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## ADP1109AAR-5 Datasheet(PDF) 7 Page - Analog Devices

 Part No. ADP1109AAR-5 Description Micropower Low Cost Fixed 3.3 V, 5 V, 12 V and Adjustable DC-to-DC Converter Download 8 Pages Scroll/Zoom 100% Maker AD [Analog Devices] Homepage http://www.analog.com Logo

## ADP1109AAR-5 Datasheet(HTML) 7 Page - Analog Devices

 7 / 8 page ADP1109A–7–REV. 0In practice, the inductor value is easily selected using the equa-tions above. For example, consider a supply that will generate12 V at 120 mA from a +5 V source. The inductor power re-quired is, from Equation 1:PL = (12 V + 0.5 V – 5 V) × (120 mA) = 900 mWOn each switching cycle, the inductor must supply:PLf OSC=900 mW120 kHz= 7.5 µJThe required inductor power is fairly low in this example, sothe peak current can also be low. Assuming a peak current of600 mA as a starting point, Equation 4 can be rearranged torecommend an inductor value:L=VINIL MAX()t=5 V600 mA5.5µs = 45.8 µHSubstituting a standard inductor value of 33µH, with 0.2 Ω dcresistance, will produce a peak switch current of:IPEAK =5 V1.0Ω1− e–1.0Ω× 5.5 µs33µH = 768 mAOnce the peak current is known, the inductor energy can becalculated from Equation 5:EL =1233µH() × 768 mA()2 = 9.7 µJThe inductor energy of 9.7µJ is greater than the PL/fOSC re-quirement of 7.5µJ, so the 33 µH inductor will work in thisapplication. By substituting other inductor values into the sameequations, the optimum inductor value can be selected. Whenselecting an inductor, the peak current must not exceed themaximum switch current of 1.2 A. If the calculated peak currentis greater than 1.2 A, either the input voltage must be increasedor the load current decreased.Output Voltage SelectionThe output voltage is fed back to the ADP1109A via resistorsR1 and R2 (Figure 5). When the voltage at the comparator’sinverting input falls below 1.25 V, the oscillator turns “on” andthe output voltage begins to rise. The output voltage is thereforeset by the formula:VOUT = 1. 25 V ×1+R2R1Resistors R1 and R2 are provided internally on fixed-voltageversions of the ADP1109A. In this case, a complete dc-dc con-verter requires only four external components.Capacitor SelectionFor optimum performance, the ADP1109A’s output capacitormust be carefully selected. Choosing an inappropriate capacitorcan result in low efficiency and/or high output ripple.Ordinary aluminum electrolytic capacitors are inexpensive, butoften have poor Equivalent Series Resistance (ESR) and Equiva-lent Series Inductance (ESL). Low ESR aluminum capacitors,specifically designed for switch mode converter applications, arealso available, and these are a better choice than general purposedevices. Even better performance can be achieved with tantalumcapacitors, although their cost is higher. Very low values of ESRcan be achieved by using OS-CON capacitors (Sanyo Corpora-tion, San Diego, CA). These devices are fairly small, availablewith tape-and-reel packaging, and have very low ESR.Diode SelectionIn specifying a diode, consideration must be given to speed,forward voltage drop and reverse leakage current. When theADP1109A switch turns off, the diode must turn on rapidly ifhigh efficiency is to be maintained. Schottky rectifiers, as well asfast signal diodes such as the 1N4148, are appropriate. Theforward voltage of the diode represents power that is notdelivered to the load, so VF must also be minimized. Again,Schottky diodes are recommended. Leakage current is especiallyimportant in low current applications, where the leakage can bea significant percentage of the total quiescent current.For most circuits, the 1N5818 is a suitable companion to theADP1109A. This diode has a VF of 0.5 V at 1 A, 4µA to 10 µAleakage, and fast turn-on and turn-off times. A surface mountversion, the MBRS130T3, is also available.For switch currents of 100 mA or less, a Schottky diode such asthe BAT85 provides a VF of 0.8 V at 100 mA and leakage lessthan 1µA. A similar device, the BAT54, is available in anSOT-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 suitablefor ADP1109A circuits. These devices, which have turn-ontimes of 10µs or more, are far too slow for switching powersupply applications. Using such a diode “just to get started” willresult in wasted time and effort. Even if an ADP1109A circuitappears to function with a 1N4001, the resulting performancewill not be indicative of the circuit performance when the cor-rect diode is used.

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