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ADP1073AR 数据表(PDF) 8 Page - Analog Devices |
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ADP1073AR 数据表(HTML) 8 Page - Analog Devices |
8 / 16 page ADP1073 –8– REV. 0 Capacitor Selection For optimum performance, the ADP1073’s output capacitor must be carefully selected. Choosing an inappropriate capacitor can result in low efficiency and/or high output ripple. Ordinary aluminum electrolytic capacitors are inexpensive, but often have poor Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL). Low ESR aluminum capacitors, specifically designed for switch mode converter applications, are also available, and these are a better choice than general purpose devices. Even better performance can be achieved with tantalum capacitors, although their cost is higher. Very low values of ESR can be achieved by using OS-CON capacitors (Sanyo Corporation, San Diego, CA). These devices are fairly small, available with tape-and-reel packaging and have very low ESR. The effects of capacitor selection on output ripple are demon- strated in Figures 12, 13 and 14. These figures show the output of the same ADP1073 converter, which was evaluated with three different output capacitors. In each case, the peak switch current is 500 mA and the capacitor value is 100 µF. Figure 12 shows a Panasonic HF-series radial aluminum electrolytic. When the switch turns off, the output voltage jumps by about 90 mV and then decays as the inductor discharges into the capacitor. The rise in voltage indicates an ESR of about 0.18 Ω. In Figure 13, the aluminum electrolytic has been replaced by a Sprague 593D-series device. In this case the output jumps about 35 mV, which indicates an ESR of 0.07 Ω. Figure 14 shows an OS-CON SA series capacitor in the same circuit, and ESR is only 0.02 Ω. Figure 12. Aluminum Electrolytic Figure 13. Tantalum Electrolytic Figure 14. OS-CON Capacitor If low output ripple is important, the user should consider using the ADP3000. This device switches at 400 kHz, and the higher switching frequency simplifies the design of the output filter. Consult the ADP3000 data sheet for additional details. All potential current paths must be considered when analyzing very low power applications, and this includes capacitor leakage current. OS-CON capacitors have leakage in the 5 µA to 10 µA range, which will reduce efficiency when the load is also in the microampere range. Tantalum capacitors, with typical leakage in the 1 µA to 5 µA range, are recommended for very low power applications. Diode Selection In specifying a diode, consideration must be given to speed, forward voltage drop and reverse leakage current. When the ADP1073 switch turns off, the diode must turn on rapidly if high efficiency is to be maintained. Schottky rectifiers, as well as fast signal diodes such as the 1N4148, are appropriate. The forward voltage of the diode represents power that is not deliv- ered to the load, so VF must also be minimized. Again, Schottky diodes are recommended. Leakage current is especially impor- tant in low current applications, where the leakage can be a significant percentage of the total quiescent current. For most circuits, the 1N5818 is a suitable companion to the ADP1073. This diode has a VF of 0.5 V at 1 A, 4 µA to 10 µA leakage and fast turn-on and turn-off times. A surface mount version, the MBRS130T3, is also available. For applications where the ADP1073 is “off” most of the time, such as when the load is intermittent, a silicon diode may provide higher overall efficiency due to lower leakage. For example, the 1N4933 has a 1 A capability, but with a leakage current of less than 1 µA. The higher forward voltage of the 1N4933 reduces efficiency when the ADP1073 delivers power, but the lower leakage may outweigh the reduction in efficiency. For switch currents of 100 mA or less, a Schottky diode such as the BAT85 provides a VF of 0.8 V at 100 mA and leakage less than 1 µA. A similar device, the BAT54, is available in an SOT-23 package. Even lower leakage, in the 1 nA to 5 nA range, can be obtained with a 1N4148 signal diode. General purpose rectifiers, such as the 1N4001, are not suitable for ADP1073 circuits. These devices, which have turn-on times of 10 µs or more, are too slow for switching power supply appli- cations. Using such a diode “just to get started” will result in wasted time and effort. Even if an ADP1073 circuit appears to function with a 1N4001, the resulting performance will not be indicative of the circuit performance when the correct diode is used. |
类似零件编号 - ADP1073AR |
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类似说明 - ADP1073AR |
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