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LT1111 Datasheet(PDF) 7 Page - Linear Technology |
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LT1111 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  7 LT1111 1111fd Inductor Selection — General A DC/DC converter operates by storing energy as mag- netic flux in an inductor core, and then switching this energy into the load. Since it is flux, not charge, that is stored, the output voltage can be higher, lower, or oppo- site in polarity to the input voltage by choosing an appropriate switching topology. To operate as an efficient energy transfer element, the inductor must fulfill three requirements. First, the inductance must be low enough for the inductor to store adequate energy under the worst case condition of minimum input voltage and switch-on time. The inductance must also be high enough so maxi- mum current ratings of the LT1111 and inductor are not exceeded at the other worst case condition of maximum input voltage and ON time. Additionally, the inductor core must be able to store the required flux; i.e., it must not saturate. At power levels generally encountered with LT1111 based designs, small surface mount ferrite core units with saturation current ratings in the 300mA to 1A range and DCR less than 0.4 Ω (depending on application) are adequate. Lastly, the inductor must have sufficiently low DC resistance so excessive power is not lost as heat in the windings. An additional consideration is Electro- Magnetic Interference (EMI). Toroid and pot core type inductors are recommended in applications where EMI must be kept to a minimum; for example, where there are sensitive analog circuitry or transducers nearby. Rod core types are a less expensive choice where EMI is not a problem. Minimum and maximum input voltage, output voltage and output current must be established before an inductor can be selected. Inductor Selection — Step-Up Converter In a step-up, or boost converter (Figure 4), power gener- ated by the inductor makes up the difference between input and output. Power required from the inductor is determined by: PV V V I L OUT D IN MIN OUT =+ ()( ) –( ) 1 where VD is the diode drop (0.5V for a 1N5818 Schottky). Energy required by the inductor per cycle must be equal or greater than: S APPLICATI I FOR ATIO P f L OSC /( ) 2 in order for the converter to regulate the output. When the switch is closed, current in the inductor builds according to: It V R e L IN Rt L () – ( ) – = ′ ′ 13 where R ′ is the sum of the switch equivalent resistance (0.8 Ω typical at 25°C) and the inductor DC resistance. When the drop across the switch is small compared to VIN, the simple lossless equation: It V L t L IN ()= () 4 can be used. These equations assume that at t = 0, inductor current is zero. This situation is called “discon- tinuous mode operation” in switching regulator parlance. Setting “t” to the switch-on time from the LT1111 speci- fication table (typically 7 µs) will yield IPEAK for a specific “L” and VIN. Once IPEAK is known, energy in the inductor at the end of the switch-on time can be calculated as: ELI L PEAK = 1 2 5 2 () EL must be greater than PL/fOSC for the converter to deliver the required power. For best efficiency IPEAK should be kept to 1A or less. Higher switch currents will cause excessive drop across the switch resulting in reduced efficiency. In general, switch current should be held to as low a value as possible in order to keep switch, diode and inductor losses at a minimum. As an example, suppose 12V at 60mA is to be generated from a 4.5V to 8V input. Recalling equation (1), P V V V mA mW L =+ ()( )= 12 0 5 4 5 60 480 6 .– . ( ) Energy required from the inductor is P f mW kHz J L OSC == 480 72 67 7 .( ) µ |
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Similar Description - LT1111_15 |
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