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NCV8853 Datasheet(PDF) 11 Page - ON Semiconductor |
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NCV8853 Datasheet(HTML) 11 Page - ON Semiconductor |
11 / 12 page NCV8853 www.onsemi.com 11 The maximum reverse voltage the diode sees is the maximum input voltage (with some margin in case of ringing on the switch node). The maximum forward current is the peak current limit of the NCV8853, or 150% of ICL . (6) Output Inductor Selection Both mechanical and electrical considerations influence the selection of an output inductor. From a mechanical perspective, smaller inductor values generally correspond to smaller physical size. Since the inductor is often one of the largest components in the power supply, a minimum inductor value is particularly important in space− constrained applications. From an electrical perspective, an inductor is chosen for a set amount of current ripple and to assure adequate transient response. The output inductor controls the current ripple that occurs over a switching period. A high current ripple will result in excessive power loss and ripple current requirements. A low current ripple will result in a poor control signal and a slow current slew rate in the event of a load transient. A good starting point for peak−to−peak ripple is around 10% of the inductor current.To choose the inductor value based on the peak−to−peak ripple current, use the following equation: i L + V OUT @ (1 * DMIN) L @ F SW where: iL: peak−to−peak output current ripple [Ap−p] From this equation it is clear that the ripple current increases as L decreases, emphasizing the trade−off between dynamic response and ripple current. The peak and valley values of the triangular current waveform are as follows: I L(pk) + IOUT ) i L 2 I L(vly) + IOUT * i L 2 where: IL(pk): peak (maximum) value of ripple current [A] IL(vly): valley (minimum) value of ripple current [A] Saturation current is specified by inductor manufacturers as the current at which the inductance value has dropped from the nominal value, typically 10%. For stable operation, the output inductor must be chosen so that the inductance is close to the nominal value even at the peak output current, IL(pk), it is recommended to choose an inductor with saturation current sufficiently higher than the peak output current, such that the inductance is very close to the nominal value at the peak output current. This allows for a safety factor and allows for more optimized compensation. Inductor efficiency is another consideration when selecting an output inductor. Inductor losses include dc and ac winding losses, which are very low due to high core resistance, and magnetic hysteresis losses, which increase with peak−to−peak ripple current. Core losses also increase as switching frequency increases. Ac winding losses are based on the ac resistance of the winding and the RMS ripple current through the inductor, which is much lower than the dc current. The ac winding losses are due to skin and proximity effects and are typically much less than dc losses, but increase with frequency. Dc winding losses account for a large percentage of output inductor losses and are the dominant factor at switching frequencies at or below 500 kHz. The dc winding losses in the inductor can be calculated with the following equation: P L(dc) + IOUT 2 @ R dc where: PL(dc): dc winding losses in the output inductor Rdc: dc resistance of the output inductor (DCR) (7) Output Capacitor Selection The output capacitor is a basic component for the fast response of a power supply. In fact, for the first few microseconds of a load transient, they supply the current to the load. The controller recognizes the load transient and proceeds to increase the duty cycle to its maximum. Neglecting the effect of the ESL, the output voltage has a first drop due to ESR of the bulk capacitor(s). DV OUT(ESR) + DIOUT @ ESR A lower ESR produces a lower ΔV during load transient. In addition, a lower ESR produces a lower output voltage ripple. In the case of stepping into a short, the inductor current approaches zero with the worst case initial current at the current limit and the initial voltage at the output voltage set point, calculating the voltage overshoot as follows: DV OS + L @ I CL 2 C ) V OUT 2 * V OUT Accordingly, a minimum amount of capacitance can be chosen for maximum allowed output voltage overshoot: C MIN + L @ I CL 2 V OUT ) DVOS(max) 2 * V OUT 2 where: CMIN: minimum amount of capacitance to minimize voltage overshoot to ΔVOS(max) [F] ΔVOS(max): maximum allowed voltage overshoot during a short [V] (8) Select Compensator Components The Current Mode control method employed by the NCV8853 allows the use of a simple, Type II compensation to optimize the dynamic response according to system requirements. Using a simulation tool such as CompCalc can assist in the selection of these components. |
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