TYPICAL CHARACTERISTICS at T
A
= T
J
= 25
o
C (cont.)
Multiplier Output vs Multiplier Inputs with Mult Out=0V
600
500
VA Out=3.5V
UC1854
UC2854
UC3854
V
RMS
=1.5V
250
V
RMS
=3V
VA Out=5V
200
400
150
300
VA Out=2.5V
VA Out=3V
Mult Out
µA
Mult Out
µA
100
VA Out=2V
200
VA Out=1.25V
100
0
0
100
200
300
400
500
50
VA Out=1.25V
0
0
100
200
300
400
500
I
AC
,
µA
I
AC
,
µA
160
140
120
140
V
RMS
=4V
VA Out=5V
V
RMS
=5V
VA Out=5V
120
100
80
60
40
VA Out=4V
100
Mult Out
µ
A
80
60
VA Out=3V
Mult Out,
µA
VA Out=3V
VA Out=2V
40
20
0
0
100
200
300
400
500
VA Out=1.25V
VA Out=1.5V
20
0
0
100
200
300
400
500
I
AC
,
µ
A
I
AC
,
µA
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
cycle of this output is simultaneously controlled by four
separate inputs to the chip:
INPUT
PIN #
FUNCTION
V
SENSE
........................ 11 .......... Output DC Voltage
I
AC
.................................6 .......... LineVoltage Waveform
I
SENSE
/Mult Out .........4/5 .......... Line Current
V
RMS
.............................8 .......... RMS Line Voltage
Additional controls of an auxiliary nature are provided.
They are intended to protect the switching power MOS-
FETS from certain transient conditions, as follows:
INPUT
PIN #
FUNCTION
ENA ............................10 .......... Start-Up Delay
SS ...............................13 .......... Soft Start
PKLIM ...........................2 .......... Maximum Current Limit
6
TI [ TEXAS INSTRUMENTS ]
