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ISL3298E Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL3298E Datasheet(HTML) 11 Page - Intersil Corporation |
11 / 17 page 11 FN6544.0 September 19, 2007 The VL supply current (IL) is typically much less than 20µA, as shown in Figure 9, when DE and DI are above/below VIH/VIL. Hot Plug Function When a piece of equipment powers-up, there is a period of time where the processor or ASIC driving the RS-485 control line (DE) is unable to ensure that the RS-485 Tx outputs are kept disabled. If the equipment is connected to the bus, a driver activating prematurely during power up may crash the bus. To avoid this scenario, the ISL329xE family incorporates a “Hot Plug” function. During power-up, circuitry monitoring VCC ensures that the Tx outputs remain disabled for a period of time, regardless of the state of DE. This gives the processor/ASIC a chance to stabilize and drive the RS-485 control lines to the proper states. ESD Protection All pins on these devices include class 3 (8kV) Human Body Model (HBM) ESD protection structures, but the RS-485 pins (driver outputs) incorporate advanced structures allowing them to survive ESD events in excess of ±16.5kV HBM and ±7kV to the IEC61000 contact test method. The RS-485 pins are particularly vulnerable to ESD damage because they typically connect to an exposed port on the exterior of the finished product. Simply touching the port pins, or connecting a cable, can cause an ESD event that might destroy unprotected ICs. These new ESD structures protect the device whether or not it is powered up, and without degrading the RS-485 common mode range of -7V to +12V. This built-in ESD protection eliminates the need for board level protection structures (e.g., transient suppression diodes), and the associated, undesirable capacitive load they present. Data Rate, Cables, and Terminations RS-485/RS-422 are intended for network lengths up to 4000’, but the maximum system data rate decreases as the transmission length increases. Devices operating at 20Mbps are limited to lengths less than 100’, while the 250kbps versions can operate at full data rates with lengths of several 1000’. Twisted pair is the cable of choice for RS-485/RS-422 networks. Twisted pair cables tend to pick up noise and other electromagnetically induced voltages as common mode signals, which are effectively rejected by the differential receivers in these ICs. Proper termination is imperative, when using the 20Mbps devices, to minimize reflections. Short networks using the 250kbps versions need not be terminated, but, terminations are recommended unless power dissipation is an overriding concern. In point-to-point, or point-to-multipoint (single driver on bus) networks, the main cable should be terminated in its characteristic impedance (typically 120 Ω) at the end farthest from the driver. In multi-receiver applications, stubs connecting receivers to the main cable should be kept as short as possible. Multipoint (multi-driver) systems require that the main cable be terminated in its characteristic impedance at both ends. Stubs connecting a transmitter or receiver to the main cable should be kept as short as possible. Driver Overload Protection As stated previously, the RS-485 specification requires that drivers survive worst case bus contentions undamaged. These drivers meet this requirement, for VCC ≤ 3.6V, via driver output short circuit current limits, and on-chip thermal shutdown circuitry. The driver output stages incorporate short circuit current limiting circuitry which ensures that the output current never exceeds the RS-485 specification, for VCC ≤ 3.6V, even at the common mode voltage range extremes. Additionally, these devices utilize a foldback circuit which reduces the short circuit current, and thus the power dissipation, whenever the contending voltage exceeds either VCC or GND. In the event of a major short circuit condition, devices also include a thermal shutdown feature that disables the drivers whenever the die temperature becomes excessive. This eliminates the power dissipation, allowing the die to cool. The drivers automatically re-enable after the die temperature drops about +20°C. If the contention persists, the thermal shutdown/re-enable cycle repeats until the fault is cleared. At VCC > 3.6V, the instantaneous short circuit current is high enough that output stage damage may occur during short circuit conditions to voltages outside of GND to VCC, before the short circuit limiting and thermal shutdown activate. For VCC = 5V operation, if output short circuits are a possibility (e.g., due to bus contention), it is recommended that a 5 Ω resistor be inserted in series with each output. This resistor limits the instantaneous current below levels that can cause damage. The driver VOD at VCC = 5V is so large that this small added resistance has little impact. TABLE 2. VIH AND VIL vs VL FOR VCC = 3.3V OR 5V VL (V) VIH (V) VIL (V) 1.35 0.7 0.4 1.5 0.8 0.5 1.8 0.9 0.7 2.3 1.1 1.0 2.7 1.3 1.1 3.3 1.5 1.4 5.0 (i.e., VCC)2.7 2.3 ISL3293E, ISL3294E, ISL3295E, ISL3296E, ISL3297E, ISL3298E |
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