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NCP5425DBR2G Datasheet(PDF) 14 Page - ON Semiconductor |
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NCP5425DBR2G Datasheet(HTML) 14 Page - ON Semiconductor |
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14 / 22 page  NCP5425 http://onsemi.com 14 Output Inductor Selection The inductor should be selected based on the criteria of 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 inductors 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, such as ferrites, molypermalloy cores (MPP), and amorphous and powdered iron cores. Powdered iron cores are particularly suitable due to high saturation flux density and low loss at high frequencies, a distributed gap, and they produce very low EMI. The minimum value of inductance to prevent 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 by: DIL + VOUT (1 * D) L fSW where: DIL = inductor ripple current; VOUT = output voltage; L = inductor value; D = duty cycle; fSW = switching frequency. After inductor selection, the designer can 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 + DVOUT ESRMAX where: ESRMAX = maximum allowable ESR; DVOUT = 1.0% ⋅ VOUT = maximum allowable output voltage ripple (budgeted by the designer); DIL = inductor ripple current; VOUT = output voltage. Rearranging, we have: ESRMAX + DVOUT DIL 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 IL(VALLEY) + IOUT ) DIL 2 where: IL(PEAK) = inductor peak current; IL(VALLEY) = inductor valley current; IOUT = load current; DIL = inductor ripple current. Output Capacitor Selection 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 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. 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 at the required slew rate. The output capacitors must therefore have a very low ESL and ESR. The voltage change during the load current transient is given by: DVOUT + DIOUT ESL Dt ) ESR ) tTR COUT where: DIOUT/DD = load current slew rate; DIOUT = load transient; Dt = load transient duration time; ESL = Maximum allowable ESL including capacitors, circuit traces, and vias; ESR = Maximum allowable ESR including capacitors and circuit traces; tTR = output voltage transient response time; COUT = output capacitance. The designer must independently assign values for the change in output voltage due to ESR, ESL, and output capacitor discharging or charging. Empirical data indicates that most of the output voltage change (droop or spike, depending on the load current transition) results from the total output capacitor ESR. |
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