UC1854J UNITRODE, UC1854J Datasheet - Page 6

• Manufacturer Part Number: UC1854J
• Description: UNITRODE
• Manufacturer: UNITRODE

TYPICAL CHARACTERISTICS at T
APPLICATIONS INFORMATION
A 250W PREREGULATOR
The circuit of Figure 1 shows a typical application of the
UC3854 as a preregulator with high power factor and effi-
ciency. The assembly consists of two distinct parts, the
control circuit centering on the UC3854 and the power
section.
The power section is a "boost" converter, with the induc-
tor operating in the continuous mode. In this mode, the
duty cycle is dependent on the ratio between input and
output voltages; also, the input current has low switching
frequency ripple, which means that the line noise is low.
Furthermore, the output voltage must be higher than the
peak value of the highest expected AC line voltage, and
all components must be rated accordingly.
In the control section, the UC3854 provides PWM pulses
(GT Drv, Pin 16) to the power MOSFET gate. The duty
Mult Out
Mult Out
A
A
160
140
120
100
600
500
400
300
200
100
80
60
40
20
0
0
0
0
100
100
200
Multiplier Output vs Multiplier Inputs with Mult Out=0V
200
I
I
AC
AC
, A
, A
VA Out=3.5V
300
VA Out=2.5V
300
VA Out=1.25V
A
VA Out=5V
= T
VA Out=1.25V
VA Out=4V
VA Out=3V
VA Out=2V
V
V
400
J
RMS
400
RMS
= 25
=1.5V
=4V
o
500
500
C (cont.)
6
cycle of this output is simultaneously controlled by four
separate inputs to the chip:
V
I
I
V
Additional controls of an auxiliary nature are provided.
They are intended to protect the switching power MOS-
FETS from certain transient conditions, as follows:
ENA ............................10 .......... Start-Up Delay
SS ...............................13 .......... Soft Start
PKLIM ...........................2 .......... Maximum Current Limit
AC
SENSE
Mult Out
Mult Out,
INPUT
INPUT
SENSE
RMS
.................................6 .......... LineVoltage Waveform
A
A
.............................8 .......... RMS Line Voltage
/Mult Out .........4/5 .......... Line Current
140
120
100
250
200
150
100
........................ 11 .......... Output DC Voltage
50
80
60
40
20
0
0
0
0
100
100
PIN #
PIN #
200
200
I
I
AC
AC
, A
, A
300
300
VA Out=3V
VA Out=5V
FUNCTION
VA Out=1.5V
FUNCTION
VA Out=5V
VA Out=3V
VA Out=2V
VA Out=1.25V
400
400
V
V
RMS
RMS
=5V
=3V
500
500
UC1854
UC2854
UC3854