UCC1858 Unitrode Semiconductor, UCC1858 Datasheet - Page 6

• Manufacturer Part Number: UCC1858
• Description: High Efficiency, High Power Factor Preregulator
• Manufacturer: Unitrode Semiconductor

APPLICATION INFORMATION (cont.)
Figure 3a. Oscillator Timing Waveforms
Figure 3c. Frequency Foldback Mode
Synchronized to Buck (DC/DC) PWM
Figure 2. Oscillator Block Diagram
UDG-97123
UDG-97122
6
the free running frequency is always lower than the syn-
chronization frequency.
When VAO falls below the threshold level set by FBL, the
oscillator goes into frequency foldback mode and dis-
ables
achieved by reducing the oscillator charging current as
the power level (and VAO voltage) falls. As shown in Fig-
ure 2, the difference between VAO and FBL regulates
current I
charging CT. The effective charge current into the capaci-
tor is given by (I
Figure 3b. Oscillator Timing Waveforms
synchronization.
f
Snom
Csub
which subtracts the current available for
19
20
CHnom
Standalone Operation
3.5
3
- I
Csub
The
R
T
1
C
). To avoid converter opera-
T
frequency
UCC1858
UCC2858
UCC3858
foldback
UDG-97121
UDG-97125
(1)
is