UC3845ADG4

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The UC3845ADG4 is a high performance current-mode control circuit, specifically designed for off-line and DC to DC converter applications offering the designer oscillator synchronization capability, frequency jittering, a buffered error amplifier, a single ramp generator and a current mode PWM comparator with programmable switching frequency. It is especially suited for those applications that require easier setting of pulse by-pass ratio, low noise, and cycle by - cycle current limit.

Application Information

The UC3845ADG4 is designed for use in a wide variety of power supplies, including switch-mode, continuous-conduction, DC-DC boost converters, off-line and DC power supplies, and constant-current/constant voltage controller circuits. These controllers can be built to control current, voltage, and frequency, and can achieve excellent noise reduction thanks to their frequency jittering, oscillator synchronization, and single ramp generation features.

The device is well suited for driving power-transistor switches, particularly Pulse Width Modulated (PWM) loads and electromechanical relays. The UC3845ADG4 can be used with a variety of inductors and capacitors to form a complete, low noise power supply system. With a wide range of options, the circuit can be tailored to many different types of power supplies and converters.

Working Principle

The UC3845ADG4 has seven pins consisting of four for the error amplifier (pins 1, 2, 3 and 4), two for the comparator (pins 5 and 6) and one for the oscillator (pin 7). The error amplifier has differential inputs that sense the output voltage from the external power supply and compares it to the reference voltage. The amplifier output is buffered before reaching the comparator where it is compared with the oscillator triangle waveform.

When the triangle wave rises above the reference voltage, the comparator output is low (logic 0 level), which allows the power transistor switches connected to the PWM comparator output to close and current flow through the external circuit. When the triangle wave falls below the reference voltage, the comparator output is high (logic 1 level) and the power transistor switches connected to the PWM comparator pin opens, thus stopping current flow through the external circuit.

The frequency of the triangle wave is set by the internal oscillator and can be varied from 16 kHz up to 500 kHz. The oscillator can be synchronized with an external frequency for noise reduction and for applications requiring a fixed frequency. The UC3845ADG4 also features a adjustable pulse by-pass ratio (PBPR) and can be adjusted by varying either the Peak Current Threshold (PCT) voltage or the PBPR voltage.

The UC3845ADG4 features cycle-by-cycle current-limit protection and overtemperature protection (OTP). When the current fed through the external circuit exceeds the current-limit threshold, current-limit protection is activated and the power transistor switches connected to the PWM comparator output are opened, thus stopping current flow through the external circuit. The OTP feature monitors the die temperature and initiates a thermal shutdown procedure to protect the device from thermal damage.

Conclusion

The UC3845ADG4 is a high performance current-mode control circuit specifically designed for off-line and DC to DC converter applications. It is suitable for driving power-transistor switches, particularly PWM loads and electromechanical relays. The device features oscillator synchronization capability and frequency jittering for low noise operation. It also features a single ramp generator, a buffered error amplifier, and a current mode PWM comparator with programmable switching frequency. With its wide range of options and protection features, the UC3845ADG4 is an ideal choice for use in a variety of switch-mode, continuous-conduction, DC-DC boost converters, off-line and DC power supplies, and constant-current/constant voltage controller circuits.

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