UC3842 / UC3843 / UC3844 / UC3845 CURRENT-MODE CONTROL Datasheet

UC3842 / UC3843 / UC3844 / UC3845 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 / UC3843 / UC3844 / UC3845 is an integrated pulse width modulator (PWM) designed with both these objectives in mind. This IC provides designers an inexpensive controller 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 utilizing UC3842 control.

CURRENT-MODE CONTROL
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. 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 instantaneously 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.

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