LT1794CFE Linear Technology, LT1794CFE Datasheet - Page 8

• Manufacturer Part Number: LT1794CFE
• Description: IC AMP XDSL LINE DRIVER 20TSSOP
• Manufacturer: Linear Technology
• Type: Line Driver, Transmitterr

Specifications of LT1794CFE

• Number Of Drivers/receivers: 2/0
• Protocol: xDSL
• Voltage - Supply: 18V
• Mounting Type: Surface Mount
• Package / Case: 20-TSSOP Exposed Pad, 20-eTSSOP, 20-HTSSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
LT1794
8
Figure 1. Internal Current Biasing Circuitry
CIRCUITRY
START-UP
I
I
I
BIAS
SUPPLY
U
TO
=
2
5
PER AMPLIFIER (mA) = 64 • I
5I
I
SHDNREF
SHDN
U
30
25
20
15
10
45
40
35
30
25
20
15
10
SHDN
5
0
5
0
7
4
= I
V
V
2k
S
S
SHDNREF
= 12V
= 12V
2I
7
W
10
1794 F01
1k
10
2I
I
TO AMPLIFIERS
BIAS CIRCUITRY
BIAS
Figure 3. R
30
BIAS
Figure 2. R
40
50
U
70
BIAS
70
90
BIAS
to Ground Current Control
V
R
R
100
+
BIAS
BIAS
= 12V
to V
V
R
+
100
130
SHDN
SHDNREF
(k )
(k )
BIAS
= 12V
+
outputs are in a high impedance state. Part to part varia-
tions however, will cause inconsistent control of the qui-
escent current if direct voltage drive of the SHDN pin is used.
Using a single external resistor, R
two ways provides a much more predictable control of the
quiescent supply current. Figure 2 illustrates the effect on
supply current per amplifier with R
tween the SHDN pin and the 12V V
and the approximate design equations. Figure 3 illustrates
the same control with R
SHDNREF pin and ground while the SHDN pin is tied to V
Either approach is equally effective.
R
Current Control
SHDN
SHDNREF
BIAS
I
R
150
S
BIAS
I
R
PER AMPLIFIER (mA)
S
BIAS
PER AMPLIFIER (mA)
=
170
130
I
=
S
I
PER AMPLIFIER (mA)
S
PER AMPLIFIER (mA)
190
V
+
V
– 1.2V
+
210
– 1.2V
160
230
R
V
BIAS
+
R
V
– 1.2V
BIAS
+
250
• 25.6 – 2k
– 1.2V
+ 2k
190
• 64 – 5k
+ 5k
270 290
• 25.6
1794 F03
1794 F02
• 64
BIAS
connected between the
BIAS
+
supply of the LT1794
, connected in one of
BIAS
connected be-
+
.