ISL32613E, ISL32614E
7
FN7906.2
July 27, 2015
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Application Information
Driver Features
These transmitters are differential output devices that operate
with V
CC
as low as 1.8V, and up to 3.6V. Devices are RS-485
compliant with V
CC
≥ 3V, but significant power savings are
obtained by operating at V
CC
= 1.8V.
The transmitter outputs are tri-statable via the active high DE
input. If the Tx enable function is not needed, tie DE to V
CC
through a 1kΩ to 2kΩ resistor. Outputs are slew rate limited to
minimize EMI, and to reduce reflections in unterminated or
improperly terminated networks.
1.8V Operation
The ISL32613E and ISL32614E operate with V
CC
as low as 1.8V.
When coupled with the ISL32610E or ISL32611E 1.8V receivers,
they provide a differential communication link optimized for very
low power, and for slow data rates. Figures 9
and 10 illustrate the
static and dynamic power savings from using these transmitters
at low supply voltages. With V
CC
= 1.8V rather than 3.3V, using
the ISL32613E at 128kbps reduces the operating supply current
from 9.9mA to 56µA (a factor of 177).
5.5V Tolerant Logic Pins
Logic input pins (DI, DE) contain no ESD or parasitic diodes to
V
CC
, so they withstand input voltages exceeding 5.5V, regardless
of the V
CC
voltage.
Hot Plug Function
When a piece of equipment powers up, there is a period of time
during which 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, these transmitters incorporate a “Hot Plug”
function. During power-up, circuitry monitoring V
CC
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 control lines to the proper states.
ESD Protection
All pins on these devices include class 3 (8kV) Human Body
Model (HBM) ESD protection structures, but the driver outputs
incorporate advanced structures that allow them 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 it is powered up or not,
and without degrading the common mode range. 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.
Driver Overload Protection
The driver output stages incorporate short-circuit, current-limiting
circuitry, which ensures that the output current never exceeds the
RS-485 specification over a ±2V (-7V to +12V for V
CC
≥ 2.7V)
common mode voltage range.
In the event of a major short-circuit condition, the device also
includes a thermal shutdown feature that disables the drivers
whenever the die temperature becomes excessive. This eliminates
power dissipation, allowing the die to cool. The drivers
automatically reenable after the die temperature drops by about
+20°C. If the condition persists, the thermal shutdown/reenable
cycle repeats until the fault is cleared.
Low Power Shutdown Mode
This BiCMOS transmitter uses a fraction of the power required by
its bipolar counterparts, but it also includes a shutdown feature
that reduces the already low quiescent I
CC
to a 10nA trickle. This
device enters shutdown whenever the driver disables (DE = GND).
