AN2941 datasheet(20/37 Pages) STMICROELECTRONICS

20 / 37 page

Functional check

AN2941

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Doc ID 15447 Rev 4

To restart the system the input power must be recycled.

Additionally, if the voltage on pin PFC_OK is tied below 0.23 V, the L6563S is shut down. In

this case both PWM_STOP and PWM_LATCH keep their high impedance status. To restart

the IC simply let the voltage at the pin go above 0.27 V. This function can be used as a

remote on/off control input.

Note that this function offers a complete protection against not only feedback loop failures or

erroneous settings, but also against a failure of the protection itself. If either resistor of the

PFC_OK divider fails short or opens or the PFC_OK (#7) pin is floating, this results in

shutting down the L6563S and stopping the controller operation of the flyback stage.

The event of an open loop is given in Figure 19. We can notice the protection intervention

stopping the operation of the L6563S 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 opens also the pin Vcc_PFC, therefore the

L6563S remains inactive after latching because it is no longer powered.

This state is latched and the input power must be recycled in order to restart the IC. The

latch is removed as the voltage on the AC_OK goes below the brownout threshold.

The flyback stage is also protected 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 is proportional to 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 1 mA 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 20). 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

Figure 19.

EVL6566-75WES4 PFC open loop at 115 Vac-60 Hz - full load

CH1: Drain voltage

CH2: PFC output voltage

CH3: PFC_OK pin voltage

CH4: PWM_LATCH pin voltage

AN2941 Datasheet(PDF) 20 Page - STMicroelectronics