11
LTC1685
APPLICATIONS INFORMATION
WUU
U
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 165Ω 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
150Ω
330Ω
330Ω
330Ω
330Ω
A 1
4
EN
RO
1685 F15
1/4 LTC1518 LTC1685LTC1685
12
3
2 B
7
6
3
2
DE
DI
RO
RE TERM POWER
122Ω CABLE
4
1
150Ω
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 0.1µF ceramic by-
pass capacitor less than 1/4 inch away from the V
DD
pin is
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
