Datasheet
14
LTC2844
sn2844 2844fs
A
A
'
B
C
B
'
C
'
GENERATOR
BALANCED
INTERCONNECTING
CABLE
LOAD
CABLE
TERMINATION
RECEIVER
2844 F18
50Ω
125Ω
50Ω
50Ω
125Ω
50Ω
Figure 18. Typical V.35 Interface
Figure 19. V.35 Receiver Configuration
V.35 Interface
A typical V.35 balanced interface is shown in Figure 18. A
V.35 differential generator with outputs A and B with
ground C is connected to a differential receiver with
ground C', inputs A' connected to A, B' connected to B. The
V.35 interface requires a T or delta network termination at
the receiver end and the generator end. The receiver
differential impedance measured at the connector must be
100Ω␣ ±10Ω, and the impedance between shorted termi-
nals (A' and B') and ground C' must be 150Ω ±15Ω.
In V.35 mode, both switches S1 and S2 inside the LTC2846
are on, connecting the T network impedance as shown in
Figure 19. 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 50Ω to
150Ω and the impedance between shorted terminals (A
and B) and ground C must be 150Ω ±15Ω. For the
generator termination, switches S1 and S2 are both on as
shown in Figure 20.
No-Cable Mode
The no-cable mode (M0 = M1 = M2 = 1) is intended for the
case when the cable is disconnected from the connector.
The 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 600µA.
LTC2846 Supplies
The LTC2846 uses an internal capacitive charge pump to
generate V
DD
and V
EE
as shown in Figure 21. A voltage
doubler generates about 8V on V
DD
and a voltage inverter
generates about – 7.5V for V
EE
. Three 1µF surface mounted
tantalum or ceramic capacitors are required for C1, C2 and
C3. The V
EE
capacitor C4 should be a minimum of 3.3µF.
All capacitors are 16V and should be placed as close as
possible to the LTC2846 to reduce EMI.
The LTC2846 has an internal boost switching regulator
which generates a 5V output from the 3.3V supply as
shown in Figure 22. The 5V V
CC
supplies its internal charge
pump and transceivers as well as its companion chip.
APPLICATIONS INFORMATION
WUU
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Figure 20. V.35 Driver
V.35 DRIVER
A
B
C
51.5Ω
S2
S1
2844 F20
51.5Ω
LTC2846
124Ω
R3
124Ω
R5
20k
LTC2846
RECEIVER
2844 F19
A
'
B
'
C
'
R1
51.5Ω
R8
6k
S2
S3
R2
51.5Ω
R6
10k
R7
10k
GND
R4
20k
S1
33
32
31
30
2844 F21
7
6
5
8
C3
1µF
C5
10µF
5V
C1
1µF
C2
1µF
C4
3.3µF
LTC2846
V
DD
C1
+
C1
–
V
CC
C2
+
C2
–
V
EE
GND
+
Figure 21. Charge Pump