STC3485E Datasheet, PDF (9/11 Pages) List of Unclassifed Manufacturers – 3.3V-Powered, ±15kV ESD-Protected, and Slew-Rate-Limited True RS-485 Transceivers

Driver Output Protection
Excessive output current and power dissipation caused by faults or by bus contention are prevented by two mechanisms. A foldback
current limit on the output stage provides immediate protection against short circuits over the whole common-mode voltage range
(see Typical Operating Characteristics). In addition, a thermal shutdown circuit forces the driver outputs into a high-impedance state
if the die temperature rises excessively.
Propagation Delay
Figures 12 and 13 show the typical propagation delays. Skew time is simply the difference between the low-to-high and high-to-low
propagation delay. Small driver/receiver skew times help maintain a symmetrical mark-space ratio (50% duty cycle).
The receiver skew time, |tPRLH - tPRHL|, is under 10ns (20ns for the STC3485E). The driver skew times are typically under 50ns
for the STC3485E.
Line Length vs. Data Rate
The RS-485 standard covers line lengths up to 4000 feet. Figure 13 shows the system differential voltage for parts driving
4000 feet of 26AWG twisted-pair wire at 125kHz into 120O loads.
±15kV ESD Protection
As with all STC devices, ESD-protection structures are incorporated on all pins to protect against electrostatic discharges encoun-
tered during handling and assembly. The driver outputs and receiver inputs of the STC3485E family of devices have extra protection
against static electricity. STC’s engineers have developed state-of-the-art structures to protect these pins against ESD of 15kV
without damage. The ESD structures withstand high.
ESD in all states: normal operation, shutdown, and powered down. After an ESD event, Union’s E versions keep working without
latchup or damage. ESD protection can be tested in various ways; the transmitter outputs and receiver inputs of this product family
are characterized for protection to the following limits:
1) ±15kV using the Human Body Model
2) ±15kV using IEC 1000-4-2’s Air-Gap method.
Human Body Model
Figure 15a shows the Human Body Model and Figure 15b shows the current waveform it generates when discharged into a
low impedance. This model consists of 100pF capacitor charged to the ESD voltage of interest, which is then discharged into
the test device through a 1.5kO r?esistor.
IEC 1000-4-2
The IEC 1000-4-2 standard covers ESD testing and performance of finished equipment; it does not specifically refer to integrated
circuits. The STC3485E family devices help you design equipment that meets Level 4 (the highest level) of IEC 1000-4-2, without
the need for additional ESD-protection components. The major 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 model. Hence, the ESD
withstand voltage measured to IEC 1000-4-2 is generally lower than that measured using the Human Body Model. Figure 16a shows
the IEC 1000-4-2 model, and Figure 16b 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 charged probe.