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# ADP1110AR-5 View Datasheet(PDF) - Analog Devices

 Part Name Description Manufacturer ADP1110AR-5 Micropower, Step-Up/Step-Down Switching Regulator; Adjustable and Fixed 3.3 V, 5 V, 12 V Analog Devices ADP1110AR-5 Datasheet PDF : 16 Pages
 1 2 3 4 5 6 7 8 9 10 Next Last ADP1110 As previously mentioned, the switch voltage is higher in step- down mode than in step-up mode. VSW is a function of switch current and is therefore a function of VIN, L, time and VOUT. For most applications, a VSW value of 1.5 V is recommended. The inductor value can now be calculated: L = V IN(MIN) –V SW I PEAK – V OUT • tON (Equation 7) where: tON = Switch ON time (10 µs) If the input voltage will vary (such as an application that must operate from a 9 V, 12 V or 15 V source), an RLIM resistor should be selected from Figure 6. The RLIM resistor will keep switch current constant as the input voltage rises. Note that there are separate RLIM values for step-up and step-down modes of operation. For example, assume that +5 V at 250 mA is required from a +9 V to +18 V source. Deriving the peak current from Equation 6 yields: I PEAK = 2 •250 mA 0.69   9 5+ 0.5  −1.5+ 0.5 = 498 mA Then, the peak current can be inserted into Equation 7 to calculate the inductor value: L = 9 –1.5 – 5 •10 µs = 50 µs 498 mA Since 50 µH is not a standard value, the next lower standard value of 47 µH would be specified. To avoid exceeding the maximum switch current when the input voltage is at +18 V, an RLIM resistor should be specified. Using the step-down curve of Figure 6, a value of 560 Ω will limit the switch current to 500 mA. INDUCTOR SELECTION—POSITIVE-TO-NEGATIVE CONVERTER The configuration for a positive-to-negative converter using the ADP1110 is shown in Figure 23. As with the step-up converter, all of the output power for the inverting circuit must be supplied by the inductor. The required inductor power is derived from the formula: ( ) ( ) PL = VOUT + VD • IOUT (Equation 8) The ADP1110 power switch does not saturate in positive-to- negative mode. The voltage drop across the switch can be modeled as a 0.75 V base-emitter diode in series with a 0.65 Ω resistor. When the switch turns on, inductor current will rise at a rate determined by: IL (t ) = VL R' 1− –R't eL   (Equation 9) where: R' = 0.65 Ω + RL(DC) VL = VIN – 0.75 V For example, assume that a –5 V output at 75 mA is to be generated from a +4.5 V to +5.5 V source. The power in the inductor is calculated from Equation 8: PL = (|– 5V|+ 0.5V ) •(75 mA) = 413 mW During each switching cycle, the inductor must supply the following energy: PL = 413 mW = 5.9 µJ f OSC 70 kHz Using a standard inductor value of 56 µH with 0.2 Ω dc resistance will produce a peak switch current of: I PEAK = 4.5V – 0.75V 0.65 Ω + 0.2 Ω  –0.85 Ω •10 µs  1− e 56 µH    = 621 mA Once the peak current is known, the inductor energy can be calculated from Equation 9: EL = 1 2 (56 µH ) • (621 mA)2 =10.8 µJ Since the inductor energy of 10.8 µJ is greater than the PL/fOSC requirement of 5.9 µJ, the 56 µH inductor will work in this application. The input voltage only varies between 4.5 V and 5.5 V in this example. Therefore, the peak current will not change enough to require an RLIM resistor and the ILIM pin can be connected directly to VIN. Care should be taken, of course, to ensure that the peak current does not exceed 800 mA. CAPACITOR SELECTION For optimum performance, the ADP1110’s output capacitor must be selected carefully. Choosing an inappropriate capacitor can result in low efficiency and/or high output ripple. Ordinary aluminum electrolytic capacitors are inexpensive but often have poor Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL). Low ESR aluminum capacitors, specifically designed for switch mode converter applications, are also available, and these are a better choice than general purpose devices. Even better performance can be achieved with tantalum capacitors, although their cost is higher. Very low values of ESR can be achieved by using OS-CON capacitors (Sanyo Corporation, San Diego, CA). These devices are fairly small, available with tape-and-reel packaging and have very low ESR. The effects of capacitor selection on output ripple are demon- strated in Figures 16, 17 and 18. These figures show the output of the same ADP1110 converter, that was evaluated with three different output capacitors. In each case, the peak switch current is 500 mA, and the capacitor value is 100 µF. Figure 16 shows a Panasonic HF-series 16-volt radial cap. When the switch turns off, the output voltage jumps by about 90 mV and then decays as the inductor discharges into the capacitor. The rise in voltage indicates an ESR of about 0.18 Ω. In Figure 17, the aluminum electrolytic has been replaced by a Sprague 293D series, a 6 V tantalum device. In this case the output jumps –8– REV. 0