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UCC1858 Datasheet

  • UCC1858

  • High Efficiency, High Power Factor Preregulator

  • 12頁

  • TI

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UCC1858
UCC2858
UCC3858
High Efficiency, High Power Factor Preregulator
FEATURES
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Programmable PWM Frequency
Foldback for Higher Efficiency at Light
Loads
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Leading Edge PWM for Reduced
Output Capacitor Ripple Current
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Controls Boost PWM to Near Unity
Power Factor
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World Wide Operation without
Switches
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Accurate Power Limiting
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Synchronizable Oscillator
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100碌A Startup Supply Current
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Low Power BCDMOS
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12V to 18V Operation
PRELIMINARY
DESCRIPTION
The UCC3858 provides all of the control functions necessary for active
power factor corrected preregulators which require high efficiency at low
power operation. The controller achieves near unity power factor by
shaping the AC input line current waveform to correspond to the AC input
line voltage using average current mode control.
The operation of the UCC3858 closely resembles that of previously de-
signed Unitrode PFC parts with additional features to allow higher effi-
ciency boost converter operation at light loads. This is accomplished by
linearly scaling back the PWM frequency when the output of the voltage
error amplifier drops below a predetermined user programmable level in-
dicating a light load condition. The frequency is scaled back by reducing
the charging current for the CT ramp (in proportion to the output power),
and increasing the dead time. There is also an instantaneous reset input
to pull the IC out of foldback mode quickly when the load comes back up.
The PWM technique used in the UCC3858 is leading edge modulation.
When combined with the more conventional trailing edge modulation on
the downstream converter, this scheme offers the benefit of reduced rip-
ple current on the bulk storage capacitor. The oscillator is designed for
easy synchronization to the downstream converter. A simple synchroni-
zation scheme can be implemented by connecting the PWM output of
the downstream converter to the SYNC pin.
(continued)
BLOCK DIAGRAM
UDG-96191-1
03/99

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