UC3845D8G4

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UC3845D8G4 Application Field and Working Principle

UC3845D8G4 is a popular hybrid integrated DC/DC controller IC which provides complete power control for switching power supplies, on-board voltage regulation, and AC/DC lamp control applications. It is suitable for sub-500W power supplies with IC power level. It is a kind of Power Management Integrated Circuit (PMIC) and a particular Voltage Regulator in DC to DC Switching Controllers family.

The UC3845D8G4 IC is generally used in AC/DC adapters and power supplies, Industrial and consumer applications such as Cellular phone, TV set, CRT monitor, and so on. It also supports a wide range of power applications included 8V-42V DC to 6V-50V DC, buck-boost, and dual output.

UC3845D8G4 is a fixed frequency PWM controller designed to minimize the external parts count. Internally, the UC3845 features a temperature compensated reference, error amplifier, oscillator, self-protection circuits and gate drive circuitry. The IC generated pulse width modulation (PWM) output signals for so controlling a power switch for regulation purposes. The PWM frequency is externally programmable from 10kHz to 1 MHz as desired.

The available Gate drive for the UC3845 is an embedded isolation terminal with a source current ranging from 4A to 6A and peak rating of 10A. The power dissipation of the IC is limited to 800mW. Connecting a diode to the output of the IC ensures the safety of the output transistor of the power supply.

The package type of the UC3845D8G4 is 8-pin PDIP, 8-pin SOIC, or 8-pin SSOP, with ambient temperature range of -40°C to +85°C. Operating voltage supply should be chosen ranging from 8-40V DC, while the quiescent current varies between 25mA at 12V and 50mA at 40V, dependent on the levels set by the adjustment of external resistors.

The basic connection diagram of the UC3845D8G4 is as Fig1 (reshowin as below). The components identified by R1 and C1 are connected to the feedback loop for regulating the output voltage. The frequency of oscillation is set with R2 and C2, while the turn-on/off of the output transistor is determined by R3 and C3.

UC3845D8G4 has a few main features. Firstly, it operates in Constant Frequency or Burst Mode Operation. It also has shut down and soft-start Voltage regulation. Other features include a high voltage start-up current source and open-loop output adjustment, leading to adaptability and accuracy of the settings.

The working principle of UC3845D8G4 is based on the comparator and zero crossing detector. The Error Amplifier compares the voltage setting on FB pin (feedback) with the voltage setting of Vref (reference), and generates a difference voltage VD. The comparator compares VD with 0V, and then translates it into a PWM signal by Zero Crossing Detector. The PWM signal controls an output switch to regulate the output voltage, fully reflecting the feedback loop’s characteristics.

In summary, UC3845D8G4 is an adjustable voltage regulator, belonging to the DC–DC Switching Controllers family of PMIC. It is designed to minimize external components count by providing accurate and reliable control over output voltage. With its broad range of supported power application, temperature compensation and self-protection, UC3845D8G4 IC is a powerful solution for AC/DC adapters and several commercial or industrial applications.

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