LTC1544 datasheet(13/20 Pages) LINER

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LTC1544

Charge Pump

The LTC1543 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. 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 LTC1543 to reduce EMI.

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 and

the top side of the center resistor is brought out to a pin so

it can be bypassed with an external capacitor to reduce

common mode noise as shown in Figure 20.

Figure 20. V.35 Driver Using the LTC1344A

V.35 DRIVER

51.5

Ω

1544 F20

51.5

Ω

LTC1344A

124

Ω

100pF

1544 F21

µF

3.3

µF

LTC1543

VDD

C1+

C1–

VCC

C2+

C2–

VEE

GND

Figure 21. Charge Pump

Any mismatch in the driver rise and fall times or skew in

the driver propagation delays will force current through

the center termination resistor to ground, causing a high

frequency common mode spike on the A and B terminals.

The common mode spike can cause EMI problems that are

reduced by capacitor C1 which shunts much of the com-

mon mode energy to ground rather than down the cable.

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 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.

Receiver Fail-Safe

All LTC1543/LTC1544 receivers feature fail-safe opera-

tion in all modes. If the receiver inputs are left floating or

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 LTC1543, 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.

APPLICATIONS INFORMATION

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