current in IEC 1000-4-2, because series resistance is
lower in the IEC 1000-4-2 model. Hence, the ESD with-
stand voltage measured to IEC 1000-4-2 is generally
lower than that measured using the Human Body Model.
Figure 10c shows the IEC 1000-4-2 model, and Figure
10d shows the current waveform for IEC 1000-4-2 ESD
Contact Discharge test.
The air-gap test involves approaching the device with a
charged probe. The contact-discharge method connects
the probe to the device before the probe is energized.
Machine Model
The machine model for ESD tests all pins using a
200pF storage capacitor and zero discharge resis-
tance. The objective is to emulate the stress caused
when I/O pins are contacted by handling equipment
during test and assembly. Of course, all pins require
this protection, not just RS-485 inputs and outputs.
Applications Information
256 Transceivers on the Bus
The standard RS-485 receiver input impedance is 12kΩ
(1-unit load), and the standard driver can drive up to 32-
unit loads. The MAX3070E family of transceivers has a
1/8-unit load receiver input impedance (96kΩ), allowing
up to 256 transceivers to be connected in parallel on one
communication line. Any combination of these devices
as well as other RS-485 transceivers with a total of 32-
unit loads or fewer can be connected to the line.
Reduced EMI and Reflections
The MAX3070E/MAX3071E/MAX3072E feature reduced
slew-rate drivers that minimize EMI and reduce reflec-
tions caused by improperly terminated cables, allowing
error-free data transmission up to 250kbps. The
MAX3073E/MAX3074E/MAX3075E offer higher driver
output slew-rate limits, allowing transmit speeds up to
500kbps. The MAX3079E with SRL = V
CC
or uncon-
nected, are slew-rate limited. With SRL unconnected,
the MAX3079E error-free data transmission is up to
250kbps; with SRL connected to V
CC
the data transmit
speeds up to 500kbps.
MAX3070E–MAX3079E
+3.3V, ±15kV ESD-Protected, Fail-Safe,
Hot-Swap, RS-485/RS-422 Transceivers
______________________________________________________________________________________ 17
(23 paginas)
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