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LT1110CN8 Datasheet(PDF) 8 Page - Linear Technology |
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LT1110CN8 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  LT1110 8 S APPLICATI I FOR ATIO Picking an inductor value of 47 µH with 0.2Ω DCR results in a peak switch current of I V emA PEAK s H =− = −• 45 10 1 862 08 10 10 47 . . .( ) . W Wm m Substituting IPEAK into Equation 05 results in EH A J L = ()( ) = 1 2 47 0 862 17 5 09 2 µµ .. . ( ) Since 17.5 µJ > 13.7µJ, the 47µH inductor will work. This trial-and-error approach can be used to select the opti- mum 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 such that 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. Inductor Selection — Step-Down Converter The step-down case (Figure 5) 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 ~800mA in this mode. Higher current can be obtained by using an external switch (see Figure 6). 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 (0.69) 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 (10µs). 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 250mA is to be generated from a 9V to 18V input. Recalling Equation (10), I mA mA PEAK = () + + = 2 250 069 50 5 91 5 0 5 498 12 . . –. . .( ) Next, inductor value is calculated using Equation (11) L mA sH =• = 91 5 5 498 10 50 13 –. – .( ) µµ Use the next lowest standard value (47 µH). Then pick RLIM from the curve. For IPEAK = 500mA, RLIM = 82Ω. Inductor Selection — Positive-to-Negative Converter Figure 7 shows hookup for positive-to-negative conver- sion. All of the output power must come from the inductor. In this case, PV V I L OUT D OUT =+ ()( ) || . ( ) 14 In this mode the switch is arranged in common collector or step-down mode. The switch drop can be modeled as a 0.75V source in series with a 0.65 Ω resistor. When the |
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