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LT1111MJ8-5 Datasheet(PDF) 9 Page - Linear Technology |
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LT1111MJ8-5 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LT1111 1111fd specifically designed for switch mode DC/DC converters which work much better than general-purpose units. Tantalum capacitors provide still better performance at more expense. 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 1 445 085 7 56 .– . .. –. –. ΩΩ Ωµ µ Substituting IPEAK into Equation (4) 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 equivalent series resistance (ESR) and ESL (inductance). There are low ESR aluminum capacitors on the market 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 (18) |
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