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Part Manufacturer: ST Microelectronics

Part Description: UC3842 PROVIDES LOW-COST CURRENT MODE CONTRO

Part Details:

APPLICATION NOTE UC3842 PROVIDES LOW-COST CURRENT-MODE CONTROL The fundamental challenge of power supply design thods of avoiding common pitfalls discussed. The fi- is to simultaneously realize two conflicting objecti- nal section presents designs of two power supplies ves : good electrical performance and low cost. The utilizing UC3842 control. UC3842 is an integrated pulse width modulator(PWM) designed with both these objectives in mind. CURRENT-MODE CONTROL This IC provides designers an inexpensive control- Figure 1 shows the two-loop current-mode control ler with which they can obtain all the performance system in a typical buck regulator application. A advantages of current-mode operation. In addition, clock signal initiates power pulses at a fixed frequen- the UC3842 is optimized for efficient power sequen- cy. The termination of each pulse occurs when an cing of off-line converters and for driving increasin- analog of the inductor current reaches a threshold gly popular POWERMOS. established by the error signal. In this way the error This application note gives a functional description signal actually controls peak inductor current. This of the UC3842 and suggests how to incorporate the contrasts with conventional schemes in which the IC into practical power supplies. A review of current- error signal directly controls pulse width without re- mode control and its benefits is included and me- gard to inductor current. Figure 1 : Two-loop Current-mode Control System. AN246/1188 1/16 APPLICATION NOTE Several performance advantages result from the gnal load changes, in which converter response is use of current-mode control. First, an input voltage limited by inductor slew rate, the error amplifier will feed-forward characteristic is achieved ; i.e., the saturate while the inductor is catching up with the control circuit instantaneously corrects for input volt- load. During this time, Ci will charge to an abnormal age variations without using up any of the error am- level. When the inductor current reaches its required plifier s dynamic range. Therefore, line regulation is level, the voltage on Ci causes a corresponding error excellent and the error amplifier can be dedicated to in supply output vol-tage. The recovery time is Riz Ci, correcting for load variations exclusively. which may be milleseconds. However, the compen- For converters in which inductor current is conti- sation network of figure 3b can be used where cur- nuous, controlling peak current is nearly equivalent rent-mode control has eliminated the inductor pole.

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Application Note Power Supply And Power Management UC3842A UC3842B UC3842T UC3843B UC3843T UC3844A UC3844B UC3844T UC3845A UC384.pdf Download PDF Datasheet