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clock verror vsense latch output
U-100A
APPLICATION NOTE
UC3842/3/4/5 PROVIDES LOW-COST
CURRENT-MODE CONTROL
INTRODUCTION
CURRENT-MODE CONTROL
The fundamental challenge of power supply design is to
simultaneously realize two conflicting objectives: good
electrical performance and low cost. The
UC3842/3/4/5
is an integrated pulse width modulator
(PWM)
designed
with both these objectives in mind. This
IC
provides de-
signers an inexpensive controller with which they can ob-
tain all the performance advantages of current mode op-
eration. In addition, the UC3842 series is optimized for ef-
ficient power sequencing of off-line converters, DC to DC
regulators and for driving power MOSFETs or transistors.
This application note provides a functional description of
the UC3842 family and highlights the features of each in-
dividual member, the UC3842, UC3843, UC3844 and
UC3845 Throughout the text, the UC3842 part number
will be referenced, however the generalized circuits and
performance characteristics apply to each member of the
UC3842 series unless otherwise noted. A review of cur-
rent mode control and its benefits is included and meth-
ods of avoiding common pitfalls are mentioned. The final
section presents designs of power supplies utilizing
UC3842 control.
Figure 1 shows the two-loop current-mode control system
in a typical buck regulator application. A clock signal initi-
ates power pulses at a fixed frequency. 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 cur-
rent. This contrasts with conventional schemes in which
the error signal directly controls pulse width without regard
to inductor current.
Several performance advantages result from the use of
current-mode control. First, an input voltage feed-forward
characteristic is achieved; i.e., the control circuit instanta-
neously corrects for input voltage variations without using
up any of the error amplifier’s dynamic range. Therefore,
line regulation is excellent and the error amplifier can be
dedicated to correcting for load variations exclusively.
For converters in which inductor current is continuous,
controlling peak current is nearly equivalent to controlling
average current. Therefore, when such converters employ
current-mode control, the inductor can be treated as an
Figure 1. Two-Loop Current-Mode Control System
3-53
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