Moteur de recherche de fiches techniques de composants électroniques |
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L6598D Fiches technique(PDF) 7 Page - STMicroelectronics |
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L6598D Fiches technique(HTML) 7 Page - STMicroelectronics |
7 / 17 page 7/17 L6598 During the soft start time the current ISS charges the capacitor CSS, generating a voltage ramp which is delivered to a transconductance amplifier, as shown in fig. 7. Thus this voltage signal is converted in a growing current which is subtracted to Ifstart. Therefore the current which drives the oscillator to set the frequency during the soft start is equal to: [1] where [2] At the start-up (t=0) the oscillator frequency is set by: [3] At the end of soft start (t = TSS) the second term of eq.1 decreases to zero and the switching frequency is set only by Imin (i.e. Rfmin): [4] Since the second term of eq.1 is equal to zero, we have: [5] Note that there is not a fixed threshold of the voltage across CSS in which the soft start finishes (i.e. the end of the frequency shifting), and TSS depends on CSS, Ifstart, gm, and ISS (eq. 5). Making TSS independent of Ifstart, the ISS current has been designed to be a fraction of Ifstart, so: [6] In this way the soft start time depends only on the capacitor CSS. The typical value of the kSS constant (Soft Start Timing Constant) is 0.15 s/ µF. The current Iosc is fed to the oscillator as shown in fig. 7. It is twice mirrored (x4 and x8) generating the triangular wave on the oscillator capacitor Cf. Referring to the internal structure of the oscillator (fig.7), a good relationship to compute an approximate value of the oscillator frequency in normal operation is: [7] The degree of approximation depends on the frequency value, but it remains very good in the range from 30kHz to 100kHz (figg.9-13) I osc I fmin I fstart g mVCss t () – () + I fm in I fstart g mIss C ss --------------t – + == I fmin V REF R fm in -------------- I fsart , V REF R fs tart ---------------- V RE F , 2V == = I osc 0 () I fm in I fstart + V REF 1 R fmin -------------- 1 R fstart ---------------- + == I osc Tss () I fm in V REF R fm in -------------- == I fsta rt g mIss C ss --------------T SS – 0T SS → C ssIfstart g mIss ------------------------ == I SS I fstart K -------------- T SS → C ssIfstart g m IfstartK -------------------------- T SS → C ss g mK ----------- T SS k SSCSS – → == = f min 1.41 R fminCf -------------------- = |
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