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.