LTC1546 Linear Technology, LTC1546 Datasheet - Page 13

• Manufacturer Part Number: LTC1546
• Description: Software-Selectable Multiprotocol Transceiver with Termination
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
V.35 Interface
A typical V.35 balanced interface is shown in Figure 26. A
V.35 differential generator with outputs A and B and
ground C is connected to a differential receiver with input
A
connected via the signal return to ground C. The V.35
interface requires a T or delta network termination at the
receiver end and the generator end. The receiver differen-
tial impedance measured at the connector must be
100
nals (A
In V.35 mode, both switches S1 and S2 inside the LTC1546
are on, connecting a T network impedance as shown in
Figure 27. The 30k input impedance of the receiver is
placed in parallel with the T network termination, but does
not affect the overall input impedance significantly.
The generator differential impedance must be 50
150 and the impedance between shorted terminals (A
and B) and ground (C) must be 150
'
connected to A, input B
GENERATOR
50
50
B
C
A
'
'
'
'
10 , and the impedance between shorted termi-
and B
Figure 27. V.35 Receiver Configuration
R1
51.5
S1
R2
51.5
125
S2
Figure 26. Typical V.35 Interface
'
) and ground (C
124
A
B
C
R3
U
INTERCONNECTING
BALANCED
GND
R8
6k
CABLE
S3
'
U
connected to B, and ground C
20k
20k
R5
R4
')
must be 150
C
B
A
'
W
'
'
R6
10k
R7
10k
TERMINATION
125
CABLE
15 .
RECEIVER
LTC1546
LOAD
50
50
1546 F27
U
RECEIVER
15 .
1546 F26
to
'
No-Cable Mode
The no-cable mode (M0 = M1 = M2 = 1) is intended for
the case when the cable is disconnected from the con-
nector. The charge pump, bias circuitry, drivers and
receivers are turned off, the driver outputs are forced into
a high impedance state, and the supply current drops to
less than 200 A. Note that the LTC1546’s R2 and R3
receivers continue to be terminated by a 103 differen-
tial impedance.
Charge Pump
The LTC1546 uses an internal capacitive charge pump to
generate V
doubler generates about 8V on V
generates about – 7.5V on V
tantalum or ceramic capacitors are required for C1, C2, C3
and C4. The V
3.3 F. All capacitors are 16V and should be placed as close
as possible to the LTC1546 to reduce EMI.
Receiver Fail-Safe
All LTC1546/LTC1544 receivers feature fail-safe opera-
tion in all modes. If the receiver inputs are left floating or
are shorted together by a termination resistor, the receiver
output will always be forced to a logic high.
DTE vs DCE Operation
The DCE/DTE pin acts as an enable for Driver 3/Receiver
1 in the LTC1546, and Driver 3/Receiver 1 and Driver 4/
Receiver 4 in the LTC1544. The INVERT pin in the LTC1544
allows the Driver 4/Receiver 4 enable to be high or low true
polarity.
5V
C3
1 F
DD
and V
EE
C1
1 F
C4
1 F
capacitor C5 should be a minimum of
Figure 28. Charge Pump
3
2
1
4
EE
V
C1
C1
V
DD
CC
as shown in Figure 28. A voltage
+
–
LTC1546
EE
. Four 1 F surface mounted
DD
GND
C2
C2
V
EE
+
–
and a voltage inverter
28
27
26
25
LTC1546
+
1546 F28
C2
1 F
C5
3.3 F
13