L6566ATR datasheet(17/51 Pages) STMICROELECTRONICS

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L6566A

Application information

17/51

Application information

The L6566A 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

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.

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

the L6566A, 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).

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

fsw

Pinmax

Input voltage

fosc

Burst-mode

Valley-skipping

mode

Quasi-resonant mode

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