U-100A UC3842/3/4/5 UC3842 UC3843 UC3844 UC3845 UC1842/3/4/5 UC1842 UC1844 - Datasheet Archive

 

APPLICATION NOTE UC3842/3/4/5 PROVIDES LOW-COST CURRENT-MODE CONTROL INTRODUCTION CURRENT-MODE CONTROL The fundamental challenge

 

U-100A U-100A APPLICATION NOTE UC3842/3/4/5 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 UC3842/3/4/5 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 UC3842 series is optimized for efficient power sequencing of off-line converters, DC to DC regulators and for driving power MOSFETs or transistors. 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. 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. This application note provides a functional description of the UC3842 UC3842 family and highlights the features of each individual member, the UC3842 UC3842, UC3843 UC3843, UC3844 UC3844 and UC3845 UC3845 Throughout the text, the UC3842 UC3842 part number will be referenced, however the generalized circuits and performance characteristics apply to each member of the UC3842 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 UC3842 control. 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 latch output Figure 1. Two-Loop Current-Mode Control System 3-53 APPLICATION NOTE U-100A U-100A 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 compensation can 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 which cancels 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 causes 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 course, inherent in the control scheme. Furthermore, an upper limit on the peak current can be established by simply clamping the error voltage. Accurate current limiting allows optimization of magnetic and power semiconductor elements while ensuring reliable supply operation. Finally, current-mode controlled power stages can be operated in parallel with equal current sharing. This opens the possibility of a modular approach to power supply design. Figure 2. Inductor Looks Like a Current Source to Small Signals Vref B) Current Mode Control A) Direct Duty Cycle Control Figure 3. Required Error Amplifier Compensation for Continuous Inductor Current Designs 3-54 APPLICATION NOTE U-100A U-100A THE UC3842/3/4/5 UC3842/3/4/5 SERIES OF CURRENT-MODE PWM IC'S DESCRIPTION FEATURES The UC1842/3/4/5 UC1842/3/4/5 family of control ICs provides the necessary features to implement off-line or DC to DC fixed frequency current mode control schemes with a minimal external parts count. Internally implemented circuits include under-voltage lockout featuring start up current less than 1 mA, a precision reference trimmed for accuracy at the error amp input, logic to insure latched operation, a PWM comparator which also provides current limit control, and a totem pole output stage designed to source or sink high peak current. The output stage, suitable for driving either N Channel MOSFETs or bipolar transistor switches, is low in the off state. l Optimized for Off-Line and DC to DC Converters l Low Start Up Current (< 1 mA) @ Automatic Feed Forward Compensation l Pulse-By-Pulse Current Limiting l Enhanced Load Response Characteristics l Under-Voltage Lockout with Hysteresis l Double Pulse Suppression l High Current Totem Pole Output Differences between members of this family are the under-voltage lockout thresholds and maximum duty cycle ranges. The UC1842 UC1842 and UC1844 UC1844 have UVLO thresholds of 16V (on) and 10V (off), ideally suited to off-line applications. The corresponding thresholds for the UC1843 UC1843 and UC1845 UC1845 are 8.5V and 7.9V. The UC1842 UC1842 and UC1843 UC1843 can operate to duty cycles approaching 100%. A range of zero to