L6566B datasheet(18/51 Pages) STMICROELECTRONICS

18 / 51 page

Application information

L6566B

18/51

If FF operation is selected:

FF mode from heavy to light load. The system operates exactly like a standard current

mode control, at a frequency fsw determined by the externally programmable oscillator:

both DCM and CCM transformer operation are possible, depending on whether the

power that it processes is greater or less than:

Equation 1

where Vin is the input voltage to the converter, VR the reflected voltage (i.e. the

regulated output voltage times the primary-to-secondary turn ratio) and Lp the

inductance of the primary winding. PinT is the power level that marks the transition from

continuous to discontinuous operation mode of the transformer.

Burst-mode with no or very light load. This kind of operation is activated in the same

way and results in the same behavior as previously described for QR operation.

The L6566B is specifically designed for applications with no PFC front-end; pin 6 (FMOD)

features an auxiliary oscillator that can modulate the switching frequency (when FF

operation is selected) in order to mitigate EMI emissions by a spread-spectrum action.

5.1

High-voltage start-up generator

Figure 5

shows the internal schematic of the high-voltage start-up generator (HV generator).

It is made up of a high-voltage N-channel FET, whose gate is biased by a 15 M

Ω resistor,

with a temperature-compensated current generator connected to its source.

Figure 5.

High-voltage start-up generator: internal schematic

Vin

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

L6566B

15 M

GND

Vcc

Icharge

Vcc_OK

IHV

CONTROL

HV_EN

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