Datasheet > LTC1689

LTC1689

更新时间： 2023-12-20 18:44:06

品牌	Logo	应用领域
亚德诺 - ADI		驱动驱动器
页数	文件大小	规格书
12页	224K
描述
100Mbps、RS485 可热插拔式 4 驱动器

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LTC1689 数据手册

LTC1688/LTC1689

W U U

APPLICATIONS INFORMATION

Hot Swap Capability

V_DD/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

>50mA.

With the LTC1688/LTC1689 outputs disabled but con-

nectedtothetransmissionline,theusercanturnon/offthe

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,

whichisnotseenasadifferentialsignal(seeFigure7). The

disabled outputs can be hooked/unhooked to a transmis-

sion line without disturbing the existing data.

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

muchresistance(userinsertedorcableparasitic)between

the physical short and the actual driver output.

Output Short-Circuit Protection

Inadditionto100MbpsoperationandHotSwapcapability,

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,ifthedriverinputis>2V,itexpectsthe“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

For cables with the recommended RS485 termination (no

DCbiasonthecable, seeFigure8), theLTC1688/LTC1689

willautomaticallycomeoutofshort-circuitmodeoncethe

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.

A OUTPUT

B OUTPUT

Figure 7. Common Mode Charge Injection During Hot Swapping

16889fa

1...4 5 678 9 10...12

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