Functional check

AN3089

14/25

DocID16493 Rev 2

comparator activates a current source that restarts charging the soft-start capacitor from the

initial 2 V level. If the voltage on this pin reaches 5 V, the L6566B stops the operation and it

entering into the so-called “hiccup mode”.

In Figure 12 we can see that, in this case, the IC supply voltage drops to the UVLO

threshold (10 V), causing controller turn-off before the SS pin signal achieves the disable

threshold (5 V). This happens because the transformer leakage inductance is very low and

once the output voltage drops, the auxiliary voltage also immediately drops. Furthermore, in

the image we can note that during the SS voltage ramping-up the transferred power is

limited.

3.3

Overvoltage and open loop protection

The L6566B OVP function monitors the voltage on the ZCD pin (#11) during the MOSFET’s

OFF-time, when 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 which

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

5 V, the L6566B is latched off (Figure 14). 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-0.5V (Figure 15), maintaining the IC latched.

Additionally, to improve 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 12. Short-circuit at full

load and 230 Vac – 50 Hz

Figure 13. Short-circuit detail at full

load and 230 Vac – 50 Hz

CH1: gate voltage

CH2: Vcc

CH1: drain voltage

CH2: Vcc

CH3: SS pin voltage

CH4: output current

CH3: SS pin voltage

CH4: output current