Application information L6566A
16/52 Doc ID 13794 Rev 4
5 Application information
The L6566A is a versatile peak-current-mode PWM controller specific to offline flyback
converters. The device allows either fixed-frequency (FF) or quasi-resonant (QR) operation,
selectable with the MODE/SC pin (12): forcing the voltage on the pin over 3 V (e.g. by tying
it to the 5 V reference externally available at the VREF pin, 10) activates QR operation,
otherwise the device is FF-operated.
Irrespective of the operating option selected by pin 12, the device is able to work in different
modes, depending on the converter's load conditions. If QR operation is selected (see
Figure 4):
1. QR mode at heavy load. Quasi-resonant operation lies in synchronizing MOSFET turn-
on to the transformer's demagnetization by detecting the resulting negative-going edge
of the voltage across any winding of the transformer. The system then works close to
the boundary between discontinuous (DCM) and continuous conduction (CCM) of the
transformer. As a result, the switching frequency is different for different line/load
conditions (see the hyperbolic-like portion of the curves in Figure 4). Minimum turn-on
losses, low EMI emission, and safe behavior in short-circuit are the main benefits of
this kind of operation.
2. Valley-skipping mode at medium/light load. The externally programmable oscillator of
the L6566A, synchronized to MOSFET turn-on, enables the designer to define the
maximum operating frequency of the converter. As the load is reduced, MOSFET turn-
on no longer occurs on the first valley but on the second one, the third one, and so on.
In this way the switching frequency no longer increases (piecewise linear portion in
Figure 4).
3. Burst-mode with no or very light load. When the load is extremely light or disconnected,
the converter enters a controlled on/off operation with constant peak current.
Decreasing the load then results in frequency reduction, which can go down even to a
few hundred hertz, therefore minimizing all frequency-related losses and making it
easier to comply with energy saving regulations or recommendations. Having the peak
current very low, no issue of audible noise arises.
Figure 4. Multi-mode operation with QR option active
0
0
f
sw
Pinmax
Input voltage
P
in
f
osc
Burst-mode
Valley-skipping
mode
Quasi-resonant mode
