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L6562AT Datasheet(PDF) 13 Page - STMicroelectronics |
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L6562AT Datasheet(HTML) 13 Page - STMicroelectronics |
13 / 25 page Application information L6562AT 13/25 When the load of a PFC pre-regulator is very low, the output voltage tends to stay steadily above the nominal value, which cannot be handled by the Dynamic OVP. If this occurs, however, the error amplifier output will saturate low; hence, when this is detected the external power transistor is switched off and the IC put in an idle state (static OVP). Normal operation is resumed as the error amplifier goes back into its linear region. As a result, the device will work in burst-mode, with a repetition rate that can be very low. When either OVP is activated the quiescent consumption of the IC is reduced to minimize the discharge of the Vcc capacitor and increase the hold-up capability of the IC supply system. 7.2 Disable function The INV pin doubles its function as a not-latched IC disable: a voltage below 0.2 V shuts down the IC and reduces its consumption at a lower value. To restart the IC, the voltage on the pin must exceed 0.45 V. The main usage of this function is a remote ON/OFF control input that can be driven by a PWM controller for power management purposes. However it also offers a certain degree of additional safety since it will cause the IC to shutdown in case the lower resistor of the output divider is shorted to ground or if the upper resistor is missing or fails open. 7.3 THD optimizer circuit The device is equipped with a special circuit that reduces the conduction dead-angle occurring to the AC input current near the zero-crossings of the line voltage (crossover distortion). In this way the THD (total harmonic distortion) of the current is considerably reduced. A major cause of this distortion is the inability of the system to transfer energy effectively when the instantaneous line voltage is very low. This effect is magnified by the high- frequency filter capacitor placed after the bridge rectifier, which retains some residual voltage that causes the diodes of the bridge rectifier to be reverse-biased and the input current flow to temporarily stop. |
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