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ADP1108AR-5 Datasheet(PDF) 7 Page - Analog Devices |
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ADP1108AR-5 Datasheet(HTML) 7 Page - Analog Devices |
7 / 12 page ADP1108 –7– REV. 0 where: DC = duty cycle (0.7 for the ADP1108) VSW = voltage drop across the switch VD = diode drop (0.5 V for a 1N5818) IOUT = output current VOUT = the output voltage VIN = the minimum input voltage 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) –VSW –VOUT IPEAK ×t ON (Equation 7) where: tON = switch ON time (36 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 4. 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 0.7 5 + 0.5 9 − 1.5 + 0.5 = 491 mA The peak current can than be inserted into Equation 7 to cal- culate the inductor value: L = 9–1.5–5 491 mA × 36 µs = 183 µH Since 183 µH is not a standard value, the next lower standard value of 150 µH would be specified. To avoid exceeding the maximum switch current when the in- put voltage is at +18 V, an RLIM resistor should be specified. Us- ing Figure 4, a value of 160 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 ADP1108 is shown in Figure 19. 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: P L |V OUT|+ V D ()× I OUT () (Equation 8) The ADP1108 power switch does not saturate in positive-to- negative mode. The voltage drop across the switch can be mod- eled 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: I L (t ) = V L R' 1 − e – R't L (Equation 9) where: R' = 0.65 Ω + R L (DC) VL = VIN – 0.75 V For example, assume that a –5 V output at 100 mA is to be gen- erated from a +4.5 V to +5.5 V source. The power in the induc- tor is calculated from Equation 8: P L = |– 5 V| + 0.5 V () × 100 mA () = 550 mW During each switching cycle, the inductor must supply the fol- lowing energy: PL f OSC = 550 mW 19 kHz = 28.9µ J Using a standard inductor value of 220 µH with 0.3 Ω dc resis- tance will produce a peak switch current of: I PEAK = 4.5V –0.75 V 0.65 Ω+ 0.3 Ω 1 − e –0.95 Ω× 36 µs 220 µH = 568 mA Once the peak current is known, the inductor energy can be cal- culated from Equation 9: EL = 1 2 220 µH × 568 mA ()2 = 35.5µ J The inductor energy of 35.5 µJ is greater than the P L/fOSC re- quirement of 28.9 µJ, so the 220 µH inductor will work in this application. To avoid exceeding the maximum switch current when the in- put voltage is at +5.5 V, an RLIM resistor should be specified. Referring to Figure 4, a value of 150 is appropriate in this application. Capacitor Selection For optimum performance, the ADP1108’s output capacitor must be carefully selected. 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 Equiva- lent 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 Corpora- tion, 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 12, 13, and 14. These figures show the output of the same ADP1108 converter, which 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 12 shows a Panasonic HF-series* radial aluminum electrolytic. When the switch turns off, the output voltage jumps by about 90 mV and then decays as the inductor discharges into the ca- pacitor. The rise in voltage indicates an ESR of about 0.18 . In Figure 13, the aluminum electrolytic has been replaced by a Sprague 593D-series* tantalum device. In this case the output jumps about 35 mV, which indicates an ESR of 0.07 . Figure 14 shows an OS-CON SA series capacitor in the same circuit, and ESR is only 0.02 . *All trademarks are the property of their respective holders. |
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