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LM2830ZSDX Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM2830ZSDX Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 24 page Design Guide (Continued) In general, ∆i L =0.1x(IOUT) → 0.2x(IOUT) If ∆i L = 20% of 1A, the peak current in the inductor will be 1.2A. The minimum guaranteed current limit over all operat- ing conditions is 1.2A. One can either reduce ∆i L, or make the engineering judgment that zero margin will be safe enough. The typical current limit is 1.75A. The LM2830 operates at frequencies allowing the use of ceramic output capacitors without compromising transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple. See the output capacitor section for more details on calculating output volt- age ripple. Now that the ripple current is determined, the inductance is calculated by: Where When selecting an inductor, make sure that it is capable of supporting the peak output current without saturating. Induc- tor saturation will result in a sudden reduction in inductance and prevent the regulator from operating correctly. Because of the speed of the internal current limit, the peak current of the inductor need only be specified for the required maxi- mum output current. For example, if the designed maximum output current is 1.0A and the peak current is 1.25A, then the inductor should be specified with a saturation current limit of > 1.25A. There is no need to specify the saturation or peak current of the inductor at the 1.75A typical switch current limit. The difference in inductor size is a factor of 5. Because of the operating frequency of the LM2830, ferrite based inductors are preferred to minimize core losses. This pre- sents little restriction since the variety of ferrite-based induc- tors is huge. Lastly, inductors with lower series resistance (R DCR) will provide better operating efficiency. For recom- mended inductors see Example Circuits. INPUT CAPACITOR An input capacitor is necessary to ensure that V IN does not drop excessively during switching transients. The primary specifications of the input capacitor are capacitance, volt- age, RMS current rating, and ESL (Equivalent Series Induc- tance). The recommended input capacitance is 22 µF.The input voltage rating is specifically stated by the capacitor manufacturer. Make sure to check any recommended derat- ings and also verify if there is any significant change in capacitance at the operating input voltage and the operating temperature. The input capacitor maximum RMS input cur- rent rating (I RMS-IN) must be greater than: Neglecting inductor ripple simplifies the above equation to: It can be shown from the above equation that maximum RMS capacitor current occurs when D = 0.5. Always calcu- late the RMS at the point where the duty cycle D is closest to 0.5. The ESL of an input capacitor is usually determined by the effective cross sectional area of the current path. A large leaded capacitor will have high ESL and a 0805 ceramic chip capacitor will have very low ESL. At the operating frequen- cies of the LM2830, leaded capacitors may have an ESL so large that the resulting impedance (2 πfL) will be higher than that required to provide stable operation. As a result, surface mount capacitors are strongly recommended. Sanyo POSCAP, Tantalum or Niobium, Panasonic SP, and multilayer ceramic capacitors (MLCC) are all good choices for both input and output capacitors and have very low ESL. For MLCCs it is recommended to use X7R or X5R type capacitors due to their tolerance and temperature character- istics. Consult capacitor manufacturer datasheets to see how rated capacitance varies over operating conditions. OUTPUT CAPACITOR The output capacitor is selected based upon the desired output ripple and transient response. The initial current of a load transient is provided mainly by the output capacitor. The output ripple of the converter is: When using MLCCs, the ESR is typically so low that the capacitive ripple may dominate. When this occurs, the out- put ripple will be approximately sinusoidal and 90˚ phase shifted from the switching action. Given the availability and quality of MLCCs and the expected output voltage of designs using the LM2830, there is really no need to review any other capacitor technologies. Another benefit of ceramic capaci- tors is their ability to bypass high frequency noise. A certain amount of switching edge noise will couple through parasitic capacitances in the inductor to the output. A ceramic capaci- tor will bypass this noise while a tantalum will not. Since the output capacitor is one of the two external components that control the stability of the regulator control loop, most appli- cations will require a minimum of 22 µF of output capaci- tance. Capacitance often, but not always, can be increased 20197405 FIGURE 3. Inductor Current www.national.com 11 |
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