7

LTC1324

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

Thermal Shutdown Protection

The LTC1324 includes a thermal shutdown circuit which

protects against prolonged shorts at the driver outputs. If

a driver output is shorted to another output, ground or to

the power supply, the current will be initially limited to a

maximum of 250mA. When the die temperature rises

above 150

°C, the thermal shutdown circuit turns off the

driver outputs. When the die cools to about 130

°C, the

outputs turn on. If the short still exists, the part will heat

again and the cycle will repeat. This oscillation occurs at

about 10Hz and prevents the part from being damaged by

excessive power dissipation. When the short is removed,

the part will return to normal operation.

Power Shutdown

The power shutdown feature of the LTC1324 is designed

for battery-powered systems. When SHDN is forced high,

the part enters shutdown mode. In shutdown, the supply

current typically drops from 1mA to 1

µAandthedriverand

receiver outputs are three-stated.

Supply Bypassing

errors. A 1

EMI Filters and Slew Rate Control

Most LocalTalk applications need to use an electromag-

netic interference (EMI) filter consisting of a resistor-

capacitor T network between each driver, receiver and the

connector. Unfortunately, the resistors will attenuate the

driver’s output signal applied to the cable. Because the

LTC1324 uses a single 5V supply, the resistors’ values

should be reduced to 5.1

Ω to ensure enough voltage

swing on the cable (Figure 8). Another way to get maxi-

mum swing and EMI immunity is to use a ferrite bead and

capacitor as the T network (Figure 9). For data rates below

250kbps, the LTC1324 features a low EMI mode which

limits the rise time of the drivers to 400ns. With a lower

rise time, the EMI network can be eliminated, allowing

more signal voltage to reach the cable. Figures 10 and 11

show the output signals of the driver with different slew

rates.

100pF

1324 F09

FERRITE BEAD

FERRITE BEAD

Figure 9

Figure 8

5.1

Ω

1324 F08

100pF

100pF

TXD

5.1

Ω

120

Ω

5.1

Ω

5.1

Ω

Figure 7. Receiver Enable and Disable

1.5V

3V

0V

1.5V

5V

0.5V

OUTPUT NORMALLY LOW

0V

1324 F07

2.3V

2.3V

OUTPUT NORMALLY HIGH

0.5V

RXEN

RXO, RXO, RXDO

RXO, RXO, RXDO