## Overview of the FST3345 The **FST3345** is a high-speed CMOS 8-bit bus switch. It is designed to connect or isolate two data buses without adding significant propagation delay or generating additional ground bounce noise. It is commonly used in high-frequency signal routing and voltage level shifting applications. ### Key Specifications | Parameter | Specification | | :--- | :--- | | **Logic Type** | 8-Bit Bus Switch | | **Input/Output Pins** | 8 Ports (A to B) | | **Propagation Delay** | < 0.25 ns (Negligible) | | **On-Resistance ($R_{on}$)** | ~5 $\Omega$ (Typical) | | **Operating Voltage ($V_{CC}$)** | 4.0V to 5.5V | | **Control Input** | Output Enable ($\overline{OE}$) | | **Package Types** | SOIC, QSOP, TSSOP | --- ### Internal Architecture and Operation The FST3345 operates as a bi-directional bridge. It does not contain an internal clock or a buffer amplifier; instead, it uses an N-channel MOSFET as a pass transistor. 1. **The Pass Transistor:** When the device is enabled, the MOSFET connects Port A to Port B. Because the "ON" resistance is extremely low (typically 5 ohms), the signal passes through with minimal distortion. 2. **Control Logic ($\overline{OE}$):** * **Low (0):** The switch is closed. Port A is connected to Port B. * **High (1):** The switch is open. Port A and Port B are in a high-impedance (Hi-Z) state, effectively isolating the buses. ### Practical Applications * **Bus Isolation:** Allows a processor to talk to multiple peripherals on the same bus without loading the bus when those peripherals are not in use. * **Voltage Level Shifting:** Because it is an N-channel switch, it can be used to translate 5V signals down to 3.3V levels by limiting the $V_{CC}$ or using a diode to drop the gate voltage. * **Hot-Swapping:** Protects sensitive components during live insertion of circuit boards by keeping the bus isolated until power is stable. --- ### Pinout Configuration (Standard 20-Pin) | Pin Name | Function | | :--- | :--- | | **$\overline{OE}$** | Output Enable (Active Low) | | **A1 - A8** | Port A Data Inputs/Outputs | | **B1 - B8** | Port B Data Inputs/Outputs | | **GND** | Ground | | **$V_{CC}$** | Power Supply |