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APPLICATION NOTE
UC3842 PROVIDES LOW-COST CURRENT-MODE CONTROL
The fundamental challenge of power supply design
is to simultaneously realize two conflicting objecti-
ves : good electrical performance and low cost. The
UC3842 is an integrated pulse width modulator
(PWM) designed with both these objectives in mind.
This IC provides designers an inexpensive control-
ler with which they can obtain all the performance
advantages of current-mode operation. In addition,
the UC3842 is optimized for efficient power sequen-
cing of off-line converters and for driving increasin-
gly popular POWERMOS.
This application note gives a functional description
of the UC3842 and suggests how to incorporate the
IC into practical power supplies. A review of current-
mode control and its benefits is included and me-
thods of avoiding common pitfalls discussed. The fi-
nal section presents designs of two power supplies
utilizing UC3842 control.
CURRENT-MODE CONTROL
Figure 1 shows the two-loop current-mode control
system in a typical buck regulator application. A
clock signal initiates power pulses at a fixed frequen-
cy. The termination of each pulse occurs when an
analog of the inductor current reaches a threshold
established by the error signal. In this way the error
signal actually controls peak inductor current. This
contrasts with conventional schemes in which the
error signal directly controls pulse width without re-
gard to inductor current.
AN246/1188
Figure 1 : Two-loop Current-mode Control System.
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