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LT1111 Datasheet(PDF) 9 Page - Linear Technology |
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LT1111 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LT1111 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 LT1111 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 0.24 Ω. Figure 2 shows the 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 0.07 Ω. Figure 3 shows the circuit with a 16V OS-CON unit. ESR is now only 0.02 Ω. S APPLICATI I FOR ATIO switch closes, current in the inductor builds according to It V R e L L Rt L () = ′ ′ 115 –( ) – where R ′ = 0.65Ω + DCRL VL = VIN – 0.75V As an example, suppose –5V at 50mA is to be generated from a 4.5V to 5.5V input. Recalling Equation (14), PL = (-5V+0.5V)(50mA) = 275mW (16) Energy required from the inductor is: P f mW kHz J L OSC == 275 72 38 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 1 445 18 085 7 56 .– . .. –. ( ) –. ΩΩ Ωµ µ Substituting IPEAK into Equation (04) results in: EH A J L = ()( ) = 1 2 56 0 445 5 54 19 2 µµ .. . ( ) Since 5.54 µJ > 3.82µJ, 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 ~650mA. 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 5 µs/DIV LT1111 • F01 Figure 1. Aluminum 5 µs/DIV LT1111 • F02 Figure 2. Tantalum 5 µs/DIV LT1111 • F01 Figure 3. OS-CON |
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