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## ADP1108 Datasheet(PDF) 6 Page - Analog Devices

 Part No. ADP1108 Description Micropower DC-DC Converter Adjustable and Fixed 3.3 V, 5 V, 12 V Download 12 Pages Scroll/Zoom 100% Maker AD [Analog Devices] Homepage http://www.analog.com Logo ## ADP1108 Datasheet(HTML) 6 Page - Analog Devices           6 / 12 page ADP1108–6–REV. 0Calculating the Inductor ValueSelecting the proper inductor value is a simple three-stepprocess:1. Define the operating parameters: minimum input voltage,maximum input voltage, output voltage and output current.2. Select the appropriate conversion topology (step-up, step-down or inverting).3. Calculate the inductor value, using the equations in the fol-lowing sections.Inductor Selection—Step-Up ConverterIn a step-up or boost converter (Figure 15), the inductor muststore enough power to make up the difference between the inputvoltage and the output voltage. The inductor power is calculatedfrom the equation:PL = V OUT + V D − V IN MIN()()× IOUT()(Equation 1)where VD is the diode forward voltage (≈ 0.5 V for a 1N5818Schottky). Energy is only stored in the inductor while theADP1108 switch is ON, so the energy stored in the inductor oneach switching cycle must be equal to or greater than:PLfOSC(Equation 2)in order for the ADP1108 to regulate the output voltage.When the internal power switch turns ON, current flow in theinductor increases at the rate of:IL (t ) =VINR'1− e– R'tL(Equation 3)where L is in henrys and R is the sum of the switch equivalentresistance (typically 0.8Ω at +25°C) and the dc resistance ofthe inductor. If the voltage drop across the switch is smallcompared to VIN, a simpler equation can be used:IL (t ) =VINLt(Equation 4)Replacing t in the above equation with the ON time of theADP1108 (36µs, typical) will define the peak current for agiven inductor value and input voltage. At this point, theinductor energy can be calculated as follows:EL =12L× I2PEAK(Equation 5)As previously mentioned, EL must be greater than PL/fOSC so theADP1108 can deliver the necessary power to the load. For bestefficiency, peak current should be limited to 1 A or less. Higherswitch currents will reduce efficiency because of increased satura-tion voltage in the switch. High peak current also increases outputripple. As a general rule, keep peak current as low as possible tominimize losses in the switch, inductor and diode.In practice, the inductor value is easily selected using the equationsabove. For example, consider a supply that will generate 12 Vat 30 mA from a 3 V battery, assuming a 2 V end-of-life voltage.The inductor power required is from Equation 1:PL = 12 V + 0.5 V –2 V()× 30 mA() = 315 mWOn each switching cycle, the inductor must supply:PLf OSC=315 mW19 kHz=16.6 µ JThe required inductor power is fairly low in this example, so thepeak current can also be low. Assuming a peak current of500 mA as a starting point, Equation 4 can be rearranged torecommend an inductor value:L=VINIL(MAX )t=2 V500 mA36µs = 144 µHSubstituting a standard inductor value of 100µH with 0.2 Ω dcresistance, will produce a peak switch current of:IPEAK =2 V1.0Ω1– e–1.0Ω×36 µs100µH = 605 mAOnce the peak current is known, the inductor energy can becalculated from Equation 5:EL =12100µH × 605 mA()2 =18.3µ JThe inductor energy of 18.3µJ is greater than the PL/fOSCrequirement of 16.6µJ, so the 100 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 1.5 A. If the calculated peak currentis greater than 1.5 A, either the ADP3000 should be consideredor an external power transistor can be used.The peak current must be evaluated for both minimum andmaximum values of input voltage. If the switch current is highwhen VIN is at its minimum, the 1.5 A limit may be exceeded at themaximum value of VIN. In this case, the current limit feature ofthe ADP1108 can be used to limit switch current. Simply selecta resistor (using Figure 3) that will limit the maximum switchcurrent to the IPEAK value calculated for the minimum value ofVIN. This will improve efficiency by producing a constantIPEAK as VIN increases. See the Limiting the Switch Currentsection of this data sheet for more information.Note that the switch current limit feature does not protect thecircuit if the output is shorted to ground. In this case, current islimited only by the dc resistance of the inductor and the forwardvoltage of the diode.Inductor Selection—Step-Down ConverterThe step-down mode of operation is shown in Figure 16. Unlikethe step-up mode, the ADP1108’s power switch does notsaturate when operating in the step-down mode. Therefore,switch current should be limited to 650 mA in this mode. If theinput voltage will vary over a wide range, the ILIM pin can beused to limit the maximum switch current. Higher switch currentis possible by adding an external switching transistor, as shownin Figure 18.The first step in selecting the step-down inductor is to calculatethe peak switch current as follows:IPEAK =2 IOUTDCVOUT +VDV IN – VSW +VD(Equation 6)

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