Rainbow Electronics MAX3430 - Manual (Page 9)

MAX3430
±80V Fault-Protected, Fail-Safe,
1/4-Unit Load, +3.3V RS-485 Transceiver
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Detailed Description
Driver
The driver accepts a single-ended, logic-level input
(DI) and transfers it to a differential, RS-485 level output
(A and B). Driving DE high enables the driver, while
pulling DE low places the driver outputs (A and B) into
a high-impedance state.
Receiver
The receiver accepts a differential, RS-485 level input
(A and B), and transfers it to a single-ended, logic-level
output (RO). Pulling RE low enables the receiver, while
driving RE high and DE low places the receiver inputs
(A and B) into a high-impedance state.
Low-Power Shutdown
Force DE low and RE high to shut down the MAX3430. A
time delay of 1µs prevents the device from accidentally
entering shutdown due to logic skews when switching
between transmit and receive modes. Holding DE low
and RE high for at least 1ms guarantees that the
MAX3430 enters shutdown. In shutdown, the device
consumes 100µA supply current.
±80V Fault Protection
The driver outputs/receiver inputs of RS-485 devices in
industrial network applications often experience voltage
faults resulting from transients that exceed the -7V to
+12V range specified in the EIA/TIA-485 standard. In
these applications, ordinary RS-485 devices (typical
absolute maximum ratings -8V to +12.5V) require costly
external protection devices. To reduce system com-
plexity and the need for external protection, the driver
outputs/receiver inputs of the MAX3430 withstand volt-
age faults of up to
±80V with respect to ground without
damage (see the Absolute Maximum Ratings section,
Note 1). Protection is guaranteed regardless of whether
the device is active, shut down, or without power.
True Fail-Safe
The MAX3430 uses a -50mV to -200mV differential
input threshold to ensure true fail-safe receiver inputs.
This threshold guarantees the receiver outputs a logic
high for shorted, open, or idle data lines. The -50mV to
-200mV threshold complies with the ±200mV threshold
EIA/TIA-485 standard.
±12kV ESD Protection
As with all Maxim devices, ESD-protection structures
are incorporated on all pins to protect against ESD
encountered during handling and assembly. The
MAX3430 receiver inputs/driver outputs (A, B) have
extra protection against static electricity found in nor-
mal operation. Maxim’s engineers have developed
state-of-the-art structures to protect these pins against
±12kV ESD without damage. After an ESD event, the
MAX3430 continues working without latchup.
ESD protection can be tested in several ways. The
receiver inputs are characterized for protection up to
±12kV using the Human Body Model.
ESD Test Conditions
ESD performance depends on a number of conditions.
Contact Maxim for a reliability report that documents
test setup, methodology, and results.
Human Body Model
Figure 8a shows the Human Body Model, and Figure
8b shows the current waveform it generates when dis-
charged into a low impedance. This model consists of
a 100pF capacitor charged to the ESD voltage of inter-
est, which is then discharged into the device through a
1.5k
Ω resistor.
Driver Output Protection
Two mechanisms prevent excessive output current and
power dissipation caused by faults or bus contention.
The first, a foldback current limit on the driver output
CHARGE-CURRENT-
LIMIT RESISTOR
DISCHARGE
RESISTANCE
STORAGE
CAPACITOR Cs
100pF
R C
1M Ω
R D
1.5k Ω
HIGH-
VOLTAGE
DC
SOURCE
DEVICE
UNDER
TEST
Figure 8a. Human Body ESD Test Model
I P 100%
90%
36.8%
t RL
TIME
t DL
CURRENT WAVEFORM
PEAK-TO-PEAK RINGING
(NOT DRAWN TO SCALE) Ir
10%
0
0
AMPERES
Figure 8b. Human Body Model Current Waveform