AN2690 datasheet(17/31 Pages) STMICROELECTRONICS

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AN2690

Functional check

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Figure 17.

EVL6566-75WADP PFC open loop at 115 Vac-60 Hz - full load

The event of an open loop is shown in Figure 17. We can notice the protection intervention

stopping the operation of the L6563 and the activation of the PWM_LATCH pin that is

connected to the L6566A pin #7 (DIS). This function of the L6566A is a latched device

shutdown. Internally the pin connects a comparator that, when the voltage on the pin

exceeds 4.5 V, shuts down the IC and brings its consumption to a value barely higher than

before startup. The internal L6566A logic also opens the pin Vcc_PFC, therefore the L6563

remain inactive after latching because it is no longer powered.

This state is latched and it is necessary to recycle the input power to restart the IC. The

latch is removed as the voltage on the Vcc pin falls below the UVLO threshold.

The flyback stage is protected too against open loop conditions that lead to losing control of

the output voltage.

The L6566A OVP function monitors the voltage on the ZCD pin (#11) during the MOSFET's

OFF-time, during which the voltage generated by the auxiliary winding tracks the converter's

output voltage. If the voltage on the pin exceeds an internal 5 V reference, an overvoltage

condition is assumed and the device is shut down. An internal current generator is activated

that sources 1mA out of the VFF pin (#15). If the VFF voltage is allowed to reach 2 VBE over

5 V, the L6566A is latched off (Figure 18). As soon as the IC is latched, Vcc starts

decreasing until it reaches a value 0.5 V below the turn-on threshold. Then the HV startup

circuit turns on and begins to operate periodically in order to keep Vcc between VccON and

VccON-0.5 V (Figure 19) maintaining the IC latched.

CH1: Drain voltage

CH2: PFC output voltage

CH3: PFC_OK pin voltage

CH4: PWM_LATCH pin voltage

AN2690 Datasheet(PDF) 17 Page - STMicroelectronics