L6668

12/23

4

Application Information

The L6668 is a versatile current-mode PWM controller specific for offline fixed-frequency, peak-current-

mode-controlled flyback converters.

The device is able to operate in different modes (fig. 34), depending on the converter's load conditions:

1) Fixed frequency at heavy load. In this region the IC operates exactly like a standard current mode control

chip: a relaxation oscillator, externally programmable with a capacitor and a resistor, generates a sawtooth

and releases a clock pulse during the falling edge of the sawtooth; the power switch is turned on by the clock

pulses and is turned off by the control loop.

2) Frequency-foldback mode at medium and light load. As the load is reduced the oscillator frequency is re-

duced as well by slowing down the charge of the timing capacitor proportionally to the load itself.

3) Burst-mode control 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. A load decrease will be then translated

into 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. Being the peak current very

low, no issue of audible noise arises.

Figure 34. Multi-mode operation of the L6668

4.1 High-voltage start-up generator

Figure 35 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

pensated current generator connected to its source.

Figure 35. High-voltage start-up generator: internal schematic

The HV generator is physically located on a separate chip, made with BCD off-line technology able to with-

stand 700V, controlled by a low-voltage chip, where all of the control functions reside.

0

0

Fixed-frequency mode

Frequency foldback mode

Pinmax

L6668

15 M

Ω

GND

HV

Vcc

5

3

1

Vcc_OK

CONTROL

HV_EN