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ADP1173AR-12 데이터시트(PDF) 8 Page - Analog Devices |
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ADP1173AR-12 데이터시트(HTML) 8 Page - Analog Devices |
8 / 16 page ADP1173 –8– REV. 0 Figure 11. Aluminum Electrolytic Figure 12. Tantalum Electrolytic Figure 13. OS-CON Capacitor If low output ripple is important, the user should consider 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 ADP1173 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 ADP1173. 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 ADP1173 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 ADP1173 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 a SOT23 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 ADP1173 circuits. These devices, which have turn-on times of 10 µs or more, are too slow for switching power supply applications. Using such a diode “just to get started” will result in wasted time and effort. Even if an ADP1173 circuit appears to function with a 1N4001, the resulting performance will not be indicative of the circuit performance when the correct diode is used. CIRCUIT OPERATION, STEP-UP (BOOST) MODE In boost mode, the ADP1173 produces an output voltage that is higher than the input voltage. For example, +12 V can be generated from a +5 V logic power supply or +5 V can be derived from two alkaline cells (+3 V). Figure 16 shows an ADP1173 configured for step-up operation. The collector of the internal power switch is connected to the output side of the inductor, while the emitter is connected to GND. When the switch turns on, pin SW1 is pulled near ground. This action forces a voltage across L1 equal to VIN–VCE(SAT), and current begins to flow through L1. This current reaches a final value (ignoring second-order effects) of: IPEAK ≅ V IN –VCE(SAT ) L ×23 µs where 23 µs is the ADP1173 switch’s “on” time. |
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