MC33370

The MC33370 series adopts an 8-pin dual in-line package (DIP-8) or a 5-pin TO-220 (divided into in-line and straight bend) packages. Its pin arrangement is shown in the figure. Taking the TO-220 package as an example, the functions of each pin are introduced. Pin 1 (Vcc): The input terminal of the working power supply voltage. When starting the chip, the working voltage below 10V must be supplied to this pin through pin 5 (D). When Vcc>8.5V (operating threshold voltage), the MOS field effect tube in the startup circuit is immediately turned off, and the power switch tube starts to work. The normal output voltage can be obtained from the secondary coil of the high-frequency transformer. At this time, it is changed to The feedback voltage supplies power to the chip. Once an overload or short-circuit fault occurs in the switching power supply, resulting in Vcc <7.5V (undervoltage threshold voltage), the power switch is turned off, while the MOS field effect transistor for startup is working, and the chip enters the self-starting working mode. When in use, the Vcc terminal is generally short-circuited with pin 2. Pin 2 (Feedback Input, FB): feedback input terminal, referred to as feedback terminal. This terminal is connected to the inverting input terminal of the error amplifier through an internal 15Ω resistor, which can periodically control the on and off of the power switch tube. The upper and lower threshold voltages of the feedback terminal are 8.5V and 7.5V respectively, with a hysteresis voltage of 1V. This terminal is usually connected to the Vcc terminal and connected to the output voltage of the feedback coil. Obviously, the feedback voltage value reflects the high and low output voltage of the switching power supply. The output voltage of the feedback coil forms a feedback output voltage after high-frequency rectification and filtering, and is then connected to the feedback terminal through the phototransistor in the optocoupler. The transmitter tube of the optocoupler is connected to the sampling circuit. The feedback terminal is grounded through R3 and C5. C5 has three functions: (1) Timing capacitor of the startup circuit; (2) It also serves as a compensation capacitor, working with R3 to perform frequency compensation on the feedback loop; (3) As a bypass capacitor at the Vcc end of the working power supply voltage, during the startup process By charging C5, Vcc is established. As a special application, a resistor can be connected between the Vcc terminal and the feedback terminal, and other components can be used to form a soft-start circuit. Pin 3 (GND): Ground. This terminal is the common ground between the control circuit and the power switch tube. When a radiator is installed on the component, it also serves as the ground terminal of the radiator. Pin 4 (State Control Input, SCI): The input terminal of the state controller, referred to as the state control terminal. It is also a multi-functional terminal. With only a few peripheral components, it can control the on/off state of the converter in a variety of ways. It has 6 status control methods as follows: (1) Use button triggering to select working mode or standby mode; (2) Equipped with a microcontroller for shutdown operation; (3) Equipped with a low-voltage protection circuit for the status controller , so that it will not cause fluctuations in the output voltage of the switching power supply during the mode conversion process; (4) Use digital signals for control; (5) Equip with an electrical delay circuit; (6) Prohibit operation of the state controller. Pin 5 (Power Switch Drain, referred to as D): the drain terminal of the power switch tube. This end can directly drive the primary of the high-frequency transformer. In addition, it is connected to the drain of the internal start-up MOS field effect transistor. It should be pointed out that products using DIP-8 packaging have 4 additional GND terminals, but they are still equivalent to five-terminal components. The internal block diagram of the MC33370 series is shown in the figure below, which mainly includes 9 parts: (1) oscillator; (2) parallel regulator/error amplifier; (3) pulse width modulation comparator and pulse width modulation flip-flop; (4) External shutdown circuit and shutdown trigger; (5) current limit comparator and power switch tube; (6) startup circuit; (7) undervoltage lockout circuit; (8) overheating protection circuit; (9) status controller.