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CS5422 데이터시트(PDF) 11 Page - ON Semiconductor |
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CS5422 데이터시트(HTML) 11 Page - ON Semiconductor |
11 / 17 page CS5422 http://onsemi.com 11 Figure 8. Switching Frequency 10 20 30 40 50 60 100 200 300 400 500 600 700 800 ROSC (kW) Selection of the Output Inductor The inductor should be selected based on its inductance, current capability, and DC resistance. Increasing the inductor value will decrease output voltage ripple, but degrade transient response. There are many factors to consider in selecting the inductor including cost, efficiency, EMI and ease of manufacture. The inductor must be able to handle the peak current at the switching frequency without saturating, and the copper resistance in the winding should be kept as low as possible to minimize resistive power loss. There are a variety of materials and types of magnetic cores that could be used for this application. Among them are ferrites, molypermalloy cores (MPP), amorphous and powdered iron cores. Powdered iron cores are very commonly used. Powdered iron cores are very suitable due to its high saturation flux density and have low loss at high frequencies, a distributed gap and exhibit very low EMI. The minimum value of inductance which prevents inductor saturation or exceeding the rated FET current can be calculated as follows: LMIN + (VIN(MIN) * VOUT)VOUT fSW VIN(MIN) ISW(MAX) where: LMIN = minimum inductance value; VIN(MIN) = minimum design input voltage; VOUT = output voltage; fSW = switching frequency; ISW(MAX) − maximum design switch current. The inductor ripple current can then be determined: DIL + VOUT (1 * D) L fSW where: ΔIL = inductor ripple current; VOUT = output voltage; L = inductor value; D = duty cycle. fSW = switching frequency The designer can now verify if the number of output capacitors will provide an acceptable output voltage ripple (1.0% of output voltage is common). The formula below is used: DIL + D VOUT ESRMAX Rearranging we have: ESRMAX + D VOUT DIL where: ESRMAX = maximum allowable ESR; ΔVOUT = 1.0% × VOUT = maximum allowable output voltage ripple ( budgeted by the designer ); ΔIL = inductor ripple current; VOUT = output voltage. The number of output capacitors is determined by: Number of capacitors + ESRCAP ESRMAX where: ESRCAP = maximum ESR per capacitor (specified in manufacturer’s data sheet). The designer must also verify that the inductor value yields reasonable inductor peak and valley currents (the inductor current is a triangular waveform): IL(PEAK) + IOUT ) DIL 2 where: IL(PEAK) = inductor peak current; IOUT = load current; ΔIL = inductor ripple current. IL(VALLEY) + IOUT * DIL 2 where: IL(VALLEY) = inductor valley current. Selection of the Output Capacitors These components must be selected and placed carefully to yield optimal results. Capacitors should be chosen to provide acceptable ripple on the regulator output voltage. Key specifications for output capacitors are their ESR (Equivalent Series Resistance), and ESL (Equivalent Series Inductance). For best transient response, a combination of low value/high frequency and bulk capacitors placed close to the load will be required. In order to determine the number of output capacitors the maximum voltage transient allowed during load transitions has to be specified. The output capacitors must hold the output voltage within these limits since the inductor current can not change with the required slew rate. The output capacitors must therefore have a very low ESL and ESR. |
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