LTC2846 Linear Technology, LTC2846 Datasheet - Page 15

• Manufacturer Part Number: LTC2846
• Description: 3.3V Software-Selectable Multiprotocol Transceiver with Termination
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
In V.28 mode, S3 is closed inside the LTC2846/LTC2844
which connects a 6k (R8) impedance to ground in parallel
with 20k (R5) plus 10k (R6) for a combined impedance of
5k as shown in Figure 25. Proper termination is only pro-
vided when the B input of the receivers is floating, since S1
of the LTC2846’s R2 and R3 receivers remains on in V.28
mode
LTC2846/LTC2844 receiver and connected to a TTL level
reference voltage to give a 1.4V receiver trip point.
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
'
4
connected to A, input B
GENERATOR
1
50
50
. The noninverting input is disconnected inside the
C
A
B
'
'
'
Figure 27. V.35 Receiver Configuration
125
R1
51.5
S1
R2
51.5
Figure 26. Typical V.35 Interface
S2
A
B
C
124
U
R3
INTERCONNECTING
BALANCED
CABLE
GND
R8
6k
S3
'
U
connected to B, and ground C
20k
20k
R5
R4
C
B
A
'
W
'
'
TERMINATION
R6
10k
R7
10k
125
CABLE
RECEIVER
LTC2846
LOAD
50
50
2846 F27
U
RECEIVER
2846 F26
'
100
nals (A
In V.35 mode, both switches S1 and S2 inside the LTC2846
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
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 V
transceiver drops to less than 300 A while its V
current drops to less than 10 A. Note that the LTC2846’s
R2 and R3 receivers continue to be terminated by a 103
differential impedance.
Charge Pump
The LTC2846 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 and
C3. The V
All capacitors are 16V and should be placed as close as
possible to the LTC2846 to reduce EMI.
5V
'
10 , and the impedance between shorted termi-
and B
C3
1 F
EE
DD
capacitor C4 should be a minimum of 3.3 F.
'
and V
) and ground (C
C1
1 F
C5
10 F
Figure 28. Charge Pump
7
6
5
8
EE
V
C1
C1
V
DD
CC
+
–
as shown in Figure 28. A voltage
LTC2846
EE
. Three 1 F surface mounted
')
GND
DD
C2
C2
V
must be 150
CC
EE
+
–
and a voltage inverter
supply current to the
33
32
31
30
15 .
+
LTC2846
2846 F28
C2
1 F
C4
3.3 F
IN
sn2846 2846fs
15
15 .
supply
to