Datasheet

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LT1794

Dual 500mA, 200MHz

xDSL Line Driver Amplifier

■

Exceeds All Requirements For Full Rate,

Downstream ADSL Line Drivers

■

±500mA Minimum I

OUT

■

±11.1V Output Swing, V

= ±12V, R

= 100Ω

■

±10.9V Output Swing, V

= ±12V, I

= 250mA

■

Low Distortion: –82dBc at 1MHz, 2V

P-P

Into 50Ω

■

Power Saving Adjustable Supply Current

■

Power Enhanced Small Footprint Packages:

20-Lead TSSOP and 20-Lead SW

■

200MHz Gain Bandwidth

■

500V/µs Slew Rate

■

Specified at ±15V, ±12V and ±5V

■

High Density ADSL Central Office Line Drivers

■

High Efficiency ADSL, HDSL2, G.lite,

SHDSL Line Drivers

■

Buffers

■

Test Equipment Amplifiers

■

Cable Drivers

The LT

1794 is a 500mA minimum output current, dual op

amp with outstanding distortion performance. The ampli-

fiers are gain-of-ten stable, but can be easily compensated

for lower gains. The extended output swing allows for

lower supply rails to reduce system power. Supply current

is set with an external resistor to optimize power dissipa-

tion. The LT1794 features balanced, high impedance in-

puts with low input bias current and input offset voltage.

Active termination is easily implemented for further sys-

tem power reduction. Short-circuit protection and thermal

shutdown insure the device’s ruggedness.

The outputs drive a 100Ω load to ±11.1V with ±12V

supplies, and ±10.9V with a 250mA load. The LT1794,

with its increased swing on lower supplies, can be used to

upgrade LT1795 line driver applications.

The LT1794 is available in the very small, thermally

enhanced, 20-lead TSSOP for maximum port density in

line driver applications. The 20-lead SW is also available.

, LTC and LT are registered trademarks of Linear Technology Corporation.

High Efficiency ±12V Supply ADSL Central Office Line Driver

1794 TA01

–

1/2

LT1794

–IN

–

1/2

LT1794

+IN

12V

SHDN

–12V

12.7Ω

BIAS

24.9k

1:2*

110Ω

1000pF

110Ω

12.7Ω

SHDNREF

100Ω

*COILCRAFT X8390-A OR EQUIVALENT

SUPPLY

= 10mA PER AMPLIFIER

WITH R

BIAS

= 24.9k

•

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

Summary of content (20 pages)

