Datasheet LT1110 (Linear Technology) - 8

LT1110
O U U W U APPLICATI S I FOR ATIO
Picking an inductor value of 47µH with 0.2Ω DCR results VOUT = output voltage in a peak switch current of VIN = minimum input voltage 4 5 . V  −1 0 . W•10 s m  VSW is actually a function of switch current which is in turn I = 1− e 47m  = m 862 A PEAK H . 08 ( ) a function of V 1 0 . W IN, L, time and VOUT. To simplify, 1.5V can   be used for VSW as a very conservative value. Substituting IPEAK into Equation 05 results in Once IPEAK is known, inductor value can be derived from 1 E 47 H 0 862 . A 2 µ 17 5 . J V – V – V IN MIN SW OUT L = ( )( ) = µ . 0 ( 9) = • 2 L t ( ) 11 I ON PEAK Since 17.5µJ > 13.7µJ, the 47µH inductor will work. This where t trial-and-error approach can be used to select the opti- ON = switch ON time (10µs). mum inductor. Keep in mind the switch current maximum Next, the current limit resistor RLIM is selected to give rating of 1.5A. If the calculated peak current exceeds this, IPEAK from the RLIM Step-Down Mode curve. The addition an external power transistor can be used. of this resistor keeps maximum switch current constant as the input voltage is increased. A resistor can be added in series with the ILIM pin to invoke switch current limit. The resistor should be picked such As an example, suppose 5V at 250mA is to be generated that the calculated IPEAK at minimum VIN is equal to the from a 9V to 18V input. Recalling Equation (10), Maximum Switch Current (from Typical Performance Characteristic curves). Then, as V 2 mA (250 ) IN increases, switch  5 + 0 5 .  I mA 498 . 12 ( ) current is held constant, resulting in increasing efficiency. PEAK =   = 0 6 . 9 9 – 1 5 . + 0 5 . 
Inductor Selection — Step-Down Converter
Next, inductor value is calculated using Equation (11) The step-down case (Figure 5) differs from the step-up in = 9 – 1 5 . – 5 •10µ = µ that the inductor current flows through the load during L s 50 H. 13 ( ) 498mA both the charge and discharge periods of the inductor. Current through the switch should be limited to ~800mA Use the next lowest standard value (47µH). in this mode. Higher current can be obtained by using an Then pick RLIM from the curve. For IPEAK = 500mA, external switch (see Figure 6). The ILIM pin is the key to RLIM = 82Ω. successful operation over varying inputs.
Inductor Selection — Positive-to-Negative Converter
After establishing output voltage, output current and input voltage range, peak switch current can be calculated by the Figure 7 shows hookup for positive-to-negative conver- formula sion. All of the output power must come from the inductor. In this case, 2I  V V  I OUT OUT D PEAK = +   10 ( ) DC V – V  + V (| | )( ) IN SW D  P = V + V I L OUT D OUT . ( ) 14 where DC = duty cycle (0.69) In this mode the switch is arranged in common collector VSW = switch drop in step-down mode or step-down mode. The switch drop can be modeled as V a 0.75V source in series with a 0.65Ω resistor. When the D = diode drop (0.5V for a 1N5818) IOUT = output current 8