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ADP1111ANZ Datasheet(PDF) 9 Page - Analog Devices |
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ADP1111ANZ Datasheet(HTML) 9 Page - Analog Devices |
9 / 15 page ADP1111 –9– REV. 0 Figure 15. Aluminum Electrolytic Figure 16. Tantalum Electrolytic Figure 17. OS-CON Capacitor If low output ripple is important, the user should consider the ADP3000. Because this device switches at 400 kHz, lower peak current can be used. Also, the higher switching frequency simplifies 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, forward voltage drop and reverse leakage current. When the ADP1111 switch turns off, the diode must turn on rapidly if high efficiency is to be maintained. Shottky rectifiers, as well as fast signal diodes such as the 1N4148, are appropriate. The forward voltage of the diode represents power that is not delivered to the load, so VF must also be minimized. Again, Schottky diodes are recommended. Leakage current is especially important 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 ADP1111. 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 Shottky 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 ADP1111 circuits. These devices, which have turn-on times of 10 μs or more, are far 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 ADP1111 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 ADP1111 produces an output voltage that is higher than the input voltage. For example, +12 V can be gener- ated from a +5 V logic power supply or +5 V can be derived from two alkaline cells (+3 V). Figure 18 shows an ADP1111 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 ≅ VIN − V CE (SAT ) L •7μs where 7 μs is the ADP1111 switch’s “on” time. ILIM VIN SW1 FB GND SW2 ADP1111 5 4 + VIN L1 D1 1N5818 C1 R2 R1 VOUT R3 (OPTIONAL) 1 2 3 8 Figure 18. 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 ADP1111 via resistors R1 and R2. When the voltage at pin FB falls below 1.25 V, SW1 turns “on” again, and the cycle repeats. The output voltage is therefore set by the formula: VOUT = 1.25 V • 1+ R2 R1 ⎛ ⎝⎜ ⎞ ⎠⎟ The circuit of Figure 18 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. REV. A |
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