Hot Swap Capability
With the LTC1688/LTC1689 outputs disabled but con-
nected to the transmission line, the user can turn on/off the
power to the LTC1688/LTC1689 without inducing a differ-
ential signal on the transmission line. Due to capacitive
coupling, however, there can be a small amount of com-
mon mode charge injected into both disabled outputs,
which is not seen as a differential signal (see Figure 7). The
disabled outputs can be hooked/unhooked to a transmis-
sion line without disturbing the existing data.
Output Short-Circuit Protection
In addition to 100Mbps operation and Hot Swap capability,
the LTC1688/LTC1689 employ voltage sensing short-
circuit protection that reduces short-circuit current by
over an order of magnitude. For a given input polarity, this
circuitry determines what the correct output level should
be. If the output level is different from the expected, the
circuitry shuts off the big output devices. Much smaller
devices are instead turned on, thus producing a much
smaller short-circuit output current (3mA typical). For
example, if the driver input is > 2V, it expects the “A” output
to be >3.25V and the “B” output to be less than 1.75V. If
the “A” output is subsequently shorted to a voltage below
/2, this circuitry shuts off the big outputs and turns on
3mA current sources instead (the converse applies to the
“B” output). Note that these 3mA current sources are
active only during a short-circuit fault. During normal
operation, the regular output drivers can sink/source
A time-out period of about 50ns is required before a short-
circuit fault is detected. This circuitry might falsely detect
a short under excess output capacitive load (>200pF).
Additionally, a short might go undetected if there is too
much resistance (user inserted or cable parasitic) between
the physical short and the actual driver output.
For cables with the recommended RS485 termination (no
DC bias on the cable, see Figure 8), the LTC1688/LTC1689
will automatically come out of short-circuit mode once the
physical short has been removed.
To prevent permanent damage to the part, the maximum
allowable short is 10V (not 12V). Note that during a short,
the voltage right at the pin should not ring to a voltage
higher than 12V. Instability could surface if the short is
made with long leads (parasitic inductance). Once the
short is removed, the instability will disappear.
Figure 7. Common Mode Charge Injection During Hot Swapping