LTC2844IG datasheet(14/20 Pages) LINER

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LTC2844

sn2844 2844fs

GENERATOR

BALANCED

INTERCONNECTING

CABLE

LOAD

CABLE

TERMINATION

RECEIVER

2844 F18

Ω

125

Ω

125

Ω

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

100

Ω ±10Ω, and the impedance between shorted termi-

nals (A

' and B') and ground C' must be 150

Ω ±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 50

Ω to

150

Ω and the impedance between shorted terminals (A

and B) and ground C must be 150

Ω ±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 600

µA.

LTC2846 Supplies

The LTC2846 uses an internal capacitive charge pump to

generate VDD and VEE as shown in Figure 21. A voltage

doubler generates about 8V on VDD and a voltage inverter

generates about – 7.5V for VEE. Three 1µF surface mounted

tantalum or ceramic capacitors are required for C1, C2 and

C3. The VEE 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 VCC supplies its internal charge

pump and transceivers as well as its companion chip.

APPLICATIONS INFORMATION

Figure 20. V.35 Driver

V.35 DRIVER

51.5

Ω

2844 F20

51.5

Ω

LTC2846

124

Ω

124

Ω

20k

LTC2846

RECEIVER

2844 F19

51.5

Ω

51.5

Ω

10k

GND

20k

2844 F21

µF

3.3

µF

LTC2846

VDD

C1+

C1–

VCC

C2+

C2–

VEE

GND

Figure 21. Charge Pump

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