L6668_06 STMICROELECTRONICS [STMicroelectronics], L6668_06 Datasheet - Page 12

• Manufacturer Part Number: L6668_06
• Description: SMART PRIMARY CONTROLLER
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

L6668
4
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
2) Frequency-foldback mode at medium and light load. As the load is reduced the oscillator frequency is re-
3) Burst-mode control with no or very light load. When the load is extremely light or disconnected, the converter
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 Ω resistor, with a temperature-com-
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.
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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.
duced as well by slowing down the charge of the timing capacitor proportionally to the load itself.
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.
Application Information
f
sw
0
0
Frequency foldback mode
L6668
Vcc_OK
15 M Ω
CONTROL
HV_EN
P
GND
3
in
1
HV
I
charge
Fixed-frequency mode
I
HV
5
Vcc
Pinmax