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Controls Boost PWM to Near Unity
Power Factor
Limits Line Current Distortion To <3%
World-Wide Operation Without Switches
Accurate Power Limiting
Fixed Frequency Average Current Mode
Control
High Bandwidth (5MHz), Low Offset
Current Amplifier
Integrated Current and Voltage Amp
Output Clamps
Multiplier Improvements: Linearity,
500mV VAC Offset (eliminates external
resistor), 0-5V Multout Common Mode
Range
V
REF
"GOOD" Comparator
Faster and Improved Accuracy ENABLE
Comparator
UVLO Threshold Options
(16/10V / 10.5/10V)
300碌A(chǔ) Startup Supply Current
UVLO Turn on UVLO Turn off
16V
10V
10.5V
10V
DESCRIPTION
The UC1854A/B products are pin compatible enhanced versions of the
UC1854. Like the UC1854, these products provide all of the functions
necessary for active power factor corrected preregulators. The
controller achieves near unity power factor by shaping the AC input line
current waveform to correspond to the AC input line voltage. To do this
the UC1854A/B uses average current mode control. Average current
mode control maintains stable, low distortion sinusoidal line current
without the need for slope compensation, unlike peak current mode
control.
The UC1854A/B products improve upon the UC1854 by offering a wide
bandwidth, low offset Current Amplifier, a faster responding and
improved accuracy enable comparator, a V
REF
"good" comparator,
UVLO threshold options (16/10V for offline, 10.5/10V for startup from
an auxiliary 12V regulator), lower startup supply current, and an
enhanced multiply/divide circuit. New features like the amplifier output
clamps, improved amplifier current sinking capability, and low offset
VAC pin reduce the external component count while improving
performance. Improved common mode input range of the Multiplier
output/Current Amp input allow the designer greater flexibility in
choosing a method for current sensing. Unlike its predecessor, R
SET
controls only oscillator charging current and has no effect on clamping
the maximum multiplier output current. This current is now clamped to
a maximum of 2 * I
AC
at all times which simplifies the design process
and provides foldback power limiting during brownout and extreme low
line conditions.
A 1% 7.5V reference, fixed frequency oscillator, PWM, Voltage
Amplifier with softstart, line voltage feedforward (V
RMS
squarer), input
supply voltage clamp, and over current comparator round out the list of
features.
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UC1854A
UC1854B
BLOCK DIAGRAM
UDG-93001-1
6/98
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