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LT1108 Datasheet(PDF) 8 Page - Linear Technology |
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LT1108 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LT1108 S APPLICATI I FOR ATIO 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 switch closes, current in the inductor builds according to It V R e L L Rt L () = ' –( ) –' 115 where: R' = 0.65 Ω + DCRL VL = VIN – 0.75V As an example, suppose – 5V at 100mA is to be generated from a 4.5V to 5.5V input. Recalling Equation (14), PL = (–5V+ 0.5V)(100mA) = 550mW. (16) Energy required from the inductor is P f mW kHz J L OSC == 550 19 28 9 17 .( ) µ Picking an inductor value of 220 µH with 0.3Ω DCR results in a peak switch current of I VV e mA PEAK s H = () + () = × 45 075 65 0 3 1 568 095 36 220 .– . .. – –. 0 ΩΩ Ωµ µ Substituting IPEAK into Equation (04) results in EH A J L = ()( ) = 1 2 220 0 568 35 5 19 2 µµ .. ( ) Since 35.5 µJ > 28.9µJ, the 220µH inductor will work. Finally, RLIM should be selected by looking at the Switch Current vs RLIM curve. In this example, RLIM = 150Ω. STEP-UP (BOOST MODE) OPERATION A step-up DC/DC converter delivers an output voltage higher than the input voltage. Step-up converters are not short-circuit protected since there is a DC path from input to output. The usual step-up configuration for the LT1108 is shown in Figure 1. The LT1108 first pulls SW1 low causing VIN – VCESAT to appear across L1. A current then builds up in L1. At the end of the switch-ON time the current in L1 is I V L t PEAK IN ON = *( ) 20 (18) *Expression 20 neglects the effect of switch and coil resistance. This is taken into account in the "Inductor Selection" section. Immediately after switch turn-off, the SW1 voltage pin starts to rise because current cannot instantaneously stop flowing in L1. When the voltage reaches VOUT + VD, the inductor current flows through D1 into C1, increasing VOUT. This action is repeated as needed by the LT1108 to keep VFB at the internal reference voltage of 1.245V. R1 and R2 set the output voltage according to the formula V R R V OUT =+ () 1 2 1 1 245 21 .( ) STEP-DOWN (BUCK MODE) OPERATION A step-down DC/DC converter converts a higher voltage to a lower voltage. The usual hookup for an LT1108 based step-down converter is shown in Figure 2. When the switch turns on, SW2 pulls up to VIN – VSW. This puts a voltage across L1 equal to VIN – VSW – VOUT, causing a current to build up in L1. At the end of the switch- ON time, the current in L1 is equal to Figure 1. Step-Up Mode Hookup L1 LT1108 • F01 GND SW2 SW1 LIM I IN V D1 R3 LT1108 + VOUT R2 R1 C1 VIN FB |
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