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L6598 Datasheet(PDF) 7 Page - STMicroelectronics |
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L6598 Datasheet(HTML) 7 Page - STMicroelectronics |
7 / 16 page 7/16 L6598 Figure 4. Soft Start and frequency shifting block 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. 4. 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 L6598 oscillates at fstart, 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 Css Ifmin Ifstart Iss gm OSC Iosc I osc I fmin I fstart g mVCss t () – () + I fmin I fstart g mIss C ss -------------- t – + == I fmin VREF R fmin -------------- I fsart , VREF R fstart ---------------- V REF , 2V == = Iosc 0 () Ifmin Ifstart + VREF 1 R fm in -------------- 1 R fst art ---------------- + == I osc Tss () I fm in V REF Rfmin -------------- == I fst art g mIss C ss -------------- T SS – 0T SS → C ssIfstart g m Iss ------------------------ == I SS I fstart K -------------- T SS → C ssIfstart g mIfstartK -------------------------- T SS → C ss g mK ----------- T SS k SSCSS – → == = |
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