11

LTC1323

An external load may be connected between ground and

will sink up to a maximum of 10mA while maintaining the

pin voltage between – 4.5V and – 5.5V. If an external load

creased to 4.7

µF. Both LTC1323 and the external chip

EMI Filter

Most LocalTalk applications use an electromagnetic inter-

ference (EMI) filter consisting of a resistor-capacitor T

network between each driver and receiver and the connec-

tor. Unfortunately, the resistors significantly attenuate the

drivers output signals before they reach the cable. Because

APPLICATIO S I FOR ATIO

the LTC1323 uses a single supply differential driver, the

resistor values should be reduced to 5

Ω to 10Ω to guaran-

tee adequate voltage swing on the cable (Figure 16a). In

most applications, removing the resistors completely does

not cause an increase in EMI as long as a shielded connec-

tor and cable are used (Figure 16b). With the resistors

removed the only DC load is the primary resistance of the

LocalTalk transformer. This will increase the DC standby

current when the driver outputs are active, but does not

adversely affect the drivers because they can handle a

direct indefinite short circuits without damage. Trans-

former primary resistance should be above 15

Ωtokeepthe

LTC1323 operating normally and prevent it from entering

thermal shutdown. For maximum swing and EMI immu-

nity, a ferrite bead and capacitor T network can be used

(Figure 16c).

Figure 16. EMI Filters

EXTERNAL

CHIP

GND

12

24

21

1

µF

4.7

µF

LTC1323 • F15

–5.5V

13

LTC1323

C1

+

Figure 15

TX

15

16

0.33

µF

14

13

12

11

10

9

8

1

2

5

6

7

3

4

CHARGE PUMP

LTC1323CS

5V

LocalTalk

TRANSFORMER

RX

100pF

100pF

120

Ω

100pF

100pF

LTC1323 • TA02

0.33

µF

SHDN

DATA IN

TX ENABLE

RX ENABLE

DATA OUT

1

µF

1

µF

+

Typical LocalTalk Connection

5

Ω TO 10Ω

5

Ω TO 10Ω

(a)

(b)

(c)

100pF

FERRITE BEAD

FERRITE BEAD

100pF

LTC1323 • F16

100pF

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.