7

LTC1688/LTC1689

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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

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

> 50mA.

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

A OUTPUT

B OUTPUT