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TEA2018A Datasheet(PDF) 4 Page - STMicroelectronics |
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TEA2018A Datasheet(HTML) 4 Page - STMicroelectronics |
4 / 7 page GENERAL DESCRIPTION (see application note AN-086) Operating Principles (Figure 1) On every period, the beginning of the conduction time of the transistor is triggered by the fall of the oscillator sawtooth which acts as clock signal. The period Tosc is given by : Tosc ≅ 0.66 Ct (Rt + 200) (Tosc in seconds, Ct in Farad, Rt in Ω) The end of the conduction time is determined by a signal issued from comparing the following sig- nals : a) the sawtooth waveform representing the collector current of the switching transistor, sampled across the emitter shunt resistor, b) the output of the error amplifier. Base Drive - Fast turn-on : On each period, a current pulse ensures fast transistor switch-on. This pulse performs also the ton(min) function at the beginning of the conduction. - Proportional base drive : In order to save power, the positive base current after the starting pulse becomes an image of the collector current. The ratio IC IB is programmed as follows Figure 2) : IC IB = RB Re - Efficient and fast switch-off : When the positive base drive is removed, 1ms (typically) will elapse before the application of negative current there- fore allowing a safe and rapid collector current fall. Safety Functions - Overload & short-circuit protection : When the voltage applied to pin 3 exceeds the current limitation threshold voltage [Vth(Ic)], the output flip-flop is reset and the transistor is turned off. The shunt resistor Re must be calculated so as to obtain the current limitation threshold on pin 3 at the maximum allowable collector current. - Demagnetization sensing : This function disables any new conduction cycle of the transistor as long as the core is not completely demagnetized. When not used, pin 7 must be grounded. -ton(max) : Outside the regulation area and in the absence of current limitation, the maximum con- duction time is set at about 70 % of the period. -ton(min) : A minimum conducting time is ensured during each period (see Figure 2) - Supply voltage monitoring : The TEA2018A will stop operating if VCC + on pin 6 falls below the threshold level VCC(stop) OSCILLATOR SAW-TOOTH Error signal I (sample) C t FLIP-FLOP OUTPUT t t TEST CIRCUIT C I SENSE OS CILLATOR ERROR SIGNAL ERROR AMPLIFIER COMPARATO R S R Q FLIP-FLOP VREF I C Vi OUTPUT FILTER LO AD Re TEA2018A 4/7 |
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