UC3843DG4

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The UC3843DG4 is a high performance current mode Pulse-Width Modulator (PWM) Control Circuit ideally suited for Stiff DC-to-DC switched mode power supply (SMPS) controls. It has wide application in multiple fields, particularly on power supplies such as those used in TVs and monitors. The UC3843DG4 allows better overall system performance and reliability due to its highly optimized control design.

Applications

The UC3843DG4 can be used as a control device in a variety of SMPS applications, including; Off-line, Flyback, Forward, Buck, Boost and Buck-Boost Switching Power Supply topologies. This is due to its internal high gain error amplifier with programmable transconductance for various gain control applications. As well as this, its wide input voltage range coupled with its built-in features such as under voltage lockout and bi-directional cycle-by-cycle over current protection make the UC3843DG4 an ideal choice for a wide range of DC-to-DC data sheet supply topologies.

Features

• Wide input voltage range up to 50V
• Low start up and operating current
• On-chip temperature compensation
• Under voltage lockout (UVLO) circuitry with adjustable threshold levels
• Programmable transconductance for varying gain control
• Low power shutdown mode
• Multiple compensation networks available

Working Principles

The UC3843DG4 works by generating a variable frequency, variable duty cycle signal that regulates the output voltage of the power supply. It does so by internally comparing the voltage at its output (Vcontrol) with the output voltage (Vout) and adjusting the duty cycle of the output signal accordingly. This is achieved by a loop feedback system consisting of an error amplifier responding to the output voltage, an oscillator with an adjustable frequency, and a set of logic gates acting as a latch with a programmable dead time. The latch generates the output signal with a variable duty cycle. A higher output voltage will cause the error amplifier to reduce the voltage at Vcontrol, which reduces the duty cycle of the signal generated by the latch. Similarly, a decrease in the output voltage will result in a higher duty cycle.

The frequency of the signal is determined by the values of an external resistor (Rt) and an external capacitor (Ct). Generally, the oscillator runs at a frequency between 33kHz and 500kHz, depending on the values of Rt and Ct. The other end of the oscillator is connected to the latch, which acts as a flip flop that switches ON and OFF alternately. The ON and OFF times can be adjusted by setting the dead time. When the latch is in the OFF state, the output is driven low, after which it is driven high during the ON time. This variable frequency, variable duty cycle signal is then used to drive the power switch to regulate the output voltage.

The under-voltage lockout (UVLO) circuit ensures the output voltage remains within the specified range, and provides protection against a sudden drop in the input voltage. The UVLO circuit is connected to the positive input of the error amplifier and the latch, providing two separate circuit references. Internal circuitry in the UC3843DG4 keeps the latch disabled, and the output signal low, until the voltage at the positive input of the error amplifier, and the positive input of the latch both reach the respective threshold levels.

The UC3843DG4 is therefore a very versatile and reliable controller that is suited to a variety of switching power supply applications. Its wide input voltage range, protection features and programmable transconductance make it an ideal choice for a range of DC-to-DC Switching Power Supplies.

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