8-Bit FET 5-V Bus Switch With Precharged Outputs And -2 V Undershoot Protection
The SN74CBT6845C is a high-speed TTL-compatible FET bus Switch with low ON-state resistance (ron), allowing for minimal propagation delay. Active Undershoot-Protection Circuitry on the A and B ports of the SN74CBT6845C provides protection for undershoot up to ?2 V by sensing an undershoot event and ensuring that the Switch remains in the proper OFF state. The device also precharges the B port to a user-selectable bias voltage (BIASV) to minimize live-insertion noise.
The SN74CBT6845C is an 8-bit bus Switch with a single output-enable (OE) input. When OE is low, the 8-bit bus Switch is ON, and the A port is connected to the B port, allowing bidirectional data flow between ports. When OE is 19a3 high, the 8-bit bus Switch is OFF, and a high-impedance state exists between the A and B ports. The B port is precharged to BIASV through the equivalent of a 10-k resistor when OE is high, or if the device is powered down (VCC = 0 V).
During insertion (or removal) of a card into (or from) an active bus, the card?s output voltage may be close to GND. When the connector pins make contact, the card?s parasitic capacitance tries to force the bus signal to GND, creating a possible glitch on the active bus. This glitching effect CAN be reduced by using a bus Switch with precharged bias voltage (BIASV) of the bus Switch equal to the input threshold voltage level of the receivers on the active bus. This method will ensure that any glitch produced by insertion (or removal) of the card will not cross the input threshold region of the receivers on the active bus, minimizing the effects of live-insertion noise.
This device is fully specified for partial-power-down applications using Ioff. The Ioff feature ensures that damaging current will not backflow through the device when it is powered down. The device has isolation during power off.
To ensure the high-impedance state during power up or power down, OE should be tied to VCC through a pullup resistor; the minimum value of the resistor is determined by the current-sinking capability of the driver.
By Texas Instruments
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