Functional check

AN2521

14/33

Figure 17.

Adapter circuit primary side waveforms 265 V, 50 Hz

3.2

Standby and no-load operation

In Figure 18 and Figure 19, some no-load waveforms of the circuit are shown. As illustrated,

the L6668 works in burst mode to achieve optimal efficiency. The burst mode threshold can

be adjusted by setting the divider connected to the SKIPADJ pin (#9). When the voltage at

the COMP pin falls 50 mV below the voltage on the SKIPADJ pin, the IC is shut down and

consumption is reduced. The chip is re-enabled as the voltage on the COMP pin exceeds

the voltage on the threshold set by pin 9 with its hysteresis. Additionally, in order to achieve

the best efficiency during light load operation the PFC stage is turned off. In fact, the L6668

pin #14 (PFC_STOP) is dedicated to enabling or disabling PFC operation according to the

CH1: Q7 drain voltage

CH2: L6668 Vpin #12 (ISEN)

Figure 18.

EVAL6668-75 W no-load operation

waveforms @90 V, 60 Hz

Figure 19.

EVAL6668-75 W no-load operation

waveforms @265 V, 50 Hz

CH1: Q7 drain voltage

CH2: L6668 COMP (pin #10)

CH1: Q7 drain voltage

CH2: L6668 COMP (pin #10)