The **FS243** is a high-performance, single-chip CMOS **Primary-Side Regulation (PSR)** PWM controller designed primarily for low-power offline SMPS (Switched-Mode Power Supply) applications, such as battery chargers and LED drivers. --- ### 1. Key Technical Specifications The FS243 is designed to eliminate the need for an optocoupler and a TL431 on the secondary side, reducing overall component count and cost. | Parameter | Typical Value | | :--- | :--- | | **Control Method** | Primary Side Regulation (PSR) | | **Operating Mode** | CC (Constant Current) & CV (Constant Voltage) | | **Max Power Output** | Typically 5W - 12W (depending on package) | | **Start-up Current** | Very Low (approx. 5μA) | | **Switching Frequency** | Variable (Frequency Shuffling for EMI) | | **Package Types** | SOT23-6, SOP-8 | --- ### 2. Core Functional Parts The internal circuitry of the FS243 consists of several critical electronic blocks: * **Error Amplifier & Feedback:** It senses the output voltage via the auxiliary winding of the transformer. By monitoring the "knee" of the voltage waveform during the flyback period, it regulates the output without direct secondary feedback. * **Current Sensing (CS):** A resistor connected to the CS pin senses the primary peak current. This is used for cycle-by-cycle current limiting and determining the Constant Current (CC) output. * **Frequency Shuffling:** To minimize Electromagnetic Interference (EMI), the chip oscillates with a jittered frequency, spreading the energy across a wider spectrum. * **Built-in Protections:** * **UVLO:** Under Voltage Lock Out to ensure the chip only operates when the supply is stable. * **OVP:** Over Voltage Protection for the VDD pin. * **SCP:** Short Circuit Protection. --- ### 3. Pin Configuration (Typical SOT23-6) Understanding the pins is essential for circuit integration: | Pin Name | Description | | :--- | :--- | | **VDD** | Power supply pin for the controller. | | **GND** | Ground reference. | | **FB** | Feedback pin; senses the auxiliary winding voltage. | | **CS** | Current Sense; connects to a resistor to monitor primary current. | | **GATE/OUT** | Driver output for the external Power MOSFET gate. | | **COMP** | Compensation pin for stability (sometimes internal). | --- ### 4. Typical Application Diagram In a standard flyback converter using the FS243, the electronic layout follows this flow: 1. **Input:** AC Line → Bridge Rectifier → Pi Filter. 2. **Switching:** The FS243 drives a **High-Voltage MOSFET**. 3. **Transformer:** Energy is transferred to the secondary and the **Auxiliary Winding**. 4. **Feedback:** The FS243 reads the Auxiliary Winding to adjust the PWM duty cycle.