LTC2850/LTC2851/LTC2852
11
285012fe
For more information www.linear.com/LTC2850
APPLICATIONS INFORMATION
Driver
The driver provides full RS485/RS422 compatibility. When
enabled, if DI is high, Y-Z is positive for the full-duplex
devices (LTC2851, LTC2852) and A-B is positive for the
half-duplex device (LTC2850).
When the driver is disabled, both outputs are high im-
pedance. For the full-duplex devices, the leakage on the
driver output pins is guaranteed to be less
than 10µA
over the entire common mode range of –7V to 12V. On
the half-duplex LTC2850, the impedance is dominated by
the receiver input resistance, R
IN
.
Driver Overvoltage and Overcurrent Protection
The driver outputs are protected from short-circuits
to any voltage within the Absolute Maximum range of
(V
CC
– 15V) to 15V. The typical peak current in this con-
dition does not exceed 180mA
.
If a high driver output is shorted to a voltage just above
V
CC
, a reverse current will flow into the supply. When
this voltage exceeds V
CC
by about 1.4V, the reverse
current turns off. Preventing the driver from turning off
with outputs shorted to output voltages just above V
CC
keeps the driver active even for receiver loads that have
a positive common mode with respect
to the driver—
a valid condition.
The worst-case peak reverse short-circuit current can be as
high as 300mA in extreme cold conditions. If this current
can not be absorbed by the supply, a 3.6V Zener diode can
be added in parallel with the supply to sink this current.
All devices also feature thermal shutdown protection that
disables the driver and receiver in case of excessive
power
dissipation (see Note 4 in the Electrical Characteristics
section).
Receiver and Failsafe
With the receiver enabled, when the absolute value of
the differential voltage between the A and B pins is
greater than 200mV, the state of RO will reflect the
polarity of (A-B)
These parts have a failsafe feature that guarantees the
receiver output to be in a logic-high state when the inputs
are either shorted, left open, or terminated but not driven.
This failsafe feature is guaranteed to work for inputs
spanning the entire common mode range of –7V to 12V.
The receiver output is internally driven high (to V
CC
) or
low (to ground) with no external pull-up needed. When the
receiver is disabled the RO pin becomes Hi-Z with leakage
of less than ±1µA for voltages
within the supply range.
Receiver Input Resistance
The receiver input resistance from A or B to ground is
guaranteed to be greater than 96k (C-, I-grade). This is 8x
higher than the requirements for the RS485 standard and
thus this receiver represents a one-eighth unit load. This,
in turn, means that 8x the standard number of receivers,
or 256 total, can be connected to a line
without loading
it beyond what is specified in the RS485 standard. The
receiver input resistance from A or B to ground on high
temperature H-grade parts is greater than 48k providing
a one-quarter unit load. The high input resistance of the
receiver is maintained whether it is enabled or disabled,
powered or unpowered.
Supply Current
The unloaded static supply currents in these devices are
very low
, typically under 500µA for all modes of opera-
tion. In applications with resistively terminated cables,
the supply current is dominated by the driver load. For
example, when using two 120Ω terminators with a dif-
ferential driver output voltage of 2V, the DC load current
is 33mA, which is sourced by the positive voltage supply.
Power supply current increases with toggling data due to
capacitive loading and this
term can increase significantly
at high data rates. Figure 13 shows supply current vs
data rate for two different capacitive loads for the circuit
configuration of Figure 4.
High Speed Considerations
A ground plane layout is recommended. A 0.1µF bypass
capacitor less than one-quarter inch away from the V
CC
pin
is also recommended. The PC board traces connected to
signals A/B and Z/Y should be symmetrical
and as short
as possible to maintain good differential signal integrity.
