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LT1108 Datasheet(PDF) 7 Page - Linear Technology |
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LT1108 Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page 7 LT1108 S APPLICATI I FOR ATIO Energy required from the inductor is P f mW kHz J L OSC == 315 19 16 6 07 .( ) µ Picking an inductor value of 100 µH with 0.2Ω DCR results in a peak switch current of I V emA PEAK s H = = × 2 10 1 605 08 10 36 100 . –( ) –. Ω Ωµ µ Substituting IPEAK into Equation 04 results in EH A J L = ()( ) = 1 2 100 6 605 18 3 09 2 µµ .. ( ) Since 18.3 µJ > 16.6µJ, the 100µH inductor will work. This trial-and-error approach can be used to select the optimum inductor. Keep in mind the switch current maximum rating of 1.5A. If the calculated peak current exceeds this, an external power transistor can be used. A resistor can be added in series with the ILIM pin to invoke switch current limit. The resistor should be picked so the calculated IPEAK at minimum VIN is equal to the Maximum Switch Current (from Typical Performance Characteristic curves). Then, as VIN increases, switch current is held constant, resulting in increasing efficiency. Step-Down Converter The step-down case (Figure 2) differs from the step-up in that the inductor current flows through the load during both the charge and discharge periods of the inductor. Current through the switch should be limited to ~650mA in this mode. Higher current can be obtained by using an external switch (see Figure 3). The ILIM pin is the key to successful operation over varying inputs. After establishing output voltage, output current and input voltage range, peak switch current can be calculated by the formula: I I DC VV VV V PEAK OUT OUT D IN SW D = + + 2 10 – () where DC = duty cycle (0.60) VSW = switch drop in step-down mode VD = diode drop (0.5V for a 1N5818) IOUT = output current VOUT = output voltage VIN = minimum input voltage VSW is actually a function of switch current which is in turn a function of VIN, L, time, and VOUT. To simplify, 1.5V can be used for VSW as a very conservative value. Once IPEAK is known, inductor value can be derived from L VV V I t IN MIN SW OUT PEAK ON = −− × () 11 where tON = switch-ON time (36µs). Next, the current limit resistor RLIM is selected to give IPEAK from the RLIM Step-Down Mode curve. The addition of this resistor keeps maximum switch current constant as the input voltage is increased. As an example, suppose 5V at 300mA is to be generated from a 12V to 24V input. Recalling Equation (10), I mA mA PEAK = () + + = 2 300 060 50 5 12 15 05 500 12 . . –. . () Next, inductor value is calculated using Equation (11) L mA sH == 12 1 5 5 500 36 396 13 –. – () µµ Use the next lowest standard value (330 µH). Then pick RLIM from the curve. For IPEAK = 500mA, RLIM = 220Ω. Positive-to-Negative Converter Figure 4 shows hookup for positive-to-negative conver- sion. All of the output power must come from the inductor. In this case, PL = (VOUT+ VD)(IOUT) (14) |
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