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ISL3170EIUZ-T7A

±15kV ESD Protected, 3.3V, Full Fail-Safe, Low Power, High Speed or Slew Rate Limited, RS-485/RS-422 Transceivers

Renesas Electronics 

ISL3170E, ISL3171E, ISL3172E, ISL3173E, ISL3174E, ISL3175E, ISL3176E, ISL3177E, ISL3178E

Application Information

RS-485 and RS-422 are differential (balanced) data

transmission standards for use in long haul or noisy

environments. RS-422 is a subset of RS-485, so RS-485

transceivers are also RS-422 compliant. RS-422 is a

point-to-multipoint (multidrop) standard, which allows only

one driver and up to 10 receivers on each bus, assuming

one unit load devices.. RS-485 is a true multipoint standard,

which allows up to 32 one-unit load devices (any

combination of drivers and receivers) on each bus. To allow

for multipoint operation, the RS-485 specification requires

that drivers must handle bus contention without sustaining

any damage.

Another important advantage of RS-485 is the extended

common-mode range (CMR), which specifies that the driver

outputs and receiver inputs withstand signals that range from

+12V to -7V. RS-422 and RS-485 are intended for runs as

long as 4000ft, so the wide CMR is necessary to handle

ground potential differences, as well as voltages induced in

the cable by external fields.

Receiver Features

These devices utilize a differential input receiver for maximum

noise immunity and common mode rejection. Input sensitivity

is better than ±200mV, as required by the RS-422 and RS-485

specifications.

Receiver input resistance of 96kΩ surpasses the RS-422

specification of 4kΩ and is eight times the RS-485 “Unit Load

(UL)” requirement of 12kΩ minimum. Thus, these products

are known as “one-eighth UL” transceivers and there can be

up to 256 of these devices on a network while still complying

with the RS-485 loading specification.

Receiver inputs function with common-mode voltages as

great as +9V/-7V outside the power supplies (that is, +12V

and -7V), making them ideal for long networks where induced

voltages and ground potential differences are realistic

concerns.

All the receivers include a “Full Fail-Safe” function that

guarantees a high level receiver output if the receiver inputs

are unconnected (floating) or shorted. Fail-safe with shorted

inputs is achieved by setting the Rx upper switching point to

-50mV, thereby ensuring that the Rx sees 0V differential as a

high input level.

Receivers easily meet the data rates supported by the

corresponding driver, and all receiver outputs (except on the

ISL3171E, ISL3174E, and ISL3177E) are tri-statable using

the active low RE input.

Driver Features

The RS-485/422 driver is a differential output device that

delivers at least 1.5V across a 54Ω load (RS-485) and at

least 2V across a 100Ω load (RS-422). The drivers feature

low propagation delay skew to maximize bit width and to

minimize EMI.

All drivers are tri-statable through the active high DE input,

except on the ISL3171E, ISL3174E, and ISL3177E.

The 250kbps and 500kbps driver outputs are slew rate

limited to minimize EMI and to reduce reflections in

unterminated or improperly terminated networks. Outputs of

the ISL3176E through ISL3178E drivers are not limited, so

faster output transition times allow data rates of at least

20Mbps.

Hot Plug Function

When a piece of equipment powers up, a period of time

occurs in which the processor or ASIC driving the RS-485

control lines (DE, RE) is unable to ensure that the RS-485

Tx and Rx outputs are kept disabled. If the equipment is

connected to the bus, a driver activating prematurely during

power up may crash the bus. To avoid this scenario, the

ISL317XE versions with output enable pins incorporate a

“Hot Plug” function. During power up, circuitry monitoring VCC

ensures that the Tx and Rx outputs remain disabled for a period

of time, regardless of the state of DE and RE. This gives the

processor/ASIC a chance to stabilize and drive the RS-485

control lines to the proper states.

ESD Protection

All pins on these devices include Class 3 (>7kV) Human

Body Model (HBM) ESD protection structures, but the

RS-485 pins (driver outputs and receiver inputs)

incorporate advanced structures allowing them to survive

ESD events in excess of ±15kV HBM and ±15kV

IEC61000. The RS-485 pins are particularly vulnerable to

ESD damage because they typically connect to an exposed

port on the exterior of the finished product. Simply touching

the port pins, or connecting a cable, can cause an ESD

event that might destroy unprotected ICs. These new ESD

structures protect the device whether or not it is powered

up, and without degrading the RS-485 common mode

range of -7V to +12V. This built-in ESD protection

eliminates the need for board level protection structures

(for example, transient suppression diodes), and the

associated, undesirable capacitive load they present.

IEC61000-4-2 Testing

The IEC61000 test method applies to finished equipment,

rather than to an individual IC. Therefore, the pins most likely

to suffer an ESD event are those that are exposed to the

outside world (the RS-485 pins in this case), and the IC is

tested in its typical application configuration (power applied)

rather than testing each pin-to-pin combination. The lower

current limiting resistor coupled with the larger charge

storage capacitor yields a test that is much more severe than

the HBM test. The extra ESD protection built into this

device’s RS-485 pins allows the design of equipment

meeting Level 4 criteria without the need for additional board

level protection on the RS-485 port.

FN6307 Rev.6.00

Aug 31, 2017

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