L6566B View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6566B' PDF : 51 Pages View PDF
L6566B
5
Application information
Application information
The L6566B is a versatile peak-current-mode PWM controller specific for offline flyback
converters. The device allows either fixed-frequency (FF) or quasi-resonant (QR) operation,
selectable with the pin MODE/SC (12): forcing the voltage on the pin over 3 V (e.g. by tying
it to the 5 V reference externally available at pin VREF, 10) will activate QR operation,
otherwise the device will be 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's
turn-on to the transformer’s demagnetization by detecting the resulting negative-going
edge of the voltage across any winding of the transformer. Then the system works
close to the boundary between discontinuous (DCM) and continuous conduction
(CCM) of the transformer. As a result, the switching frequency will be 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 L6566B, synchronized to MOSFET’s turn-on, enables the designer to define the
maximum operating frequency of the converter. As the load is reduced MOSFET’s turn-
on will not any more occur on the first valley but on the second one, the third one and
so on. In this way the switching frequency will no longer increase (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 will enter a controlled on/off operation with constant peak current.
Decreasing the load will then result in frequency reduction, which can go down even to
few hundred hertz, thus minimizing all frequency-related losses and making it easier to
comply with energy saving regulations or recommendations. Being the peak current
very low, no issue of audible noise arises.
Figure 4. Multi-mode operation with QR option active
fosc
Valley-s kipping
f sw
mode
Input voltage
Burs t-m ode
0
0
Quasi-resonant mode
Pin
Pinmax
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