UC3825Q

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UC3825Q Application Field and Working Principle

UC3825Q is a pulse width modulation control circuit specifically designed for flyback converters, Buck, Buck-Boost and PFC circuits. It integrates several features on to one chip with the purpose of simplifying the design process. This DC-DC controller is part of the Voltage Regulators range in Power Management Integrated Circuits (PMIC) and is classified under DCDC Switching Controllers.

Features

The UC3825Q standby current is very low, ideal for high efficiency circuits requiring lowest possible power dissipation. It also integrates features such as:

• Voltage controlled oscillator with fixed frequency or frequency modulation
• Over current protection
• 25V working voltage maximum
• Enhanced surge current capability
• Programmable under voltage protection
• Output paralleling boost diode drivers
• Low operating current of 6mA
• Single Resistor Frequency Programming
• Over temperature protection
• Internal control gates for high side MOSFET and synchronous rectifier control

These together with its improved performances, makes it suitable for a wide variety of applications including: notebook adapter, WIFI and Bluetooth, TV, Computer and Mobile Applications, Industrial Application to name a few.

Working Principle

UC3825Q is a PWM control circuit designed to work in Flyback Converter Topology, which creates a regulated DC output voltage through the use of an inductor and transformer. The circuit works according to the following principles:

• The switching signal is generated by the IC through a voltage-controlled oscillator. This voltage-control signal is generated by comparing the Error Amplifier output with a triangle wave signal generated inside the IC.
• To provide under-voltage protection at the output and prevent operation of the Flyback topology below a certain voltage, the IC includes an undervoltage lockout circuit.
• At power-up, UC3825Q applies a start-up signal for a fixed duration. This duration is adjusted externally by selecting a capacitor and resistor connected to the START pin
• To ensure that the output voltage remains within the desired range, and to indicate an undervoltage or over-voltage situation at the output, an Error Amplifier compares the output voltage with the reference voltage.
• To prevent an overcurrent situation in the flyback circuit, UC3825Q provides a logic signal at the P em pin, indicating that a certain maximum primary current has been reached and causes the MOSFET to be switched off for a fixed duration.

Conclusion

The UC3825Q is a very versatile DC-DC controller which provides excellent performance over a wide range of conditions including voltage, current and power. Its synchronous rectifier control, low operating current and programmable under-voltage protection make it ideal for a wide range of applications in the power management industry. Additionally, with its low standby current, integrated features and simple external system requirements, the UC3825Q is a reliable and efficient solution for current and future power management systems.

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