SO16N datasheet(17/51 Pages) STMICROELECTRONICS

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L6566BH

Application information

Doc ID 16610 Rev 2

17/51

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 operations 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 L6566BH 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 startup generator

Figure 5 shows the internal schematic of the high voltage startup 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 startup generator: internal schematic

With reference to the timing diagram of

Figure 6, when power is first applied to the converter

the voltage on the bulk capacitor (Vin) builds up and, at about 80 V, the HV generator is

enabled to operate (HV_EN is pulled high) so that it draws about 1 mA. This current, minus

the device consumption, charges the bypass capacitor connected from the Vcc pin (5) to

ground and makes its voltage rise almost linearly.

AM11481v1

L6566BH

15 MW

GND

Vcc

Icharge

Vcc_OK

IHV

CONTROL

HV_EN

SO16N Datasheet(PDF) 17 Page - STMicroelectronics

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