AN920-D.PDF - AN920\/D Theory and Applications of the MC34063 and mA78S40 Switching Regulator Control Circuits http\/onsemi.com APPLICATION NOTE A78S40

AN920-D.PDF - AN920/D Theory and Applications of the...

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 Semiconductor Components Industries, LLC, 2013 December, 2013   Rev. 6 1 Publication Order Number: AN920/D AN920/D Theory and Applications of the MC34063 and mA78S40 Switching Regulator Control Circuits This paper describes in detail the principle of operation of the MC34063 and  A78S40 switching regulator subsystems. Several converter design examples and numerous applications circuits with test data are included. INTRODUCTION The MC34063 and  A78S40 are monolithic switching regulator subsystems intended for use as dc to dc converters. These devices represent a significant advancement in the ease of implementing highly efficient and yet simple switching power supplies. The use of switching regulators is becoming more pronounced over that of linear regulators because the size reductions in new equipment designs require greater conversion efficiency. Another major advantage of the switching regulator is that it has increased application flexibility of output voltage. The output can be less than, greater than, or of opposite polarity to that of the input voltage. PRINCIPLE OF OPERATION In order to understand the difference in operation between linear and switching regulators we must compare the block diagrams of the two step down regulators shown in Figure 1. The linear regulator consists of a stable reference, a high gain error amplifier, and a variable resistance series pass element. The error amplifier monitors the output voltage level, compares it to the reference and generates a linear control signal that varies between two extremes, saturation and cutoff. This signal is used to vary the resistance of the series pass  element in a corrective fashion in order to maintain a constant output voltage under varying input voltage and output load conditions. The switching regulator consists of a stable reference and a high gain error amplifier identical to that of the linear regulator. This system differs in that a free running oscillator and a gated latch have been added. The error amplifier again monitors the output voltage, compares it to the reference level and generates a control signal. If the output voltage is below nominal, the control signal will go to a high state and turn on the gate, thus allowing the oscillator clock pulses to drive the series pass element alternately from cutoff to saturation. This will continue until the output voltage is pumped up slightly above its nominal value. At this time, the control signal will go low and turn off the gate, terminating any further switching of the series pass element. The output voltage will eventually decrease to below nominal due to the
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