Datasheet ISL32704E (Intersil) - 9

link to page 9 link to page 9 link to page 2 link to page 9 link to page 9 link to page 9 ISL32704E Dynamic Power Consumption where: The isolator within the ISL32704E achieves its low power • LS is the stub length (ft) consumption from the way it transmits data across the barrier. • t By detecting the edge transitions of the input logic signal and r is the driver rise time (s) converting these to narrow current pulses, a magnetic field is • c is the speed of light (9.8 x 108 ft/s) created around the GMR Wheatstone bridge. Depending on the • v is the signal velocity as a percentage of c. direction of the magnetic field, the bridge causes the output comparator to switch following the input signal. Since the current To ensure the receiver output of the ISL32704E is high when the pulses are narrow, about 2.5ns, the power consumption is bus is not actively driven, fail-safe biasing of the bus lines is independent of the mark-to-space ratio and solely depends on recommended. Figure 8 shows the proper termination of a frequency. high-speed data link with fail-safe biasing. TABLE 2. SUPPLY CURRENT INCREASE WITH DATA RATE
VS
DATA RATE IDD1 IDD2 (Mbps) (mA) (mA)
RB 542R
1 0.15 0.15 4 0.6 0.6
RT2 135R 120R RT1
Power Supply Decoupling
RB 542R
Both supplies, VDD1 and VDD2, must be bypassed with 100nF ceramic capacitors. These should be placed as close as possible to the supply pins for proper operation. FIGURE 8. FAIL-SAFE BIASING FOR HIGH-SPEED DATA LINKS DC CORRECTNESS Here the termination resistor value at the cable end without The ISL32704E incorporates a patented refresh circuit to maintain fail-safe biasing equals the characteristic cable impedance: the correct output state with respect to data input. At power-up, RT1 = Z0. The values for RB and RT2 are calculated using the bus outputs follow the truth tables on page 2. The DE input Equations 2 and 3. should be held low during power-up to prevent false drive data V Z pulses on the bus. This can be accomplished by connecting a R S  ------ 0  --- B (EQ. 2) 10kΩ pull-down resistor between DE and GND1. VAB 4 Data Rate, Cables, and Terminations 2R  R B Z0 = ------------ T2 (EQ. 3) RS-485 is intended for network lengths up to 4000 feet, but the 2RB – Z0 maximum system data rate decreases as the transmission where: length increases. Devices operating at 4Mbps are typically limited to lengths less than 100 feet, but are capable of driving • RB are the fail-safe biasing resistors up to 350 feet of cable when allowing for some jitter of 5%. • RT2 is the termination resistor in the fail-safe biasing network Twisted pair is the cable of choice for RS-485 networks. Twisted • V pair cables tend to pick up noise and other electromagnetically S is the minimum transceiver supply voltage induced voltages as common-mode signals, which are effectively • VAB is the minimum bus voltage of the undriven bus rejected by the differential receivers in these ICs. • Z0 is the characteristic cable impedance To minimize reflections, proper termination is imperative when Note, the resistor values in Figure 8 have been calculated for using this high data rate transceiver. In point-to-point or VS = 4.5V, VAB = 0.25V, and Z0 = 120Ω. point-to-multipoint (single driver on bus) networks, the main cable should be terminated in its characteristic impedance Transient Protection (typically 120Ω for RS-485) at the end farthest from the driver. In Protecting the ISL32704E against transients exceeding the multireceiver applications, stubs connecting receivers to the device’s transient immunity requires the addition of an external main cable should be kept as short as possible. Multipoint TVS. For this purpose, Semtech’s RClamp0512TQ was chosen (multidriver) systems require that the main cable be terminated due to its high transient protection levels, low junction in its characteristic impedance at both ends. Stubs connecting a capacitance, and small form factor. transceiver to the main cable should be kept as short as possible. A useful guideline for determining the maximum stub lengths is TABLE 3. RCLAMP0512 TVS FEATURES given with Equation 1. PARAMETER SYMBOL VALUE UNIT t ESD (IEC61000-4-2) Air V L r  ---  v  c (EQ. 1) ESD ±30 kV S 10 Contact VESD ±30 kV EFT (IEC61000-4-4) VEFT ±4 kV FN8860 Rev.3.00 Page 9 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