), are pro- SHDN 20 V+ – 1.2V I ≈ vided to control quiescent power consumption and allow S PER AMPLIFIER (mA) • 25.6 RBIAS + 2k 15 for the complete shutdown of the driver. The quiescent R V+ – 1.2V BIAS = • 25.6 – 2k current should be set high enough to prevent distortion PER AMPLIFIER (mA) I 10 S PER AMPLIFIER (mA) SHDNREF induced errors in a particular application, but not so high I SUPPLY 5 that power is wasted in the driver unnecessarily. A good 0 starting point to evaluate the LT1739 is to set the quiescent 7 10 40 70 100 130 160 190 R current to 10mA per amplifier. BIAS (kΩ) 1739 F02
Figure 2. R
The internal biasing circuitry is shown in Figure 1. Ground-
BIAS to V+ Current Control
ing the SHDNREF pin and directly driving the SHDN pin with 45 V a voltage can control the operating current as seen in the S = ±12V V+ = 12V 40 Typical Performance Characteristics. When the SHDN pin SHDN 35 V+ – 1.2V I ≈ S PER AMPLIFIER (mA) • 64 R is less than SHDNREF + 0.4V, the driver is shut down and 30 BIAS + 5k consumes typically only 100µA of supply current and the 25 R V+ – 1.2V BIAS = • 64 – 5k 20 IS PER AMPLIFIER (mA) outputs are in a high impedance state. Part to part varia- PER AMPLIFIER (mA) SHDNREF 15 tions, however, will cause inconsistent control of the qui- RBIAS I SUPPLY 10 escent current if direct voltage drive of the SHDN pin is used. 5 Using a single external resistor, R 0 BIAS, connected in one of 4 7 10 30 50 70 90 100 130 150 170 190 210 230 250 270 290 two ways provides a much more predictable control of the RBIAS (kΩ) 1739 F03 quiescent supply current. Figure 2 illustrates the effect
Figure 3. RBIAS to Ground Current Control
1739fas, sn1739 7
U U W U APPLICATIO S I FOR ATIO
The LT1739 is a high speed, 200MHz gain bandwidth on supply current per amplifier with RBIAS connected product, dual voltage feedback amplifier with high output between the SHDN pin and the 12V V+ supply of the current drive capability, 500mA source and sink. The LT1739 and the approximate design equations. Figure 3 LT1739 is ideal for use as a line driver in xDSL data illustrates the same control with RBIAS connected between communication applications. The output voltage swing the SHDNREF pin and ground while the SHDN pin is tied has been optimized to provide sufficient headroom when to V+. Either approach is equally effective. operating from ±12V power supplies in full-rate ADSL applications. The LT1739 also allows for an adjustment of SHDN the operating current to minimize power consumption. In addition, the LT1739 is available in small footprint 5I 2k 3mm × 4mm DFN and 20-lead TSSOP surface mount I 2I 2I package to minimize PCB area in multiport central office DSL cards. 1k To minimize signal distortion, the LT1739 amplifiers are TO decompensated to provide very high open-loop gain at START-UP CIRCUITRY IBIAS high frequency. As a result each amplifier is frequency TO AMPLIFIERS BIAS CIRCUITRY stable with a closed-loop gain of 10 or more. If a closed- SHDNREF 1739 F01 loop gain of less than 10 is desired, external frequency 2 IBIAS = I 5 SHDN = ISHDNREF compensating components can be used. ISUPPLY PER AMPLIFIER (mA) = 64 • IBIAS
Figure 1. Internal Current Biasing Circuitry Setting the Quiescent Operating Current
30 Power consumption and dissipation are critical concerns VS = ±12V V+ = 12V in multiport xDSL applications. Two pins, Shutdown 25 RBIAS (SHDN) and Shutdown Reference (SHDNREF), are pro- SHDN 20 V+ – 1.2V I ≈ vided to control quiescent power consumption and allow S PER AMPLIFIER (mA) • 25.6 RBIAS + 2k 15 for the complete shutdown of the driver. The quiescent R V+ – 1.2V BIAS = • 25.6 – 2k current should be set high enough to prevent distortion PER AMPLIFIER (mA) I 10 S PER AMPLIFIER (mA) SHDNREF induced errors in a particular application, but not so high I SUPPLY 5 that power is wasted in the driver unnecessarily. A good 0 starting point to evaluate the LT1739 is to set the quiescent 7 10 40 70 100 130 160 190 R current to 10mA per amplifier. BIAS (kΩ) 1739 F02
Figure 2. R
The internal biasing circuitry is shown in Figure 1. Ground-
BIAS to V+ Current Control
ing the SHDNREF pin and directly driving the SHDN pin with 45 V a voltage can control the operating current as seen in the S = ±12V V+ = 12V 40 Typical Performance Characteristics. When the SHDN pin SHDN 35 V+ – 1.2V I ≈ S PER AMPLIFIER (mA) • 64 R is less than SHDNREF + 0.4V, the driver is shut down and 30 BIAS + 5k consumes typically only 100µA of supply current and the 25 R V+ – 1.2V BIAS = • 64 – 5k 20 IS PER AMPLIFIER (mA) outputs are in a high impedance state. Part to part varia- PER AMPLIFIER (mA) SHDNREF 15 tions, however, will cause inconsistent control of the qui- RBIAS I SUPPLY 10 escent current if direct voltage drive of the SHDN pin is used. 5 Using a single external resistor, R 0 BIAS, connected in one of 4 7 10 30 50 70 90 100 130 150 170 190 210 230 250 270 290 two ways provides a much more predictable control of the RBIAS (kΩ) 1739 F03 quiescent supply current. Figure 2 illustrates the effect
Figure 3. RBIAS to Ground Current Control
1739fas, sn1739 7
