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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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LM2574N-5 데이터시트(PDF) 15 Page - ON Semiconductor |
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LM2574N-5 데이터시트(HTML) 15 Page - ON Semiconductor |
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15 / 26 page  LM2574, NCV2574 http://onsemi.com 15 EXTERNAL COMPONENTS Input Capacitor (Cin) The Input Capacitor Should Have a Low ESR For stable operation of the switch mode converter a low ESR (Equivalent Series Resistance) aluminium or solid tantalum bypass capacitor is needed between the input pin and the ground pin, to prevent large voltage transients from appearing at the input. It must be located near the regulator and use short leads. With most electrolytic capacitors, the capacitance value decreases and the ESR increases with lower temperatures. For reliable operation in temperatures below −25 °C larger values of the input capacitor may be needed. Also paralleling a ceramic or solid tantalum capacitor will increase the regulator stability at cold temperatures. RMS Current Rating of Cin The important parameter of the input capacitor is the RMS current rating. Capacitors that are physically large and have large surface area will typically have higher RMS current ratings. For a given capacitor value, a higher voltage electrolytic capacitor will be physically larger than a lower voltage capacitor, and thus be able to dissipate more heat to the surrounding air, and therefore will have a higher RMS current rating. The consequences of operating an electrolytic capacitor beyond the RMS current rating is a shortened operating life. In order to assure maximum capacitor operating lifetime, the capacitor’s RMS ripple current rating should be: Irms u 1.2 x d x ILoad where d is the duty cycle, for a continuous mode buck regulator d + ton T + Vout V in and d + ton T + |Vout| |Vout| ) Vin for a buck−boost regulator. Output Capacitor (Cout) For low output ripple voltage and good stability, low ESR output capacitors are recommended. An output capacitor has two main functions: it filters the output and provides regulator loop stability. The ESR of the output capacitor and the peak−to−peak value of the inductor ripple current are the main factors contributing to the output ripple voltage value. Standard aluminium electrolytics could be adequate for some applications but for quality design, low ESR types are recommended. An aluminium electrolytic capacitor’s ESR value is related to many factors, such as the capacitance value, the voltage rating, the physical size and the type of construction. In most cases, the higher voltage electrolytic capacitors have lower ESR value. Often capacitors with much higher voltage ratings may be needed to provide low ESR values, that are required for low output ripple voltage. The Output Capacitor Requires an ESR Value that has an Upper and Lower Limit As mentioned above, a low ESR value is needed for low output ripple voltage, typically 1% to 2% of the output voltage. But if the selected capacitor’s ESR is extremely low (below 0.03 W), there is a possibility of an unstable feedback loop, resulting in oscillation at the output. This situation can occur when a tantalum capacitor, that can have a very low ESR, is used as the only output capacitor. At Low Temperatures, Put in Parallel Aluminium Electrolytic Capacitors with Tantalum Capacitors Electrolytic capacitors are not recommended for temperatures below −25 °C. The ESR rises dramatically at cold temperatures and typically rises 3 times at −25 °C and as much as 10 times at −40 °C. Solid tantalum capacitors have much better ESR spec at cold temperatures and are recommended for temperatures below −25 °C. They can be also used in parallel with aluminium electrolytics. The value of the tantalum capacitor should be about 10% or 20% of the total capacitance. The output capacitor should have at least 50% higher RMS ripple current rating at 52 kHz than the peak−to−peak inductor ripple current. Catch Diode Locate the Catch Diode Close to the LM2574 The LM2574 is a step−down buck converter, it requires a fast diode to provide a return path for the inductor current when the switch turns off. This diode must be located close to the LM2574 using short leads and short printed circuit traces to avoid EMI problems. Use a Schottky or a Soft Switching Ultra−Fast Recovery Diode Since the rectifier diodes are very significant source of losses within switching power supplies, choosing the rectifier that best fits into the converter design is an important process. Schottky diodes provide the best performance because of their fast switching speed and low forward voltage drop. They provide the best efficiency especially in low output voltage applications (5.0 V and lower). Another choice could be Fast−Recovery, or Ultra−Fast Recovery diodes. It has to be noted, that some types of these diodes with an abrupt turnoff characteristic may cause instability or EMI troubles. A fast−recovery diode with soft recovery characteristics can better fulfill some quality, low noise design requirements. Table 1 provides a list of suitable diodes for the LM2574 regulator. Standard 50/60 Hz rectifier diodes, such as the 1N4001 series or 1N5400 series are NOT suitable. |
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