Datasheet ISL32704E (Intersil) - 8

link to page 8 link to page 8 link to page 7 ISL32704E Digital Isolator Principle
60
The ISL32704E utilizes a giant magneto-resistance (GMR) isolation. Figure 5 shows the principle operation of a single
) 50 FCC-B < 1GHz 3m
channel GMR isolator.
40 EN55022 < 1GHz 3m (dBµV/m EXTERNAL B-FIELD 30 UDE LABORATORY VDD2 20 NOISE FLOOR INTERNAL Qp- B-FIELD GMR1 GMR2 AMPLIT 10 MEASUREMENTS IN OUT 010MHz 100MHz 1GHz GMR3 GMR4
FIGURE 7. UNDETECTABLE EMISSIONS OF GMR ISOLATORS
GND2
Low EMI Susceptibility FIGURE 5. SINGLE CHANNEL GMR ISOLATOR Because GMR isolators have no pulse trains or carriers to interfere with, they also have very low EMI susceptibility. The input signal is buffered and drives a primary coil, which creates a magnetic field that changes the resistance of the GMR For the list of compliance tests conducted on GMR isolators, refer resistors 1 to 4. GMR1 to GMR4 form a Wheatstone bridge in to the “Electromagnetic Compatibility” on page 7. order to create a bridge output voltage that only reacts to magnetic field changes from the primary coil. Large external Receiver (Rx) Features magnetic fields, however, are treated as common-mode fields This transceiver utilizes a differential input receiver for maximum and are therefore suppressed by the bridge configuration. The noise immunity and common-mode rejection. Input sensitivity is bridge output is fed into a comparator whose output signal is ±200mV, as required by the RS-485 specification. identical in phase and shape to the input signal. The receiver input resistance meets the RS-485 Unit Load (UL) GMR Resistor in Detail requirement of 12kΩ minimum. The receiver includes a “fail-safe Figure 6 shows a GMR resistor consisting of ferromagnetic alloy if open” function that guarantees a high level receiver output if layers, B1, B2, sandwiched around an ultra thin, nonmagnetic the receiver inputs are unconnected (floating). The receiver conducting middle layer A, typically copper. The GMR structure is output is tri-statable via the active low RE input. designed so that, in the absence of a magnetic field, the Driver (Tx) Features magnetic moments in B1 and B2 face opposite directions, thus causing heavy electron scattering across layer A, which increases The RS-485 driver is a differential output device that delivers at its resistance for current C drastically. When a magnetic field D is least 1.5V across a 54Ω purely differential load. The driver applied, the magnetic moments in B1 and B2 are aligned and features low propagation delay skew to maximize bit width and electron scattering is reduced. This lowers the resistance of layer to minimize EMI. A and increases current C. The driver in the ISL32704E is tri-statable via the active high DE input. The outputs of the ISL32704E driver are not slew rate
HIGH LOW
limited, so faster output transition times allow data rates of at
RESISTANCE RESISTANCE
least 4Mbps.
B1 B1 C C C C
Built-In Driver Overload Protection
A A
As stated previously, the RS-485 specification requires that
B2 B2
drivers survive worst-case bus contentions undamaged. The ISL32704E transmitters meet this requirement via driver output
D
short-circuit current limits and on-chip thermal shutdown circuitry.
APPLIED MAGNETIC FIELD
The driver output stage incorporates short-circuit current limiting FIGURE 6. MULTI-LAYER GMR RESISTOR circuitry, which ensures that the output current never exceeds the RS-485 specification. In the event of a major short-circuit Low Emissions condition, the device also includes a thermal shutdown feature Because GMR isolators do not use fancy encoding schemes, such that disables the driver whenever the die temperature becomes as RF carriers or high-frequency clocks, and do not include power excessive. This eliminates the power dissipation, allowing the die transfer coils or transformers, their radiated emission spectrum to cool. The driver automatically re-enables after the die is virtually undetectable. temperature drops about 15°C. If the contention persists, the thermal shutdown/re-enable cycle repeats until the fault is cleared. The receiver stays operational during thermal shutdown. FN8860 Rev.3.00 Page 8 of 13 Mar 29, 2017 Document Outline Related Literature Features Applications Ordering Information Pin Configurations Truth Table Truth Table Pin Descriptions Typical Operating Circuits Absolute Maximum Ratings (Note 16) Thermal Information Recommended Operating Conditions Electrical Specifications Insulation Specifications Magnetic Field Immunity (Note 16) Safety and Approvals VDE V 0884-11 (Certification Pending) UL 1577 Electromagnetic Compatibility Application Information RS-485 and Isolation Digital Isolator Principle GMR Resistor in Detail Low Emissions Low EMI Susceptibility Receiver (Rx) Features Driver (Tx) Features Built-In Driver Overload Protection Dynamic Power Consumption Power Supply Decoupling DC Correctness Data Rate, Cables, and Terminations Transient Protection Pinout Differences Between Packages Revision History About Intersil Package Outline Drawing M16.15B M16.3A