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ZHCS2000 Datasheet(PDF) 11 Page - Zetex Semiconductors |
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ZHCS2000 Datasheet(HTML) 11 Page - Zetex Semiconductors |
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11 / 19 page  ZXSC100 ISSUE 1 - JANUARY 2001 11 Inductor selection The inductor value must be chosen to satisfy performance, cost and size requirements of the overall solution. For the reference designs we recommend an inductor value of 22µH with a core saturation current rating greater than the converter peak current value. Inductor selection has a significant impact on the converter efficiency. For applications where efficiency is critical, a 5% improvement can be achieved with a high performance inductor. This should be selected with a core saturation current rating much higher than the peak current of the converter, say 3 times greater. The resultant reduction in core losses brings about the efficiency improvement. Peak current definition The peak current rating is a design parameter whose value is dependent upon the overall application. For the reference designs, a peak current of 1.2A was chosen to ensure that the converter could provide the required output power. In general, the IPK value must be chosen to ensure that the switching transistor, Q1, is in full saturation with maximum output power conditions, assuming worse-case input voltage and transistor gain under all operating temperature extremes. Once IPK is decided the value of RSENSE can be determined by: R V I SENSE ISENSE PK = Output power definition By making the above assumptions for the inductor and IPK the output power can be determined by: Output Power (V V x I x T 2x(T T OUT IN PK DIS On OFF = − + ) ) where T IxL V ON PK IN = and T IxL VV DIS PK OUT IN = − () Note: VOUT = output voltage + rectifier diode VF Figure 3 shows the discontinuous inductor current and the relationship between output power, TON,TDIS and TOFF. Figure 3 Discontinuous inductor current Output capacitors Output capacitors are a critical choice in the overall performance of the solution. They are required to filter the output and supply load transient currents. There are three parameters which are paramount in the selection of the output capacitors; their capacitance value, IRIPPLE and ESR. The capacitance value is selected to meet the load transient requirements. The capacitors IRIPPLE rating must meet or exceed the current ripple of the solution. The ESR of the output capacitor can also affect loop stability and transient performance. The capacitors selected for the solution, and indicated in the reference designs, are optimised to provide the best overall performance. |
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