ISL3298EMRTEP

FN7607 Rev.2.00

Page 6 of 11

Oct 24, 2019

Hot Plug Function

When a piece of equipment powers-up, there is a period of time

where the processor or ASIC driving the RS-485 control line (DE)

is unable to ensure that the RS-485 Tx 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 ISL3298EMRTEP incorporates a “Hot Plug” function.

During power-up, circuitry monitoring VCC ensures that the Tx

outputs remain disabled for a period of time, regardless of the state

of DE. 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 this device includes class 3 (8kV) Human Body

Model (HBM) ESD protection structures, but the RS-485 pins

(driver outputs) incorporate advanced structures allowing it to

survive ESD events in excess of ±16.5kV HBM and ±7kV to the

IEC61000 contact test method. 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 (e.g., transient

suppression diodes), and the associated, undesirable capacitive

load they present.

Data Rate, Cables, and Terminations

RS-485/RS-422 are intended for network lengths up to 4000’,

but the maximum system data rate decreases as the

transmission length increases. Devices operating at 16Mbps are

limited to lengths less than 100’.

Twisted pair is the cable of choice for RS-485/RS-422 networks.

Twisted pair cables tend to pick up noise and other

electromagnetically induced voltages as common mode signals,

which are effectively rejected by the differential receivers in

these ICs. Proper termination is imperative, to minimize

reflections.

In point-to-point, or point-to-multipoint (single driver on bus)

networks, the main cable should be terminated in its

characteristic impedance (typically 120Ω) at the end farthest

from the driver. In multi-receiver applications, stubs connecting

receivers to the main cable should be kept as short as possible.

Multipoint (multi-driver) systems require that the main cable be

terminated in its characteristic impedance at both ends. Stubs

connecting a transmitter or receiver to the main cable should be

kept as short as possible.

Driver Overload Protection

As stated previously, the RS-485 specification requires that

drivers survive worst case bus contentions undamaged. These

drivers meet this requirement, for VCC ≤ 3.6V, via driver output

short circuit current limits, and on-chip thermal shutdown

circuitry.

The driver output stages incorporate short circuit current limiting

circuitry which ensures that the output current never exceeds the

RS-485 specification, for VCC  3.6V, even at the common mode

voltage range extremes. Additionally, these devices utilize a

foldback circuit which reduces the short circuit current, and thus

the power dissipation, whenever the contending voltage exceeds

either VCC or GND.

In the event of a major short circuit condition, devices also include

a thermal shutdown feature that disables the drivers whenever the

die temperature becomes excessive. This eliminates the power

dissipation, allowing the die to cool. The drivers automatically

re-enable after the die temperature drops about +20°C. If the

contention persists, the thermal shutdown/re-enable cycle repeats

until the fault is cleared.

At VCC > 3.6V, the instantaneous short circuit current is high enough

that output stage damage may occur during short circuit conditions

to voltages outside of GND to VCC, before the short circuit limiting

and thermal shutdown activate. For VCC = 5V operation, if output

short circuits are a possibility (e.g., due to bus contention), it is

recommended that a 5Ω resistor be inserted in series with each

output. This resistor limits the instantaneous current below levels

that can cause damage. The driver VOD at VCC = 5V is so large that

this small added resistance has little impact.

High Temperature Operation

Due to power dissipation and instantaneous output short circuit

current levels at VCC = 5V, these transmitters may not be

operated at +125°C with VCC > 3.6V.

At VCC = 3.6V, the device may be operated at +125°C, while

driving a 100’, double terminated, CAT 5 cable at 16Mbps,

without triggering the thermal SHDN circuit.

Low Power Shutdown Mode

This BiCMOS transmitter uses a fraction of the power required by

its bipolar counterparts and it also includes a shutdown feature

that reduces the already low quiescent ICC to a 1µA trickle. This

device enters shutdown whenever the driver disables (DE = GND).