L6566ATR datasheet(24/51 Pages) STMICROELECTRONICS

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Application information

L6566A

24/51

5.3

Burst-mode operation at no load or very light load

When the voltage at the COMP pin (9) falls 20 mV below a threshold fixed internally at a

value, VCOMPBM, depending on the selected operating mode, the L6566A is disabled with

the MOSFET kept in OFF state and its consumption reduced at a lower value to minimize

Vcc capacitor discharge.

The control voltage now will increase as a result of the feedback reaction to the energy

delivery stop (the output voltage will be slowly decaying), the threshold will be exceeded and

the device will restart switching again. In this way the converter will work in burst-mode with

a nearly constant peak current defined by the internal disable level. A load decrease will

then cause a 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. This kind of operation, shown in the timing diagrams of Figure 11 along with the

others previously described, is noise-free since the peak current is low.

If it is necessary to decrease the intervention threshold of the burst-mode operation, this can

be done by adding a small DC offset on the current sense pin as shown in Figure 12 on

page 25

Note:

The offset reduces the available dynamics of the current signal; thereby, the value of the

sense resistor must be determined taking this offset into account.

Figure 11. Load-dependent operating modes: timing diagrams

(pin 4)

VCOMPBM

COMP

(pin 9)

20 mV

hyster.

QR Mode

Valley-skipping Mode

Burst-mode

fsw

fosc

QR Mode

FF Mode

Burst-mode

FF Mode

MODE/SC=Open

MODE/SC=VREF

MODE/SC=Open

MODE/SC=VREF

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