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LT1110-5 Datasheet(PDF) 9 Page - Linear Technology |
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LT1110-5 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  LT1110 9 switch closes, current in the inductor builds according to I V R e L L Rt L + () = ' –( ) –' 115 where R' = 0.65 Ω + DCRL VL = VIN – 0.75V As an example, suppose –5V at 75mA is to be generated from a 4.5V to 5.5V input. Recalling Equation (14), PV V mA mW L =− + ()( ) = || .. ( ) 5 0 5 75 413 16 Energy required from the inductor is P f mW kHz J L OSC == 413 70 59 17 .. ( ) µ Picking an inductor value of 56 µH with 0.2Ω DCR results in a peak switch current of I VV emA PEAK s H = () + () = • 45 075 065 0 2 1 621 18 085 10 56 .– . .. –. ( ) –. ΩΩ Ωµ µ Substituting IPEAK into Equation (04) results in EH A J L = ()( ) = 1 2 56 0 621 10 8 19 2 µµ .. . ( ) Since 10.8 µJ > 5.9µJ, the 56µH inductor will work. With this relatively small input range, RLIM is not usually necessary and the ILIM pin can be tied directly to VIN. As in the step-down case, peak switch current should be limited to ~800mA. Capacitor Selection Selecting the right output capacitor is almost as important as selecting the right inductor. A poor choice for a filter capacitor can result in poor efficiency and/or high output ripple. Ordinary aluminum electrolytics, while inexpensive and readily available, may have unacceptably poor Equiva- lent Series Resistance (ESR) and ESL (inductance). There are low ESR aluminum capacitors on the market specifi- cally designed for switch mode DC-DC converters which work much better than general-purpose units. Tantalum capacitors provide still better performance at more ex- pense. We recommend OS-CON capacitors from Sanyo Corporation (San Diego, CA). These units are physically quite small and have extremely low ESR. To illustrate, Figures 1, 2 and 3 show the output voltage of an LT1110 based converter with three 100 µF capacitors. The peak switch current is 500mA in all cases. Figure 1 shows a Sprague 501D, 25V aluminum capacitor. VOUT jumps by over 120mV when the switch turns off, followed by a drop in voltage as the inductor dumps into the capacitor. This works out to be an ESR of over 240m Ω.Figure2showsthe same circuit, but with a Sprague 150D, 20V tantalum capacitor replacing the aluminum unit. Output jump is now about 35mV, corresponding to an ESR of 70m Ω. Figure 3 shows the circuit with a 16V OS-CON unit. ESR is now only 20m Ω. 5 s/DIV LT1110 • TA19 µ Figure 1. Aluminum 5 s/DIV LT1110 • TA20 µ Figure 2. Tantalum 5 s/DIV LT1110 • TA21 µ Figure 3. OS-CON S APPLICATI I FOR ATIO |
Similar Part No. - LT1110-5 |
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Similar Description - LT1110-5 |
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