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參數(shù)資料
| 型號： | L6668TR |
| 廠商： | 意法半導(dǎo)體 |
| 英文描述： | SMART PRIMARY CONTROLLER |
| 中文描述： | 智能主控制器 |
| 文件頁數(shù)： | 12/23頁 |
| 文件大小： | 671K |
| 代理商： | L6668TR |
L6668
12/23
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
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.
Application Information
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
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.
0
0
f
sw
P
in
Fixed-frequency mode
Frequency foldback mode
Pinmax
L6668
15 M
GND
HV
Vcc
5
3
1
I
charge
Vcc_OK
I
HV
CONTROL
HV_EN
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