## Technical Overview of the FSTU3384WM The **FSTU3384WM** is a high-speed CMOS 10-bit bus switch designed for 5V or 3.3V power supply applications. It is part of the Fairchild (now ON Semiconductor) **FSTU** series, featuring "Undershoot Hardened" circuitry. --- ### 1. Key Features and Specifications The following table summarizes the primary electrical characteristics of the FSTU3384WM: | Parameter | Specification | | :--- | :--- | | **Type** | 10-Bit Bus Switch | | **Operating Voltage ($V_{CC}$)** | 4.0V to 5.5V | | **On-State Resistance ($R_{on}$)** | 4$\Omega$ (Typical) | | **Propagation Delay** | < 0.25 ns | | **Package Type** | SOIC (Wide), 24-lead | | **Special Feature** | Undershoot Hardened (to -2V) | | **Enable Control** | Low-Enable ($\overline{OE}$) | --- ### 2. Functional Description The device consists of ten high-speed CMOS switches. It is organized as two banks of five switches (1-5 and 6-10). Each bank is controlled by an Output Enable ($\overline{OE}$) signal. * **When $\overline{OE}$ is LOW:** The switches are closed, allowing a low-resistance connection between Port A and Port B. * **When $\overline{OE}$ is HIGH:** The switches are open, creating a high-impedance (Hi-Z) state between the ports. --- ### 3. Critical Electronic Characteristics #### A. Undershoot Hardened Circuitry The "U" in FSTU stands for Undershoot Hardened. Standard CMOS switches can experience "latch-up" or data corruption if the input signal drops below ground (negative voltage spikes). The FSTU3384WM includes specialized circuitry to prevent the switch from turning on during undershoot events down to **-2.0V**. #### B. Low Propagation Delay Because the device acts as a simple FET switch rather than a buffer/driver, it does not add significant propagation delay. The delay is primarily determined by the RC time constant of the On-Resistance ($R_{on}$) and the load capacitance ($C_L$). #### C. Bus Swapping and Isolation It is commonly used for: * **Hot-Plugging:** Isolating a bus during live insertion. * **Voltage Translation:** With a diode in series with $V_{CC}$, it can be used to interface 5V signals with 3.3V logic levels. --- ### 4. Pinout Configuration The FSTU3384WM typically follows this pin arrangement in its 24-pin SOIC package: ```text +--\/--+ /OE1 |1 24| VCC B1 |2 23| B10 A1 |3 22| A10 A2 |4 21| A9 B2 |5 20| B9 B3 |6 19| B8 A3 |7 18| A8 A4 |8 17| A7 B4 |9 16| B7 B5 |10 15| B6 A5 |11 14| A6 GND |12 13| /OE2 +------+ ```