11

LTC1685

single-ended trace over an adjacent ground plane. Then

set the (bypassed) negative input of the receiver to roughly

2.5V. Note that single-ended operation might not reach

maximum speeds.

High Speed Differential SCSI (Fast-20, Fast-40 HVD)

The LTC1685’s high speed, tight propagation delay win-

dow and matched driver/receiver propagation delays make

it a natural choice as the external transceiver in high speed

differential SCSI applications. Note that the

±3.5ns propa-

gation delay window covers the entire commercial tem-

perature range. If, for example, a group of 16 transceivers

is placed on the same board, their temperature difference

will be much smaller. Hence, the difference in their propa-

gation delays should be even better than the

±3.5ns

specification (typically better than

±2ns). The LTC1685 is

the most efficient and reliable implementation that meets

the Fast-20 and Fast-40 HVD driver and receiver skew

specifications.

Power-Up Requirements

The LTC1685 has unique short-circuit protection that

shuts off the big output devices (and keeps them off) when

a short is detected. When the LTC1685 is powered up with

the driver outputs enabled (Figure 15 shows a typical

connection), the part will power up in short-circuit mode.

After power-up, the user must hold the DE pin of the

LTC1685 low for at least 200ns in order to start normal

operation. Note also that turning the termination power

on/off might induce the LTC1685 to see a “short.” Conse-

quently, the DE pin should be held low for 200ns after

cable termination power is turned on.

This requirement is solely due to the cable termination

(the 165

Ω parallel resistance to both power and ground).

For applications whose connections to the cable are

made exclusively with RS485 devices, the cable can be

terminated

only across the two signal wires (as in Figure

10). With cable distances covering under 25 meters, the

common mode range of the LTC1685 should be more

than sufficient to account for any ground differences

between any two communicating devices. The fact that

transmission is differential should greatly improve noise

TERM POWER

150

Ω

330

Ω

330

Ω

330

Ω

330

Ω

A 1

4

EN

RO

1685 F15

1/4 LTC1518

LTC1685

LTC1685

12

3

2 B

7

6

3

2

DE

DI

RO

RE

TERM POWER

122

Ω CABLE

4

1

150

Ω

7

6

3

2

DE

DI

RO

RE

4

1

EN

Figure 15. Fast-20, Fast-40 Differential SCSI Application

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.

margin. Furthermore, the good high frequency CMRR of

the receiver will serve to reject any common mode

interference.

DE, DI Inputs

It is not necessary that the driver input (DI) have 0V to 3V

signal levels. The DI input can be driven by CMOS levels

(0V to 5V) and still achieve 40Mbps operation. However,

duty cycle will be slightly compromised when driven by a

CMOS device. Care should be taken to minimize the

ringing on the DI input in order to achieve a driver

propagation delay within the

±3.5ns window. This also

improves the package-to-package matching of propaga-

tion delays.

The DE pin should be held low for 200ns after the power-

up sequence has been completed. After fault conditions

such as an output short or thermal shutdown, the DE pin

should be held low for at least 200ns after the fault has

been removed. This is usually necessary only if the driver

outputs are connected to DC-biased cable terminations

(as in Figure 15).

Layout Considerations

A ground plane is recommended when using a high

frequency device like the LTC1685. A 0.1

µF ceramic by-

recommended. Good bypassing is especially needed when

operating at maximum frequency or when package-to-

package matching is very important. The PC board traces

connected to the “A” and “B” outputs must be kept as

symmetrical and short as possible to obtain the same