The TL2218-285 is a current-mode 9-channel monolithic terminator specially designed for single-ended small-computer-systems-interface (SCSI) bus termination. A user-controlled disable function is provided to reduce standby power. No impedance-matching resistors or other external components are required for its operation as a complete terminator.
The device operates over a wide termination-voltage (Vterm[dagger]) range of 3.5 V to 5.5 V, offering an extra 0.5 V of operating range when compared to the minimum termination voltage of 4 V required by other integrated active terminators. The TL2218-285 functions as a current-sourcing terminator and supplies a constant output current of 23 mA into each asserted line. When a line is deasserted, the device senses the rising voltage level and begins to function as a voltage source, supplying a fixed output voltage of 2.85 V. The TL2218-285 features compatibility with active negation drivers and has a typical sink current capability of 20 mA.
The TL2218-285 is able to ensure that maximum current is applied at the first high-level step. This performance means that the device should provide a first high-level step exceeding 2 V even at a 10-MHz rate. Therefore, noise margins are improved considerably above those provided by resistive terminators.
A key difference between the TL2218-285 current-mode terminator and a Boulay terminator is that the TL2218-285 does not incorporate a low dropout regulator to set the output voltage to 2.85 V. In contrast with the Boulay termination concept, the accuracy of the 2.85 V is not critical with the current-mode method used in the TL2218-285 because this voltage does not determine the driver current. Therefore, the primary device specifications are not the same as with a voltage regulator but are more concerned with output current.
The DISABLE\ terminal is TTL compatible and must be taken low to shut down the outputs. The device is normally active, even when DISABLE\ is left floating. In the disable mode, only the device startup circuits remain active, thereby reducing the supply current to just 500 uA. Output capacitance in the shutdown mode is typically 6 pF.
The TL2218-285 has on-board thermal regulation and current limiting, thus eliminating the need for external protection circuitry. A thermal regulation circuit that is designed to provide current limiting, rather than an actual thermal shutdown, is included in the individual channels of the TL2218-285. When a system fault occurs that leads to excessive power dissipation by the terminator, the thermal regulation circuit causes a reduction in the asserted-line output current sufficient to maintain operation. This feature allows the bus to remain active during a fault condition, which permits data transfer immediately upon removal of the fault. A terminator with thermal shutdown does not allow for data transfer until sufficient cooling has occurred. Another advantage offered by the TL2218-285 is a design that does not require costly laser trimming in the manufacturing process.
The TL2218-285 is characterized for operation over the virtual junction temperature range of 0°C to 125°C.
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