UC3844NG

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The UC3844NG is a high performance fixed frequency current mode switching controller featuring a frequency selector circuit, an internal oscillator, a digital soft start circuit, an error amplifier, a driver for external power transistors, a current fold back circuit, a comparator for monitoring output voltage (OV) / output current (OC) protection, and a latch circuit for over temperature protection (OTP). It can be used for a wide range of applications, from AC/DC power supplies to high power DC/DC converters, and is also suitable for other switched mode power supplies.

The UC3844NG uses a current control mode to regulate the output voltage. It compares the reference voltage, VREF, with the signal voltage at the error amplifier output, which is produced by subtracting the feedback voltage from the external voltage reference, VREF. The error amplifier output is then feedback to the pulse width modulation (PWM) comparator, which will generate a pulse width modulated signal. This modulated signal is then used to turn on and off the power switch transistor to regulate the output voltage.

In addition to the current mode control, the frequency selection is a major factor for the UC3844NG. This is achieved by using an internal oscillator, which is powered from a resistor divider network connected to the current sense input. The frequency selection range can be adjusted from 5kHz to 150kHz by changing the external resistor divider network. This is extremely useful for applications such as AC/DC power supplies, where frequency jitter must be eliminated to reduce conducted EMI emissions.

At the start of the switching cycle the UC3844NG features a soft-start circuit, which prevents large charging current transients during turn-on. It is enabled via an external capacitor, which is charged via the current sense input. Once the soft-start time period has expired, the normal operation resumes.

The UC3844NG can also prevent over temperature conditions by utilizing the internal latch circuit. It is enabled by connecting a resistor between the OTP input and the current sense input. When the temperature rises too high, the latch will engage to shut down the power switch.

Finally, the UC3844NG has several other features built-in to ensure maximum performance and avoid destruction due to failed output control. First, the fold-back circuit will reduce the output current at light load conditions. Second, the over-voltage and over-current comparators will detect any abnormal output conditions and shut down the power switch. Finally, the output stage features a single-ended driver for even better protection.

In summary, the UC3844NG is an ideal solution for a wide range of switched mode power supplies and DC/DC converters in both AC and DC applications. It is an extremely reliable, high performance controller with a wide frequency selection range and comprehensive protection features. Most importantly, it is highly cost effective, which makes it a great choice for applications that require maximum cost efficiency.

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