ISL32600E_14 Datasheet, PDF (12/23 Pages) Intersil Corporation – 1.8V to 3.3V, Micro-Power, ±15kV ESD, +125°C, Slew Rate Limited, RS-485/RS-422 Transceivers

ISL32600E, ISL32601E, ISL32602E, ISL32603E
eliminate the duty cycle distortion that occurs on receivers with
full fail safe (FFS) functionality and with slowly transitioning
input signals (see Figure 9). FFS receiver switching points have a
1
A-B
0
-1
4
ISL3260XE
0
4
ISL3172E
0
V_CC = 3.3V, DATA RATE = 125kbps
TIME (4µs/DIV)
FIGURE 9. COMPARED WITH A FULL-FAILSAFE ISL3172E
RECEIVER, THE SYMMETRICAL RX THRESHOLDS OF
THE ISL3260XE DELIVER LESS OUTPUT DUTY CYCLE
DISTORTION WHEN DRIVEN WITH SLOW INPUT
SIGNALS
negative offset, so the RO high time is naturally longer than the
low time. The ISL3260XE’s larger receiver input sensitivity range
enables an increase of the receiver input hysteresis. The 40mV
to 65mV receiver hysteresis increases the noise immunity,
which is a big advantage for noisy networks, or networks with
slow bus transitions.
Receiver input resistance of 96kΩ surpasses the RS-422 spec
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 (i.e., +12V and -7V) at
V_CC = 3V, making them ideal for long networks where induced
voltages and ground potential differences are realistic concerns.
The positive CMR is limited to +2V when the ISL32602E or
ISL32603E is operated with V_CC = 1.8V.
All the receivers include a “Fail-Safe if open” function that
guarantees a high level receiver output if the receiver inputs
are unconnected (floating). Because the Rx is not full failsafe,
terminated networks may require bus biasing resistors (pull-up
on noninverting input, pull-down on inverting input) to preserve
the bus idle state when the bus is not actively driven.
Receivers operate at data rates from 128kbps to 460kbps -
depending on the supply voltage - and all receiver outputs are
tri-statable via the active low RE input. There are no parasitic
nor ESD diodes to V_CC on the RE input, so it is tolerant of input
voltages up to 5.5V, even with the ISL3260XE powered down
(i.e., V_CC = 0V).
Driver Features
These drivers are differential output devices that deliver at
least 1.4V with V_CC ≥ 3V across a 54Ω load (RS-485) and at
least 1.95V with V_CC ≥ 3.15V across a 100Ω load (RS-422).
The 1.8V transmitters deliver a 1.1V unloaded, differential
level. Drivers operate at data rates from 128kbps to 460kbps -
depending on the supply voltage - and they feature low
propagation delay skews to maximize bit width. Driver outputs
are slew rate limited to minimize EMI and to reduce reflections
in unterminated or improperly terminated networks.
All drivers are tri-statable via the active high DE input. There
are no parasitic nor ESD diodes to V_CC on the DI and DE inputs,
so these inputs are tolerant of input voltages up to 5.5V, even
with the ISL3260XE powered down (i.e., V_CC = 0V).
1.8V Operation
The ISL32602E and ISL32603E are specifically designed to
operate with supply voltages as low as 1.8V. Termination
resistors should be avoided at this operating condition, and the
unterminated driver is guaranteed to deliver a healthy 1.1V
differential output voltage. This low supply voltage limits the
+CMR to +2V, but the CMR increases as V_CC increases.
To get good 1.8V operation, the ISL32602E and ISL32603E
have to run at a higher operating current. Thus, their I_CC with
V_CC = 3.3V is considerably higher than the I_CC of the
ISL32600E and ISL32601E, which are optimized for low I_CC at
3.3V (see Figures 1 and 2).
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
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 ISL3260XE devices
incorporate a “Hot Plug” function. During power up, circuitry
monitoring V_CC 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 (>8kV) 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 transceiver’s
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.
12
FN7967.0
June 22, 2012