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AN458 Datasheet(PDF) 4 Page - STMicroelectronics |
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AN458 Datasheet(HTML) 4 Page - STMicroelectronics |
4 / 19 page AN458 APPLICATION NOTE 4/19 The system is working in a variable frequency mode, with a switching frequency that is depending on the current delivered to the load. Bigger is the load current higher is the stored energy and longer is the time during which the comparator will block the discharge of the timing capacitor Ct, decreasing the system operating frequency. In fig.4 are shown the waveforms associated with this mode of operation. CALCULATION OF THE INDUCTANCE VALUE, L To calculate the inductance, that is a critical element in the circuit, we have to consider that: s The switching frequency increases reducing the load current Iout and increasing Vin. s The switching frequency decreases increasing the load Current Iout and decreasing Vin. So to calculate the inductor value we have to Figure 4. Self-oscillating Operation specify the minimum operation frequency (higher than 20KHz to avoid audible noise), for the minimum input voltage Vin at full load. The equation to calculate the maximum inductor value is: where: L V in min () V ces at – V o – () D ⋅ 2I out m ax () fmin ⋅⋅ ---------------------------------------------------------------------- ≤ D V o V F + V in m in () V ce sa V F + – ------------------------------------------------------- = |
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