AN2690 datasheet(18/31 Pages) STMICROELECTRONICS

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Functional check

AN2690

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If R37 is shorted, the impedance externally connected to pin #15 (VFF) is lower and the

voltage in case of OVP cannot reach the 5+2VBE threshold so the L6566A restarts after the

Vcc has dropped below 5 V. In case of L6566A OVP intervention, the L6563 operation is

stopped too because the L6566A stops the PFC via the Vcc_PFC pin.

In Figure 18 it is also possible to note that the signal at ZCD pin has on the leading edge the

typical spike at MOSFET turn-off due to the transformer leakage inductance. Thanks to the

digital circuitry integrated in the L6566A it doesn't influence the correct behavior of the IC. In

fact an internal 2

µs (typ.) blanking time at MOSFET turn-off delays the sampling (0.5 µs) of

the signal that in that region is clean and therefore perfectly linked to the output voltage.

Additionally, to improve the immunity against temporary disturbances (needed for example

in case of immunity tests), an internal logic activates the protection after the OVP has been

detected for 4 consecutive switching cycles.

Figure 18.

Flyback open loop at 230 Vac-50 Hz

- half load

Figure 19.

Flyback open loop at 230 Vac-50 Hz

- half load

CH1: VFF voltage

CH2: Vcc

CH3: ZCD voltage

CH4: Output voltage

CH1: VFF voltage

CH2: Vcc

CH3: ZCD voltage

CH4: Output voltage

AN2690 Datasheet(PDF) 18 Page - STMicroelectronics