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ISL43485 Datasheet(PDF) 8 Page - Intersil Corporation |
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ISL43485 Datasheet(HTML) 8 Page - Intersil Corporation |
8 / 11 page 8 Driver Features The RS-485, RS-422 driver is a differential output device that delivers at least 1.5V across a 54 Ω load (RS-485), and at least 2V across a 100 Ω load (RS-422) even with VCC = 3V. The driver features low propagation delay skew to maximize bit width, and to minimize EMI, and it is tri-statable via the active high DE input. Outputs of the ISL43485 driver are not slew rate limited, so faster output transition times allow data rates of at least 30Mbps. Data Rate, Cables, and Terminations 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 this IC. 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 30Mbps are often limited to lengths of less than one hundred feet. Figure 6 details the ISL43485’s 30Mbps performance driving 200’ of “CAT5” cable terminated in 120 Ω at both ends. Note that the differential signal delivered to the receiver at the end of the cable (A-B) still exceeds 1.5V peak. Longer cable lengths are possible by reducing the data rate, as shown in Figure 7 for a data rate of 20Mbps. To minimize reflections, proper termination is imperative when using this 30Mbps device. 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 transceiver to the main cable should be kept as short as possible. Built-In Driver Overload Protection As stated previously, the RS-485 spec requires that drivers survive worst case bus contentions undamaged. The ISL43485 meets this requirement 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 spec, even at the common mode voltage range extremes. Additionally, it utilizes a foldback circuit which reduces the short circuit current, and thus the power dissipation, whenever the contending voltage exceeds either supply. In the event of a major short circuit condition, this device also includes 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 reenable after the die temperature drops about 15 degrees. If the contention persists, the thermal shutdown/reenable cycle repeats until the fault is cleared. Receivers stay operational during thermal shutdown. Low Power Shutdown Mode This BiCMOS transceiver uses a fraction of the power required by its bipolar counterparts, nevertheless, the ISL43485 includes a shutdown feature that reduces the already low quiescent ICC to a 15nA trickle. They enter shutdown whenever the receiver and driver are simultaneously disabled (RE =VCC and DE = GND) for a period of at least 300ns. Disabling both the driver and the receiver for less than 80ns guarantees that shutdown is not entered. Note that receiver and driver enable times increase when these devices enable from shutdown. Refer to Notes 5-9, at the end of the Electrical Specification table, for more information. ISL43485 |
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