## Overview of the LP3669CS The **LP3669CS** is a high-performance, highly integrated **Primary Side Regulation (PSR)** controller designed for offline AC/DC flyback converter applications. It is commonly used in low-power adapters and battery chargers because it eliminates the need for an optocoupler and a TL431 shunt regulator, reducing overall system cost and size. --- ### 1. Key Technical Specifications | Parameter | Description | | :--- | :--- | | **Package Type** | SOP-7 (Small Outline Package) | | **Control Method** | Primary Side Regulation (PSR) | | **Switching Element** | Integrated Power BJT (typically 800V rated) | | **Maximum Output Power** | Approx. 5W - 7.5W (depending on thermal design) | | **Operating Mode** | CV (Constant Voltage) and CC (Constant Current) | | **Standby Power** | < 75mW (ultra-low consumption) | --- ### 2. Functional Electronic Components (Internal) The LP3669CS integrates several critical stages into a single silicon chip to simplify circuit design: * **Integrated Power BJT:** Unlike controllers that require an external MOSFET, the LP3669CS includes a high-voltage power transistor capable of handling the primary switching current. * **Error Amplifier:** Processes the feedback signal from the auxiliary winding to maintain accurate output voltage. * **PWM/PFM Controller:** Uses Pulse Width Modulation at high loads and Pulse Frequency Modulation at light loads to optimize efficiency. * **Cable Compensation:** Built-in circuitry to compensate for the voltage drop across the output DC cable, ensuring the voltage at the end of the wire remains stable. --- ### 3. Pin Configuration and Definitions | Pin Number | Name | Function | | :--- | :--- | :--- | | 1 | **VCC** | Power supply pin for the controller. | | 2 | **GND** | Ground reference. | | 3 | **FB** | Feedback pin; senses output voltage via auxiliary winding. | | 4 | **CS** | Current Sense; monitors the primary peak current. | | 5, 6 | **COL** | Collector of the internal power BJT; connects to the transformer. | | 7 | **NC/Gate** | Internal connection or No Connect (depending on sub-version). | --- ### 4. Application Circuit Features When designing with the LP3669CS, the following electronic behaviors are observed: 1. **Elimination of Optocouplers:** By sensing the "knee" voltage of the auxiliary winding during the flyback period, it calculates the output voltage without direct electrical connection to the secondary side. 2. **Protective Features:** * **Short Circuit Protection (SCP):** Reduces frequency to protect the chip during a short. * **Over Temperature Protection (OTP):** Shuts down the IC if internal temperatures exceed safe limits. * **Open Loop Protection:** Prevents runaway voltage if the feedback path fails. 3. **Start-up Mechanism:** Uses a high-resistance startup resistor from the high-voltage DC bus to charge the VCC capacitor, ensuring low power loss during normal operation. ---