Datasheet
TYPICAL CHARACTERISTICS at T
A
= T
J
= 25
o
C (cont.)
UC1854
UC2854
UC3854
I
AC
, µA
Mult Out
µA
0
100
200
300
400
500
600
0 100 200 300 400 500
V
RMS
=1.5V
VA Out=1.25V
VA Out=2.5V
VA Out=3.5V
I
AC
, µA
Mult Out
µA
0
50
100
150
200
250
0 100 200 300 400 500
VRMS=3V
VA Out=1.25V
VA Out=2V
VA Out=3V
VA Out=5V
I
AC
, µA
Mult Out,
µA
0
20
40
60
80
100
120
140
0
100 200 300 400 500
V
RMS
=5V
VA Out=5V
VA Out=1.5V
VA Out=3V
IAC,
µ
A
Mult Out
µA
0
20
40
60
80
100
120
140
160
0 100 200 300 400 500
V
RMS
=4V
VA Out=1.25V
VA Out=2V
VA Out=3V
VA Out=4V
VA Out=5V
Multiplier Output vs Multiplier Inputs with Mult Out=0V
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