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LM2736YMKX датащи(PDF) 10 Page - National Semiconductor (TI) |
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LM2736YMKX датащи(HTML) 10 Page - National Semiconductor (TI) |
10 / 22 page Design Guide (Continued) The LM2736 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 or ripple ratio is determined, the inductance is calculated by: where f s is the switching frequency and IO is the output current. When selecting an inductor, make sure that it is capable of supporting the peak output current without satu- rating. Inductor saturation will result in a sudden reduction in inductance and prevent the regulator from operating cor- rectly. Because of the speed of the internal current limit, the peak current of the inductor need only be specified for the required maximum output current. For example, if the de- signed maximum output current is 0.5A and the peak current is 0.7A, then the inductor should be specified with a satura- tion current limit of >0.7A. There is no need to specify the saturation or peak current of the inductor at the 1.5A typical switch current limit. The difference in inductor size is a factor of 5. Because of the operating frequency of the LM2736, ferrite based inductors are preferred to minimize core losses. This presents little restriction since the variety of ferrite based inductors is huge. Lastly, inductors with lower series resistance (DCR) will provide better operating efficiency. For recommended 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 10µF, al- though 4.7µF works well for input voltages below 6V. 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: 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 ce- ramic chip capacitor will have very low ESL. At the operating frequencies of the LM2736, certain 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 or Cornell Dubilier ESR, and multilayer ceramic capacitors (MLCC) are all good choices for both input and output ca- pacitors and have very low ESL. For MLCCs it is recom- mended to use X7R or X5R dielectrics. Consult capacitor manufacturer datasheet 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 LM2736, 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 at 10 µF of output capaci- tance. Capacitance can be increased significantly with little detriment to the regulator stability. Like the input capacitor, recommended multilayer ceramic capacitors are X7R or X5R. Again, verify actual capacitance at the desired operat- ing voltage and temperature. Check the RMS current rating of the capacitor. The RMS current rating of the capacitor chosen must also meet the following condition: CATCH DIODE The catch diode (D1) conducts during the switch off-time. A Schottky diode is recommended for its fast switching times and low forward voltage drop. The catch diode should be chosen so that its current rating is greater than: I D1 =IO x (1-D) The reverse breakdown rating of the diode must be at least the maximum input voltage plus appropriate margin. To im- prove efficiency choose a Schottky diode with a low forward voltage drop. BOOST DIODE A standard diode such as the 1N4148 type is recommended. For V BOOST circuits derived from voltages less than 3.3V, a small-signal Schottky diode is recommended for greater ef- ficiency. A good choice is the BAT54 small signal diode. BOOST CAPACITOR A ceramic 0.01µF capacitor with a voltage rating of at least 6.3V is sufficient. The X7R and X5R MLCCs provide the best performance. www.national.com 10 |
Аналогичный номер детали - LM2736YMKX |
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Аналогичное описание - LM2736YMKX |
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