FM2827 Shanghai Fudan Microelectronics Co., Ltd., FM2827 Datasheet - Page 5

• Manufacturer Part Number: FM2827
• Description: Fm2827 Power Factor Correction Controller
• Manufacturer: Shanghai Fudan Microelectronics Co., Ltd.

2.
2.1.
2.2.
FM2827 Power Factor Correction Controller
State Description
Multiplier
Current Sense Comparator
The rectified AC line voltage is connected to an external resistor divider, and gets a rectified
voltage range from 0 to 3.8V. The signal is sent to MULT pin. The other input is internally driven by
a DC voltage which is the difference between error amplifier output (Pin 2) and reference voltage,
Vref.
The multiplier is designed to have an extremely linear transfer curve over a wide dynamic range,
0V to 3.8V for Pin 3, and 2.25V to 7.3V for error amplifier output under all line and load conditions.
The multiplier output controls the current sense comparator threshold voltage as the ac voltage
traverses sinusoidally from zero to peak line. This allows the inductor peak current to follow the ac
line thus forcing the average input current to be sinusoidal. In other words, this has the effect of
forcing the MOSFET on-time to track the input line voltage, resulting in a fixed drive output on-time,
thus making the pre-converter load appear to be resistive to the ac line.
The current sense comparator adopts the RS latch configuration to ensure that only a single pulse
appears at the drive output during a given cycle. MOSFET drain current is sensed using an
external sense resistor in series with the external MOSFET. When the sensed voltage exceeds the
threshold set by the multiplier output, the current sense comparator turns off the MOSFET and
resets the PWM latch. The latch insures that the output remains in a low state after the MOSFET
drain current falls back to zero. The peak inductor current under the normal operating condition is
controlled by the multiplier output, Vmo. The abnormal operating condition occurs during
pre-converter start-up at extremely high line or as output voltage sensing is lost. Under these
conditions, the multiplier output and current sense threshold will be internally clamped to 1.8V.
Therefore, the maximum peak switch current is limited to:
An internal R/C filter has been included to attenuate any high frequency noise that may be present
on the current waveform. This circuit block eliminates the need for an external R/C filter otherwise
required for proper operation of the circuit.
Ipk (max) = 1.8V / Rsense
Ver 2.1
Specification
5