UC3845AD Integrated Circuits (ICs)

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UC3845AD is a fixed frequency current-mode control circuit with a minimum of external parts designed for off-line and DC-to-DC converter applications. It is especially suitable for high performance switch mode power supplies. It combines a high current totem pole output with a fixed frequency implementation of a current-mode PWM control. This unique configuration leads to a high degree of accuracy and flexibility in applications covering a wide range of power supplies design.

UC3845AD belongs to the category of DC-DC switching controllers, specifically to the subcategory of voltage regulators. As a DC-DC switching controller, it uses current sensing to regulate the output voltage in a relative stable fashion. Employing a linked combination of power switching spectral and topology, the controller can measure and adapt an output voltage that is supplied by a given voltage source.

UC3845AD has a few driving pins that provide excellent input noise immunity and system flexibility. When the supply voltage is greater than 15V and lower than 40V, the maximum gate-driving voltage can reach up to 30V, allowing larger power switching transistors. The pins on this IC include the error and timing amplifier pins, output drivers pins, current sense pins and latch pins.

The error amplifier is used to compare the supplied voltage with the reference voltage. The output of the error amplifier is then fed into the PWM comparator, and the oscillator of the PWM oscillator is basically an above reference voltage. On detecting an input signal above the reference voltage, the oscillator triggers and shifts the PWM signal which is then used by the output drivers.

The current sense pins are event detectors that detect the over-current signal, generated by the power transistor during load transients. If the signal satisfies the trigger level defined by the VREF pin, a latch pulse would be sent to the power transistor to shut off, preventing further damages to the components. The VFB pin is the feed-back pin which readjusts the output voltage.

The output drivers, including HIN and LIN, provide a totem-pole output source to charge and discharge an external capacitance. A logic high output is generated when the VILL switch is turned on, and a logic low is generated when the VILH is switched on.

The internal oscillator can run at a switch frequency from 8kHz to 90 kHz, and the dual output drivers also consist of a low-side driver with a maximum current sink of 50mA and a high-side driver with a maximum current source of 50mA. This high speed PWM can also be used in other applications like motor speed control and light dimming.

UC3845AD requires a minimum of external components, making it relatively easy to use and therefore making it highly suitable for applications like high frequency, high efficiency current mode power supplies, improved AC to DC conversion, buck/boost converter, flyback converter and forward converter.

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