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PDF LT1794 Data sheet ( Hoja de datos )
| Número de pieza | LT1794 | |
| Descripción | xDSL Line Driver Amplifier | |
| Fabricantes | Linear | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LT1794 (archivo pdf) en la parte inferior de esta página. Total 20 Páginas | ||
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FEATURES
s Exceeds All Requirements For Full Rate,
Downstream ADSL Line Drivers
s ±500mA Minimum IOUT
s ±11.1V Output Swing, VS = ±12V, RL = 100Ω
s ±10.9V Output Swing, VS = ±12V, IL = 250mA
s Low Distortion: – 82dBc at 1MHz, 2VP-P Into 50Ω
s Power Saving Adjustable Supply Current
s Power Enhanced Small Footprint Packages:
20-Lead TSSOP and 20-Lead SW
s 200MHz Gain Bandwidth
s 500V/µs Slew Rate
s Specified at ±15V, ±12V and ±5V
U
APPLICATIO S
s High Density ADSL Central Office Line Drivers
s High Efficiency ADSL, HDSL2, G.lite,
SHDSL Line Drivers
s Buffers
s Test Equipment Amplifiers
s Cable Drivers
LT1794
Dual 500mA, 200MHz
xDSL Line Driver Amplifier
DESCRIPTIO
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.
TYPICAL APPLICATIO
High Efficiency ±12V Supply ADSL Central Office Line Driver
12V
+IN
110Ω
+
1/2
LT1794
–
1k
RBIAS
24.9k
SHDN
12.7Ω
1:2*
1000pF
110Ω
1k
100Ω
–
1/2
LT1794
12.7Ω
*COILCRAFT X8390-A OR EQUIVALENT
ISUPPLY = 10mA PER AMPLIFIER
WITH RBIAS = 24.9k
–IN + SHDNREF
1794 TA01
–12V
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LT1794
Open-Loop Gain and Phase
vs Frequency
120 120
100 80
PHASE
80 40
60 0
40 –40
20 –80
GAIN
0 –120
–20 TA = 25°C
–40
VS = ±12V
AV = –10
–60 RL = 100Ω
IS PER AMPLIFIER = 10mA
–80
100k 1M 10M
FREQUENCY (Hz)
–160
–200
–240
–280
100M
1794 G07
–3dB Bandwidth
vs Supply Current
45
TA = 25°C
40 VS = ±12V
AV = 10
35 RL = 100Ω
30
25
20
15
10
5
0
2 4 6 8 10 12 14
SUPPLY CURRENT PER AMPLIFIER (mA)
1794 G08
CMRR vs Frequency
100
TA = 25°C
90 VS = ±12V
80 IS = 10mA PER AMPLIFIER
70
60
50
40
30
20
10
0
0.1 1
10 100
FREQUENCY (MHz)
1794 G10
Output Impedance vs Frequency
1000
TA = 25°C
VS ±12V
100
IS PER
AMPLIFIER = 2mA
10
IS PER
1 AMPLIFIER = 10mA
IS PER
0.1 AMPLIFIER = 15mA
0.01
0.01
0.1 1 10
FREQUENCY (MHz)
100
1734 G13
PSRR vs Frequency
100
90
VS = ±12V
AV = 10
80 IS = 10mA PER AMPLIFIER
70
60
50
(–) SUPPLY
40
30
(+) SUPPLY
20
10
0
–10
0.01
0.1 1
10
FREQUENCY (MHz)
100
1794 G11
ISHDN vs VSHDN
2.5
TA = 25°C
VS = ±12V
2.0 VSHDNREF = 0V
1.5
1.0
0.5
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
VSHDN (V)
1794 G14
Slew Rate vs Supply Current
1000
900
800
700
TA = 25°C
VS = ±12V
AV = –10
RL = 1k
RISING
600
FALLING
500
400
300
200
100
0
2 3 4 5 6 7 8 9 10 11 12 13 14 15
SUPPLY CURRENT PER AMPLIFIER (mA)
1794 G09
Frequency Response
vs Supply Current
30
VS = ±12V
25 AV = 10
20
15
2mA PER AMPLIFIER
10
10mA PER AMPLIFIER
5
15mA PER AMPLIFIER
0
–5
–10
–15
–20
1k
10k 100k 1M 10M
FREQUENCY (Hz)
100M
1794 G12
Supply Current vs VSHDN
35
TA = 25°C
30
VS = ±12V
VSHDNREF = 0V
25
20
15
10
5
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
VSHDN (V)
1794 G14
5
5 Page 
LT1794
APPLICATIO S I FOR ATIO
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.
At full power to the line the driver power dissipation is:
PD(FULL) = 24V • 8mA + (12V – 2VRMS) • 57mARMS
+ [|–12V – (– 2VRMS)|] • 57mARMS
PD(FULL) = 192mW + 570mW + 570mW = 1.332W
The junction temperature of the driver must be kept less
than the thermal shutdown temperature when processing
a signal. The junction temperature is determined from the
following expression:
TJ = TAMBIENT (°C) + PD(FULL) (W) • θJA (°C/W)
θJA is the thermal resistance from the junction of the
LT1794 to the ambient air, which can be minimized by
heat-spreading PCB metal and airflow through the enclo-
sure as required. For the example given, assuming a
maximum ambient temperature of 85°C and keeping the
junction temperature of the LT1794 to 140°C maximum,
the maximum thermal resistance from junction to ambient
required is:
θJA(MAX)
=
140°C – 85°C
1.332W
=
41.3°C /
W
Heat Sinking Using PCB Metal
Designing a thermal management system is often a trial
and error process as it is never certain how effective it is
until it is manufactured and evaluated. As a general rule,
the more copper area of a PCB used for spreading heat
away from the driver package, the more the operating
junction temperature of the driver will be reduced. The
limit to this approach however is the need for very com-
pact circuit layout to allow more ports to be implemented
on any given size PCB.
Fortunately xDSL circuit boards use multiple layers of
metal for interconnection of components. Areas of metal
beneath the LT1794 connected together through several
small 13 mil vias can be effective in conducting heat away
from the driver package. The use of inner layer metal can
free up top and bottom layer PCB area for external compo-
nent placement.
25
20
15
10
5
0
–240 –200 –160 –120 –80
–40 0 40
ILOAD (mA)
80
Figure 7. IQ vs ILOAD
120 160 200 240
1794 F07
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LT1794.PDF ] | |
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