APPLICATION NOTE UC3842/3/4/5 PROVIDES LOW-COST 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 designers an inexpensivecontroller with which they can obtain all the performance advantages of current mode operation. In addition, the UC3842 series is optimized for efficient 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 individual 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 current mode control and its benefits is included and methods of avoiding common pitfalls are mentioned. The final section presents designs of power supplies utilizingUC3842 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 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 current. Thiscontrasts 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 instantaneously corrects for input voltage variations without using up any of the error amplifier’sdynamic 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
clock verror vsense latchoutput
Figure 1. Two-Loop Current-Mode Control System
error-voltage-controlled-current-source for the purposes of small-signal analysis. This is illustrated by Figure 2. The two-pole control-to-output frequency response of these converters is reduced to a single-pole (filter capacitor in parallel with load) response. One result is that the error amplifier compensationcan be designed to yield a stable closed-loop converter response with greater gainbandwidth than would be possible with pulse-width control, giving the supply improved small-signal dynamic response to changing loads. A second result is that the error amplifier compensation circuit becomes simpler, as illustrated in Figure 3. Capacitor Ci and resistor Ri, in Figure 3a add a low frequency zero whichcancels one of the two control-to-. output poles of non-current-mode converters. For largesignal load changes, in which converter response is limited by inductor slew rate, the error amplifier will saturate while the inductor is catching up with the load. During this time, Ci will charge to an abnormal level. When the inductor current reaches its required level, the voltage on Ci
U-100Acauses a corresponding error in supply output voltage. The recovery time is R&i, which may be quite long. However, the compensation network of Figure 3b can be used where current-mode control has eliminated the inductor pole. Large-signal dynamic response is then greatly improved due to the absence of Ci. Current limiting is greatly simplified with current-mode control. Pulse-by-pulse limiting is, of...