users design equipment that meets Level 3 of IEC 1000-
4-2, without additional ESD-protection components.
The main difference between tests done using the Human
Body Model and IEC 1000-4-2 is higher peak current
in IEC 1000-4-2. Because series resistance is lower in
the IEC 1000-4-2 ESD test model (Figure 4a), the ESD-
withstand voltage measured to this standard is generally
lower than that measured using the Human Body Model.
Figure 4b shows the current waveform for the ±8kV IEC
1000-4-2 Level 4 ESD Contact Discharge test. The Air-
Gap test involves approaching the device with a charger
probe. The Contact Discharge method connects the
probe to the device before the probe is energized.
Machine Model
The Machine Model for ESD testing uses a 200pF stor-
age capacitor and zero-discharge resistance. It mimics
the stress caused by handling during manufacturing and
assembly. All pins (not just the RS-485 inputs) require this
protection during manufacturing. Therefore, the Machine
Model is less relevant to the I/O ports than are the Human
Body Model and IEC 1000-4-2.
True Fail-Safe
The MAX3280E/MAX3281E/MAX3283E/MAX3284E guar-
antee a logic-high receiver output when the receiver inputs
are shorted or open, or when they are connected to a
terminated transmission line with all drivers disabled. This
guaranteed logic high is achieved by setting the receiver
threshold between -50mV and -200mV. If the differential
receiver input voltage (VA - VB) is greater than or equal to
-50mV, RO is logic high. If (VA - VB) is less than or equal
to -200mV, RO is logic low.
In the case of a terminated bus with all transmitters dis-
abled, the receiver’s differential input voltage is pulled to
ground by the termination. This results in a logic high with
a 50mV minimum noise margin. Unlike previous fail-safe
devices, the -50mV to -200mV threshold complies with
the ±200mV EIA/TIA-485 standard.
Receiver Enable
(MAX3281E and MAX3283E only)
The MAX3281E and MAX3283E feature a receiver out-
put enable (EN, MAX3281E or EN, MAX3283E) input
that controls the receiver. The MAX3281E receiver
enable (EN) pin is active high, meaning the receiver
outputs are active when EN is high. The MAX3283E
receiver enable (EN) pin is active low. Receiver outputs
are high impedance when the MAX3281E’s EN pin is low
and when the MAX3283E’s EN pin is high.
Low-Voltage Logic Levels
(MAX3284E only)
An increasing number of applications now operate at
low-voltage logic levels. To enable compatibility with
these low-voltage logic level applications, such as digital
FPGAs, the MAX3284E VL pin is a user-defined supply
voltage that designates the voltage threshold for a logic
high.
At lower VL voltages, the data rate will also be lower.
A logic-high level of 1.65V will receive data at 20Mbps.
Table 2 gives data rates at various voltages at VL.
Applications Information
Propagation Delay Matching
The MAX3280E/MAX3281E/MAX3283E/MAX3284E (VCC
= VL) exhibit propagation delays that are closely matched
from one device to another, even between devices from
different production lots. This feature allows multiple data
lines to receive data and clock signals with minimal skew
with respect to each other. Figure 5 shows the typical
propagation delays. Small receiver skew times, the differ-
ence between the low-to-high and high-to-low propagation
delay, help maintain a symmetrical ratio (50% duty cycle).
The receiver skew time | tPLH - tPHL | is under 2ns for
either a 3.3V supply or a 5V supply.
Multidrop Clock Distribution
Low package-to-package skew (8ns max) makes the
MAX3280E/MAX3281E/MAX3283E/MAX3284E
(VCC = VL) ideal for multidrop clock distribution. When
distributing a clock signal to multiple circuits over long
transmission lines, receivers in separate locations, and
possibly at two different temperatures, would ideally
Table 1. MAX3281E/MAX3283E Enable
Table
Table 2. MAX3284E Data Rate Table
VCC = 3V TO 5.5V
VLMAXIMUM DATA RATE
1.65V 20Mbps
2.2V 33Mbps
≥3.3V 52Mbps
PART ENABLE = HIGH ENABLE = LOW
MAX3281E Active High Z
MAX3283E High Z Active
www.maximintegrated.com Maxim Integrated
│
6
MAX3280E/MAX3281E/
MAX3283E/MAX3284E
±15kV ESD-Protected 52Mbps, 3V to 5.5V,
SOT23 RS-485/RS-422 True Fail-Safe Receivers