Datasheet LT1110 (Linear Technology) - 7

LT1110
O U U W U APPLICATI S I FOR ATIO Inductor Selection — General
Energy required by the inductor per cycle must be equal or greater than A DC-DC converter operates by storing energy as mag- P netic flux in an inductor core, and then switching this L ( ) 02 energy into the load. Since it is flux, not charge, that is fOSC stored, the output voltage can be higher, lower, or oppo- in order for the converter to regulate the output. site in polarity to the input voltage by choosing an appro- When the switch is closed, current in the inductor builds priate switching topology. To operate as an efficient en- according to ergy transfer element, the inductor must fulfill three re- quirements. First, the inductance must be low enough for V  R – t '  IN the inductor to store adequate energy under the worst I (t) = – 1 e L  L (0 )  3 R' case condition of minimum input voltage and switch ON   time. The inductance must also be high enough so maxi- where R' is the sum of the switch equivalent resistance mum current ratings of the LT1110 and inductor are not (0.8Ω typical at 25°C) and the inductor DC resistance. exceeded at the other worst case condition of maximum When the drop across the switch is small compared to VIN, input voltage and ON time. Additionally, the inductor core the simple lossless equation must be able to store the required flux; i.e., it must not saturate. At power levels generally encountered with V I t IN = t ( ) L ( ) 04 LT1110 based designs, small surface mount ferrite core L units with saturation current ratings in the 300mA to 1A can be used. These equations assume that at t = 0, range and DCR less than 0.4Ω (depending on application) inductor current is zero. This situation is called “discon- are adequate. Lastly, the inductor must have sufficiently tinuous mode operation” in switching regulator parlance. low DC resistance so excessive power is not lost as heat Setting “t” to the switch ON time from the LT1110 speci- in the windings. An additional consideration is Electro- fication table (typically 10µs) will yield IPEAK for a specific Magnetic Interference (EMI). Toroid and pot core type “L” and VIN. Once IPEAK is known, energy in the inductor inductors are recommended in applications where EMI at the end of the switch ON time can be calculated as must be kept to a minimum; for example, where there are sensitive analog circuitry or transducers nearby. Rod core E = 1 LI2 L 05 ( ) PEAK types are a less expensive choice where EMI is not a 2 problem. Minimum and maximum input voltage, output EL must be greater than PL/fOSC for the converter to deliver voltage and output current must be established before an the required power. For best efficiency IPEAK should be inductor can be selected. kept to 1A or less. Higher switch currents will cause excessive drop across the switch resulting in reduced
Inductor Selection — Step-Up Converter
efficiency. In general, switch current should be held to as In a step-up, or boost converter (Figure 4), power gener- low a value as possible in order to keep switch, diode and ated by the inductor makes up the difference between inductor losses at a minimum. input and output. Power required from the inductor is As an example, suppose 12V at 120mA is to be generated determined by from a 4.5V to 8V input. Recalling equation (01), P = V ( + V – V I )( ) (12 0 5. – 4 5. )(120 ) L OUT D IN MIN OUT ( ) 01 P V V V mA = mW L = + 960 . 06 ( ) where VD is the diode drop (0.5V for a 1N5818 Schottky). Energy required from the inductor is PL mW = 960 = 13 7 . J µ . 0 ( 7) f kHz OSC 70 7