7

LTC1483

APPLICATIO S I FOR ATIO

The LTC1483 output stage of Figure 9 eliminates these

problems by adding two Schottky diodes, SD3 and SD4.

The Schottky diodes are fabricated by a proprietary modi-

fication to the standard N-well CMOS process. When the

output stage is operating normally, the Schottky diodes

are forward biased and have a small voltage drop across

them. When the output is in the high impedance state and

D1 or D2 still turns on, but SD3 or SD4 will reverse bias and

prevent current from flowing into the N-well or the sub-

strate. Thus the high impedance state is maintained even

with the output voltage beyond the supplies. With no

minority carrier current flowing into the N-well or sub-

strate, latch-up is virtually eliminated under power-up or

power-down conditions.

The LTC1483 output stage will maintain a high impedance

state until the breakdown of the N-channel or P-channel is

reached when going positive or negative respectively. The

voltage plus a Schottky diode drop, but this voltage is well

beyond the RS485 operating range. An ESD cell protects

output against multiple

±10kV human body model ESD

strikes. Because the ESD injected current in the N-well or

substrate consists of majority carriers, latch-up is pre-

vented by careful layout techniques.

Slew Rate

The LTC1483 is designed for systems that are sensitive to

electromagnetic radiation. The part features a slew rate

limited driver that reduces high frequency electromag-

netic emissions, while improving signal fidelity by reduc-

ing reflections due to misterminated cables. Figures 11

and 12 show the spectrum of the signal at the driver output

for a standard slew rate RS485 driver and the slew rate

limited LTC1483. The LTC1483 shows significant reduc-

tion of the high frequency harmonics. Because the driver

is slew rate limited, the maximum operating frequency is

limited to 250kbits/s.

Low Power Operation

The LTC1483 is designed to operate with a quiescent

current of 120

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 circuits as described herein will not infringe on existing patent rights.

05

4

3

FREQUENCY (MHz)

2

1

20

10

0

–10

–20

–30

–40

–50

–60

–70

–80

05

4

3

FREQUENCY (MHz)

2

1

20

10

0

–10

–20

–30

–40

–50

–60

–70

–80

drop to this 120

µA level. With the driver enabled there will

be additional current drawn by the internal 12k resistor.

Under normal operating conditions this additional current

is overshadowed by the current drawn by the external bus

impedance.

Figure 12. Slew Rate Limited LTC1483 Driver Output

Spectrum Transmitting at 150kHz

Figure 11. Typical RS485 Driver Output Spectrum

Transmitting at 150kHz