Datasheet

9
LT1785/LT1785A
LT1791/LT1791A
APPLICATIO S I FOR ATIO
UU W U
The high overvoltage rating of the LT1785/LT1791 facili-
tates easy extension to almost any level. Simple discrete
component networks that limit the receiver input and
driver output voltages to less than ±60V can be added to
the device to extend protection to any desired level. Figure
11 shows a protection network against faults to the
120VAC line voltage.
The LT1785/LT1791 protection is achieved by using a high
voltage bipolar integrated circuit process for the trans-
ceivers. The naturally high breakdown voltages of the
bipolar process provides protection in powered-off and
high impedance conditions. The driver outputs use a
foldback current limit design to protect against overvolt-
age faults while still allowing high current output drive.
ESD Protection
The LT1785/LT1791 I/O pins have on-chip ESD protection
circuitry to eliminate field failures caused by discharges to
exposed ports and cables in application environments.
The LT1785 pins A and B and the LT1791 driver output
pins Y and Z are protected to IEC-1000-4-2 level 2. These
pins will survive multiple ESD strikes of ±15kV air dis-
charge or ±4kV contact discharge. Due to their very high
input impedance, the LT1791 receiver pins are protected
to IEC-1000-4-2 level 2, or ±15kV air and ±4kV contact
discharges. This level of ESD protection will guarantee
immunity from field failures in all but the most severe ESD
environments. The LT1791 receiver input ESD tolerance
may be increased to IEC level 4 compliance by adding 2.2k
resistors in series with these pins.
Low Power Shutdown
The LT1785/LT1791 have RE and DE logic inputs to
control the receive and transmit modes of the transceiv-
ers. The RE input allows normal data reception when in the
low state. The receiver output goes to a high impedance
state when RE is high, allowing multiplexing the RO data
line. The DE logic input performs a similar function on the
driver outputs. A high state on DE activates the differential
driver outputs, a low state places both driver outputs into
high impedance. Tying the RE and DE logic inputs to-
gether may be done to allow one logic signal to toggle the
transceiver from receive to transmit modes. The DE input
is used as the data input in CAN bus applications.
Disabling both the driver and receiver places the device
into a low supply current shutdown mode. An internal time
delay of 3µs minimum prevents entering shutdown due to
small logic skews when a toggle between receive and
transmit is desired. The recovery time from shutdown
mode is typically 12µs. The user must be careful to allow
for this wake-up delay from shutdown mode. To allow full
250kbaud data rate transmission in CAN applications, the
RE pin should be tied low to prevent entering shutdown
mode.
Slew Limiting for EMI Emissions Control
The LT1785/LT1791 feature controlled driver output slew
rates to control high frequency EMI emissions from
equipment and data cables. The slew limiting limits data
rate operation to 250kbaud. Slew limiting also mitigates
the adverse affects of imperfect transmission line termi-
nation caused by stubs or mismatched cable. In some low
speed, short distance networks, cable termination may be
eliminated completely with no adverse effect on data
transmission.
Data Network Cable Selection and Termination
Long distance data networks operating at high data trans-
mission rates should use high quality, low attenuation
cable with well-matched cable terminations. Short dis-
tance networks at low data rates may use much less
expensive PVC cable. These cables have characteristic
impedances as low as 72Ω. The LT1785/LT1791 output
drivers are guaranteed to drive cables as low as 72Ω.