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TC110 Folha de dados(PDF) 6 Page - Microchip Technology |
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TC110 Folha de dados(HTML) 6 Page - Microchip Technology |
6 / 16 page TC110 DS21355B-page 6 © 2002 Microchip Technology Inc. 3.5 Output Capacitor The effective series resistance of the output capacitor directly affects the amplitude of the output voltage ripple. (The product of the peak inductor current and the ESR determines output ripple amplitude.) There- fore, a capacitor with the lowest possible ESR should be selected. Smaller capacitors are acceptable for light loads or in applications where ripple is not a concern. The Sprague 595D series of tantalum capacitors are among the smallest of all low ESR surface mount capacitors available. Table 4-1 lists suggested components and suppliers. 3.6 Inductor Selection Selecting the proper inductor value is a trade-off between physical size and power conversion require- ments. Lower value inductors cost less, but result in higher ripple current and core losses. They are also more prone to saturate since the coil current ramps faster and could overshoot the desired peak value. This not only reduces efficiency, but could also cause the current rating of the external components to be exceeded. Larger inductor values reduce both ripple current and core losses, but are larger in physical size and tend to increase the start-up time slightly. A22 µH inductor is recommended for the 300kHz versions and a 47 µH inductor is recommended for the 100kHz versions. Inductors with a ferrite core (or equivalent) are also recommended. For highest efficiency, use inductors with a low DC resistance (less than 20 m Ω). The inductor value directly affects the output ripple voltage. Equation 3-3 is derived as shown below, and can be used to calculate an inductor value, given the required output ripple voltage and output capacitor series resistance: EQUATION 3-1: where ESR is the equivalent series resistance of the output filter capacitor, and VRIPPLE is in volts. Expressing di in terms of switch ON resistance and time: EQUATION 3-2: Solving for L: EQUATION 3-3: Care must be taken to ensure the inductor can handle peak switching currents, which can be several times load currents. Exceeding rated peak current will result in core saturation and loss of inductance. The inductor should be selected to withstand currents greater than IPK (Equation 3-10) without saturating. Calculating the peak inductor current is straightforward. Inductor current consists of an AC (sawtooth) current centered on an average DC current (i.e., input current). Equation 3-6 calculates the average DC current. Note that minimum input voltage and maximum load current values should be used: EQUATION 3-4: Re-writing in terms of input and output currents and voltages: EQUATION 3-5: Solving for input curent: EQUATION 3-6: The sawtooth current is centered on the DC current level; swinging equally above and below the DC current calculated in Equation 3-6. The peak inductor current is the sum of the DC current plus half the AC current. Note that minimum input voltage should be used when calculating the AC inductor current (Equation 3-9). EQUATION 3-7: EQUATION 3-8: EQUATION 3-9: where: VSW =VCESAT of theswitch(note if aCMOS switch is used substitute VCESAT for rDSON xIIN) Combining the DC current calculated in Equation 3-6, with half the peak AC current calculated in Equation 3- 9, the peak inductor current is given by: EQUATION 3-10: VRIPPLE ≈ ESR(di) VRIPPLE ≈ ESR [(VIN –VSW)tON] L ≈ ESR [(VIN –VSW)tON] VRIPPLE L = Output Power Efficiency Input Power (VOUTMAX)(IOUTMAX) Efficiency (VINMIN)(IINMAX)= (VOUTMAX)(IOUTMAX) (Efficiency)(VINMAX) IINMAX = = L(di) dt V = V(dt) dt di [(VINMIN –VSW)tON] L di = IPK =IINMAX +0.5(di) |
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