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ADP1108AR датащи(PDF) 8 Page - Analog Devices |
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ADP1108AR датащи(HTML) 8 Page - Analog Devices |
8 / 12 page ADP1108 –8– REV. 0 10 0% 5 s 50mV 100 90 COUT =100 F, 16V ISW = 500mA ESR 0.18 Figure 12. Aluminum Electrolytic 10 0% 5µs 50mV 100 90 COUT =100µF, 6V ISW = 500mA ESR 0.07 Figure 13. Tantalum Electrolytic 10 0% 5 s 50mV 100 90 COUT =100 F, 16V ISW = 500mA ESR 0.02 Figure 14. OS-CON Capacitor If low output ripple is important, the user should consider using the ADP3000. This device switches at 400 kHz, which simpli- fies the design of the output filter. Consult the ADP3000 data sheet for additional details. DIODE SELECTION In specifying a diode, consideration must be given to speed, for- ward voltage drop and reverse leakage current. When the ADP1108 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 for- ward voltage of the diode represents power that is not delivered to the load, so VF must also be minimized. Again, Schottky di- odes are recommended. Leakage current is especially important in low-current applications, where the leakage can be a signifi- cant percentage of the total quiescent current. For most circuits, the 1N5818 is a suitable companion to the ADP1108. 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 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 ADP1108 circuits. These devices, which have turn-on times of 10 µs or more, are far too slow for switching power supply applica- tions. Using such a diode “just to get started” will result in wasted time and effort. Even if an ADP1108 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 ADP1108 produces an output voltage 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 15 shows an ADP1108 configured for step-up operation. The collector of the internal power switch is connected to the out- put side of the inductor, while the emitter is connected to GND. When the switch turns on, Pin SW1 is pulled near ground. This ac- tion 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: I PEAK ≅ V IN – V CE (SAT ) L × 36 µs where 36 µs is the ADP1108 switch’s “on” time. 3 5 1 8 4 2 ILIM VIN SW1 FB SW2 GND ADP1108 L1 D1 VOUT C1 R3 R1 R2 VIN Figure 15. Step-Up Mode Operation When the switch turns off, the magnetic field collapses. The polarity across the inductor changes, current begins to flow through D1 into the load and the output voltage is driven above the input voltage. The output voltage is fed back to the ADP1108 via resistors R1 and R2. When the voltage at Pin FB falls below 1.245 V, SW1 turns “on” again and the cycle repeats. The output voltage is therefore set by the formula: V OUT = 1.245 V × 1 + R1 R2 The circuit of Figure 15 shows a direct current path from VIN to VOUT, via the inductor and D1. Therefore, the boost converter is not protected if the output is short circuited to ground. Circuit Operation, Step-Down (Buck) Mode The ADP1108’s step-down mode is used to produce an output voltage lower than the input voltage. For example, the output of four NiCd cells (+4.8 V) can be converted to a +3 V logic supply. A typical configuration for step-down operation of the ADP1108 is shown in Figure 16. In this case, the collector of the internal power switch is connected to VIN and the emitter drives the inductor. |
Аналогичный номер детали - ADP1108AR |
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Аналогичное описание - ADP1108AR |
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