PAGE 1
LT1794 Dual 500mA, 200MHz xDSL Line Driver Amplifier U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LT®1794 is a 500mA minimum output current, dual op amp with outstanding distortion performance. The amplifiers are gain-of-ten stable, but can be easily compensated for lower gains. The extended output swing allows for lower supply rails to reduce system power. Supply current is set with an external resistor to optimize power dissipation.
PAGE 2
LT1794 U W W W ABSOLUTE MAXIMUM RATINGS (Note 1) Supply Voltage (V + to V –) .................................... ±18V Input Current ..................................................... ±10mA Output Short-Circuit Duration (Note 2) ........... Indefinite Operating Temperature Range ............... – 40°C to 85°C Specified Temperature Range (Note 3) .. – 40°C to 85°C Junction Temperature .......................................... 150°C Storage Temperature Range .................
PAGE 3
LT1794 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full specified temperature range, otherwise specifications are at TA = 25°C. VCM = 0V, pulse tested, ±5V ≤ VS ≤ ±15V, VSHDNREF = 0V, RBIAS = 24.9k between V + and SHDN unless otherwise noted.
PAGE 4
LT1794 ELECTRICAL CHARACTERISTICS Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: Applies to short circuits to ground only. A short circuit between the output and either supply may permanently damage the part when operated on supplies greater than ±10V.
PAGE 5
LT1794 U W TYPICAL PERFOR A CE CHARACTERISTICS Open-Loop Gain and Phase vs Frequency –3dB Bandwidth vs Supply Current 120 120 45 100 80 40 40 0 40 –40 20 –80 GAIN –160 TA = 25°C VS = ±12V AV = –10 RL = 100Ω IS PER AMPLIFIER = 10mA –40 –60 –80 100k 1M 10M FREQUENCY (Hz) –200 800 30 25 20 15 10 –240 5 –280 0 100M 2 70 60 50 40 30 20 1 10 FREQUENCY (MHz) 100 80 50 (–) SUPPLY 40 30 ISHDN (mA) OUTPUT IMPEDANCE (Ω) IS PER AMPLIFIER = 15mA 0.
PAGE 6
LT1794 U W TYPICAL PERFOR A CE CHARACTERISTICS Differential Harmonic Distortion vs Output Amplitude Differential Harmonic Distortion vs Frequency –40 f = 1MHz TA = 25°C –50 VS = ±12V AV = 10 RL = 50Ω –60 I PER AMPLIFIER = 10mA S –45 –50 DISTORTION (dBc) DISTORTION (dBc) –40 HD3 –70 –80 HD2 –55 –60 VO = 10VP-P TA = 25°C VS = ±12V AV = 10 RL = 50Ω IS PER AMPLIFIER = 10mA –65 –70 –75 HD3 –80 –90 HD2 –85 –100 0 2 4 6 8 10 12 14 16 –90 100 200 300 400 500 600 700 800 900 1000 FREQUENCY (
PAGE 7
LT1794 TEST CIRCUIT SUPPLY BYPASSING 12V 0.1µF RSHDN 2 9 10 (SHDN) 3 A – 5 4 10k 6 OUT (+) 110Ω OUT (–) 110Ω 0.1µF MINICIRCUITS ZSC5-2-2 4.7µF 0.1µF 4.7µF VOUT(P-P) –12V 7 1k 1:2* RL ≈ 50Ω SPLITTER 49.9Ω + 12.7Ω –12V EIN 4.7µF + 19 + 8 12V + 100 LINE LOAD 10k 13 0.01µF 1k 12.
PAGE 8
LT1794 U W U U APPLICATIO S I FOR ATIO outputs are in a high impedance state. Part to part variations however, will cause inconsistent control of the quiescent current if direct voltage drive of the SHDN pin is used. SHDN 5I 2k I Using a single external resistor, RBIAS, connected in one of two ways provides a much more predictable control of the quiescent supply current.
PAGE 9
LT1794 U W U U APPLICATIO S I FOR ATIO Logic Controlled Operating Current Shutdown and Recovery The DSP controller in a typical xDSL application can have I/O pins assigned to provide logic control of the LT1794 line driver operating current. As shown in Figure 4 one or two logic control inputs can control two or four different operating modes. The logic inputs add or subtract current to the SHDN input to set the operating current.
PAGE 10
LT1794 U W U 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 significant amount of power and heat compared to other components in the system. The large peak to RMS variations 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.
PAGE 11
LT1794 U W U U APPLICATIO S I FOR ATIO With no signal being placed on the line and the amplifier biased for 10mA per amplifier supply current, the quiescent 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.
PAGE 12
LT1794 U W U U APPLICATIO S I FOR ATIO Figure 8 shows four examples of PCB metal being used for heat spreading. These are provided as a reference for what might be expected when using different combinations of metal area on different layers of a PCB. These examples are with a 4-layer board using 1oz copper on each. The most effective layers for spreading heat are those closest to the LT1794 junction.
PAGE 13
LT1794 U W U U APPLICATIO S I FOR ATIO Similar results can be obtained with the LT1794CSW in the wide SO-20 package. With this package heat is conducted primarily through the V – pins, Pins 4 to 7 and 14 to 17; these pins should be soldered directly to the PCB metal plane. Important Note: The metal planes used for heat sinking the LT1794 are electrically connected to the negative supply potential of the driver, typically – 12V.
PAGE 14
LT1794 U W U U APPLICATIO S I FOR ATIO + VI RC – RF CBIG RBT VO – CC RG VI VO CABLE OR LINE WITH CHARACTERISTIC IMPEDANCE RL + VO = 1 AT LOW FREQUENCIES VI R = 1 + F AT MEDIUM FREQUENCIES RG =1+ RL RF 1794 F13 RBT = RL VO 1 = (1 + RF/RG) VI 2 RG RF AT HIGH FREQUENCIES (RC || RG) 1794 F12 Figure 12.
PAGE 15
LT1794 U W U U APPLICATIO S I FOR ATIO Note that the overall gain is increased: RP2 / (RP2 + RP1) VO = VI (1+ 1/n) / (1+ RF /RG ) − RP1/(RP2 + RP1) [ ] [ ] A simpler method of using positive feedback to reduce the back-termination is shown in Figure 15. In this case, the drivers are driven differentially and provide complementary outputs.
PAGE 16
LT1794 U W U U APPLICATIO S I FOR ATIO Using DC blocking capacitors, as shown in Figure 16, to AC couple the signal to the transformer eliminates the possibility for DC current to flow under any conditions. These capacitors should be sized large enough to not impair the frequency response characteristics required for the data transmission. Another important fault related concern has to do with very fast high voltage transients appearing on the telephone line (lightning strikes for example).
PAGE 17
LT1794 W W SI PLIFIED SCHE ATIC (one amplifier shown) V+ Q9 Q10 Q13 Q17 Q7 Q3 –IN Q1 C1 R1 Q6 Q2 Q5 C2 Q4 Q14 +IN OUT Q15 Q8 Q18 Q16 Q12 Q11 V– 1794 SS 17
PAGE 18
LT1794 U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. FE Package 20-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation CA 6.40 – 6.60* (.252 – .260) 4.95 (.195) 4.95 (.195) 20 1918 17 16 15 14 13 12 11 6.60 ±0.10 2.74 (.108) 4.50 ±0.10 2.74 6.40 (.108) BSC SEE NOTE 4 0.45 ±0.05 1.05 ±0.10 0.65 BSC 1 2 3 4 5 6 7 8 9 10 RECOMMENDED SOLDER PAD LAYOUT 1.20 (.047) MAX 4.30 – 4.50* (.169 – .177) 0° – 8° 0.09 – 0.20 (.0036 – .0079) 0.
PAGE 19
LT1794 U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. SW Package 20-Lead Plastic Small Outline (Wide 0.300) (LTC DWG # 05-08-1620) 0.496 – 0.512* (12.598 – 13.005) 20 19 18 17 16 15 14 13 12 11 0.394 – 0.419 (10.007 – 10.643) NOTE 1 0.291 – 0.299** (7.391 – 7.595) 0.010 – 0.029 × 45° (0.254 – 0.737) 1 2 3 4 5 6 7 8 9 0.093 – 0.104 (2.362 – 2.642) 10 0.037 – 0.045 (0.940 – 1.143) 0° – 8° TYP 0.009 – 0.013 (0.229 – 0.330) NOTE 1 0.016 – 0.050 (0.
PAGE 20
LT1794 U TYPICAL APPLICATIO 12V 24.9k + +IN SHDN 1/2 LT1794 13.7Ω – 1k 1:1.2* 1.65k • • 182Ω 100Ω LINE 1.65k 1000pF 182Ω 1k – 1/2 LT1794 –IN + 13.7Ω *COILCRAFT X8502-A OR EQUIVALENT 1W DRIVER POWER DISSIPATION 1.15W POWER CONSUMPTION SHDNREF 1794 F17 –12V Figure 17.