FM2827 Shanghai Fudan Microelectronics Co., Ltd., FM2827 Datasheet - Page 6

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

2.3.
2.4.
FM2827 Power Factor Correction Controller
Zero Current Detector
Voltage Protection Block
The circuit operates as a critical conduction current mode controller. The zero current detector
switches on the external MOSFET as the voltage across the boost inductor reverses, just after the
current through the boost inductor has gone to zero. The slope of the inductor current is indirectly
detected by monitoring the voltage across an auxiliary winding and connecting it to the zero current
detector Pin 5. Once the inductor current reaches ground level, the polarity of the voltage across
the winding is reversed. When the Idet input falls below 1.5V, the comparator output is triggered to
the low state. To prevent false tripping, 0.5V hysteresis is provided. The zero current detector input
is protected internally by two clamps. The upper 7.5V clamp prevents input over voltage
breakdown while the lower 0.75V clamp prevents substrate injection. An internal current limit
resistor protects the lower clamp transistor in case the Idet pin is shorted to ground accidentally. A
watchdog timer function is added to the IC to eliminate the need for an external oscillator when
used in stand-alone applications. The timer provides a means to start or restart the pre-converter
automatically if the drive output has been off for more than 500us after the inductor current reached
zero.
The chip monitors the current flowing into the EA_OUT pin. If the monitored current reaches about
30μA, the output of multiplier is forced to be decreased, thus reducing the input current drawn from
the mains (soft OVP). If the monitored current exceeds 40μA, the OVP protection is triggered
(dynamic OVP), and then the external power transistor is switched off until the current falls below
about 10μA. In this case, it disables some internal blocks reducing the quiescent current of the chip
to 2mA. However, if the over voltage lasts so long that the output of E/A goes below 2.25V, then the
protection is activated (static OVP) keeping the output stage and the external power switch turned
off. The operation of the device is re-enabled as the E/A output goes back into its linear region.
Ver 2.1
Specification
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