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LM2678SD-12 データシート(PDF) 11 Page - National Semiconductor (TI) |
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LM2678SD-12 データシート(HTML) 11 Page - National Semiconductor (TI) |
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11 / 27 page  Application Hints (Continued) Power supply design using the LM2678 is greatly simplified by using recommended external components. A wide range of inductors, capacitors and Schottky diodes from several manufacturers have been evaluated for use in designs that cover the full range of capabilities (input voltage, output voltage and load current) of the LM2678. A simple design procedure using nomographs and component tables pro- vided in this data sheet leads to a working design with very little effort. Alternatively, the design software, LM267X Made Simple (version 6.0), can also be used to provide instant component selection, circuit performance calculations for evaluation, a bill of materials component list and a circuit schematic. The individual components from the various manufacturers called out for use are still just a small sample of the vast array of components available in the industry. While these components are recommended, they are not exclusively the only components for use in a design. After a close compari- son of component specifications, equivalent devices from other manufacturers could be substituted for use in an ap- plication. Important considerations for each external component and an explanation of how the nomographs and selection tables were developed follows. INDUCTOR The inductor is the key component in a switching regulator. For efficiency the inductor stores energy during the switch ON time and then transfers energy to the load while the switch is OFF. Nomographs are used to select the inductance value re- quired for a given set of operating conditions. The nomo- graphs assume that the circuit is operating in continuous mode (the current flowing through the inductor never falls to zero). The magnitude of inductance is selected to maintain a maximum ripple current of 30% of the maximum load cur- rent. If the ripple current exceeds this 30% limit the next larger value is selected. The inductors offered have been specifically manufactured to provide proper operation under all operating conditions of input and output voltage and load current. Several part types are offered for a given amount of inductance. Both surface mount and through-hole devices are available. The inductors from each of the three manufacturers have unique charac- teristics. Renco: ferrite stick core inductors; benefits are typically lowest cost and can withstand ripple and transient peak currents above the rated value. These inductors have an external magnetic field, which may generate EMI. Pulse Engineering: powdered iron toroid core inductors; these also can withstand higher than rated currents and, being toroid inductors, will have low EMI. Coilcraft: ferrite drum core inductors; these are the smallest physical size inductors and are available only as surface mount components. These inductors also generate EMI but less than stick inductors. OUTPUT CAPACITOR The output capacitor acts to smooth the dc output voltage and also provides energy storage. Selection of an output capacitor, with an associated equivalent series resistance (ESR), impacts both the amount of output ripple voltage and stability of the control loop. The output ripple voltage of the power supply is the product of the capacitor ESR and the inductor ripple current. The capacitor types recommended in the tables were selected for having low ESR ratings. In addition, both surface mount tantalum capacitors and through-hole aluminum electrolytic capacitors are offered as solutions. Impacting frequency stability of the overall control loop, the output capacitance, in conjunction with the inductor, creates a double pole inside the feedback loop. In addition the capacitance and the ESR value create a zero. These fre- quency response effects together with the internal frequency compensation circuitry of the LM2678 modify the gain and phase shift of the closed loop system. As a general rule for stable switching regulator circuits it is desired to have the unity gain bandwidth of the circuit to be limited to no more than one-sixth of the controller switching 10088608 FIGURE 2. Basic circuit for adjustable output voltage applications www.national.com 11 |
同様の部品番号 - LM2678SD-12 |
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同様の説明 - LM2678SD-12 |
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