LTC1546C datasheet(13/20 Pages) LINER

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LTC1546

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

' connected to A, input B' connected to B, and ground C'

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

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 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

Ω to

150

Ω and the impedance between shorted terminals (A

and B) and ground (C) must be 150

Ω ±15Ω.

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

µA. Note that the LTC1546’s R2 and R3

receivers continue to be terminated by a 103

Ω differen-

tial impedance.

Charge Pump

The LTC1546 uses an internal capacitive charge pump to

generate VDD and VEE as shown in Figure 28. A voltage

doubler generates about 8V on VDD and a voltage inverter

generates about – 7.5V on VEE. Four 1µF surface mounted

tantalum or ceramic capacitors are required for C1, C2, C3

and C4. The VEE capacitor C5 should be a minimum of

3.3

µF.Allcapacitorsare16Vandshouldbeplacedasclose

as possible to the LTC1546 to reduce EMI.

Figure 28. Charge Pump

Figure 27. V.35 Receiver Configuration

Figure 26. Typical V.35 Interface

GENERATOR

BALANCED

INTERCONNECTING

CABLE

LOAD

CABLE

TERMINATION

RECEIVER

1546 F26

Ω

125

Ω

125

Ω

124

Ω

20k

LTC1546

RECEIVER

1546 F27

51.5

Ω

51.5

Ω

10k

GND

20k

1546 F28

µF

3.3

µF

LTC1546

VDD

C1+

C1–

VCC

C2+

C2–

VEE

GND

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.

APPLICATIO S I FOR ATIO

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