SO16N datasheet(16/51 Pages) STMICROELECTRONICS

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Application information

L6566BH

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Doc ID 16610 Rev 2

Application information

The L6566BH 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):

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. Then, the system 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.

Valley-skipping mode at medium/ light load. The externally programmable oscillator of

the L6566BH, synchronized to MOSFET turn-on, enables the user 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).

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

few hundred hertz, therefore minimizing all frequency-related losses and making it

easier to comply with energy saving regulations or recommendations. With the peak

current very low, no issue of audible noise arises.

Figure 4.

Multimode operation with QR option active

AM11480v1

fsw

Pinmax

Input voltage

fosc

Burst-mode

Valley-skipping

mode

Quasi-resonant mode

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