General Description
The ML4821 provides complete control for a “boost” type power factor correction system using the average current sensing method. Special care has been taken in the design of the ML4821 to increase system noise immunity. The circuit includes a precision reference, gain modulator, average current error amplifier, output error amplifier, over-voltage protection comparator, shutdown logic, as well as a high current output. In addition, start-up is simplified by an under voltage lockout circuit.
In a typical application, the ML4821 controls the AC input current by adjusting the pulse width of the output MOSFET. This modulates the line current so that its shape conforms to the shape of the input voltage. The reference for the current regulator is a product of the sinusoidal line voltage times the output of the error amplifier which is regulating the output DC voltage. Average line voltage compensation is provided in the gain modulator to ensure constant loop gain over a wide input voltage range. This compensation includes a special “brown-out” control which reduces output power below 90V RMS input.

FEATUREs
• Average current sensing for lowest possible harmonic
   distortion
• Average line compensation with brown-out control
• Precision buffered 5V reference
• 1A peak current totem-pole output drive
• Overvoltage comparator eliminates output “runaway” due
   to load removal
• Wide common mode range in current sense comparators
   for better noise immunity
• Large oscillator amplitude for better noise immunity
• Output driver internally limited to 17V
• “Sleep mode” shutdown input