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LM2590HV-AQ Datasheet(PDF) 17 Page - National Semiconductor (TI) |
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LM2590HV-AQ Datasheet(HTML) 17 Page - National Semiconductor (TI) |
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17 / 22 page  Application Information (Continued) Therefore, looking at Figure 4 we quick-select a 100µH/1A inductor (designed for 150 kHz operation) for this applica- tion. 2. We should confirm that it is rated to handle 100 µJ (see Figure 6) by the procedure shown in AN-1197 and that the losses are acceptable. (If the DC Input voltage had been greater than 40V we would need to consider e CLIM as in Example 2 above). Note that we have taken V SAT as 1.5V which includes an estimated resistive drop across the inductor. This completes the simplified inductor selection procedure. For more general applications and better optimization, the designer should refer to AN-1197. Figure 8 provides helpful contact information on suggested Inductor manufacturers who may be able to recommend suitable parts, if the require- ments are known. FEEDFORWARD CAPACITOR (Adjustable Output Voltage Version) C FF - A Feedforward Capacitor CFF, shown across R2 in Figure 1 is used when the output voltage is greater than 10V or when C OUT has a very low ESR. This capacitor adds lead compensation to the feedback loop and increases the phase margin for better loop stability. If the output voltage ripple is large (> 5% of the nominal output voltage), this ripple can be coupled to the feedback pin through the feedforward capacitor and cause the error comparator to trigger the error flag. In this situation, adding a resistor, R FF, in series with the feedforward capacitor, ap- proximately 3 times R1, will attenuate the ripple voltage at the feedback pin. INPUT CAPACITOR C IN — A low ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground pin. It must be located near the regulator using short leads. This capacitor prevents large voltage transients from appearing at the in- put, and provides the instantaneous current needed each time the switch turns on. The important parameters for the Input capacitor are the voltage rating and the RMS current rating. Because of the relatively high RMS currents flowing in a buck regulator’s input capacitor, this capacitor should be chosen for its RMS current rating rather than its capacitance or voltage ratings, although the capacitance value and voltage rating are di- rectly related to the RMS current rating. The voltage rating of the capacitor and its RMS ripple current capability must never be exceeded. OUTPUT CAPACITOR C OUT — An output capacitor is required to filter the output and provide regulator loop stability. Low impedance or low ESR Electrolytic or solid tantalum capacitors designed for switching regulator applications must be used. When select- ing an output capacitor, the important capacitor parameters are; the 100 kHz Equivalent Series Resistance (ESR), the RMS ripple current rating, voltage rating, and capacitance value. For the output capacitor, the ESR value is the most important parameter. The ESR should generally not be less than 100 m Ω or there will be loop instability. If the ESR is too large, efficiency and output voltage ripple are effected. So ESR must be chosen carefully. CATCH DIODE Buck regulators require a diode to provide a return path for the inductor current when the switch turns off. This must be a fast diode and must be located close to the LM2590HV-AQ using short leads and short printed circuit traces. Because of their very fast switching speed and low forward voltage drop, Schottky diodes provide the best performance, especially in low output voltage applications (5V and lower). Ultra-fast recovery, or High-Efficiency rectifiers are also a good choice, but some types with an abrupt turnoff charac- teristic may cause instability or EMI problems. Ultra-fast recovery diodes typically have reverse recovery times of 50 ns or less. The diode must be chosen for its average/RMS current rating and maximum voltage rating. The voltage rating of the diode must be greater than the DC input voltage (not the output voltage). SHUTDOWN /SOFT-START This reduction in start up current is useful in situations where the input power source is limited in the amount of current it can deliver. In some applications Soft-start can be used to replace undervoltage lockout or delayed startup functions. If a very slow output voltage ramp is desired, the Soft-start capacitor can be made much larger. Many seconds or even minutes are possible. If only the shutdown feature is needed, the Soft-start capaci- tor can be eliminated. www.national.com 17 |
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