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LM2595 Datasheet(PDF) 18 Page - National Semiconductor (TI) |
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LM2595 Datasheet(HTML) 18 Page - National Semiconductor (TI) |
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18 / 29 page  Application Information (Continued) lytic capacitors designed for switching regulator applications. Other capacitor manufacturers offer similar types of capaci- tors, but always check the capacitor data sheet. “Standard” electrolytic capacitors typically have much higher ESR numbers, lower RMS current ratings and typically have a shorter operating lifetime. Because of their small size and excellent performance, sur- face mount solid tantalum capacitors are often used for input bypassing, but several precautions must be observed. A small percentage of solid tantalum capacitors can short if the inrush current rating is exceeded. This can happen at turn on when the input voltage is suddenly applied, and of course, higher input voltages produce higher inrush currents. Sev- eral capacitor manufacturers do a 100% surge current test- ing on their products to minimize this potential problem. If high turn on currents are expected, it may be necessary to limit this current by adding either some resistance or induc- tance before the tantalum capacitor, or select a higher volt- age capacitor. As with aluminum electrolytic capacitors, the RMS ripple current rating must be sized to the load current. FEEDFORWARD CAPACITOR (Adjustable Output Voltage Version) C FF -A Feedforward Capacitor CFF, shown across R2 in Fig- ure 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. For C FF selection, see the de- sign procedure section. 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 output capacitor requires an ESR value that has an up- per and lower limit. For low output ripple voltage, a low ESR value is needed. This value is determined by the maximum allowable output ripple voltage, typically 1% to 2% of the out- put voltage. But if the selected capacitor’s ESR is extremely low, there is a possibility of an unstable feedback loop, re- sulting in an oscillation at the output. Using the capacitors listed in the tables, or similar types, will provide design solu- tions under all conditions. If very low output ripple voltage (less than 15 mV) is re- quired, refer to the section on Output Voltage Ripple and Transients for a post ripple filter. An aluminum electrolytic capacitor’s ESR value is related to the capacitance value and its voltage rating. In most cases, higher voltage electrolytic capacitors have lower ESR values (see Figure 14). Often, capacitors with much higher voltage ratings may be needed to provide the low ESR values re- quired for low output ripple voltage. The output capacitor for many different switcher designs of- ten can be satisfied with only three or four different capacitor values and several different voltage ratings. See the quick design component selection tables in Figure 2 and Figure 3 for typical capacitor values, voltage ratings, and manufactur- ers capacitor types. Electrolytic capacitors are not recommended for tempera- tures below −25˚C. The ESR rises dramatically at cold tem- peratures and typically rises 3X @ −25˚C and as much as 10X at −40˚C. See curve shown in Figure 15. Solid tantalum capacitors have a much better ESR spec for cold temperatures and are recommended for temperatures below −25˚C. 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 LM2595 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 re- covery diodes typically have reverse recovery times of 50 ns or less. Rectifiers such as the 1N5400 series are much too slow and should not be used. DS012565-28 FIGURE 13. RMS Current Ratings for Low ESR Electrolytic Capacitors (Typical) DS012565-29 FIGURE 14. Capacitor ESR vs Capacitor Voltage Rating (Typical Low ESR Electrolytic Capacitor) www.national.com 18 |
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