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LT1109CS8 Datasheet(PDF) 5 Page - Linear Technology |
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LT1109CS8 Datasheet(HTML) 5 Page - Linear Technology |
5 / 8 page LT1109 5 LT1109 S8 A D 8 ATIO OPER reduced by using the 8-pin version since the quiescent current flows from a lower voltage source. The SHUT- DOWN pin disables the oscillator when taken to a logic “0.” If left floating or tied high, the converter operates nor- mally. With SHUTDOWN low, quiescent current remains at 320 µA. The 8-pin versions of the LT1109 have separate pins for VIN and SENSE or FB and also have a SHUTDOWN pin. Separating the device VIN pin from the SENSE pin allows the device to be powered from the (lower) input voltage rather than the (higher) output voltage. Although quies- cent current remains constant, quiescent power will be S APPLICATI I FOR ATIO Inductor Selection A DC/DC converter operates by storing energy as mag- netic flux in an inductor core, and then switching this energy into the load. To operate as an efficient energy transfer element, the inductor must fulfill three require- ments. 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 that maxi- mum current ratings of the LT1109 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 LT1109 designs, small ferrite surface-mount inductors will function well. Lastly, the inductor must have suffi- ciently low DC resistance so that excessive power is not lost as heat in the windings. Look for DCR values in the inductors’ specification tables; values under 0.5 Ωwillgive best efficiency. 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. Specifying a proper inductor for an application requires first establishing minimum and maximum input voltage, output voltage, and output current. In a step-up converter, the inductive events add to the input voltage to produce the output voltage. Power required from the inductor is deter- mined by PL = (VOUT + VD – VIN) (IOUT) (01) P F L OSC 02 () It V L t L IN () = () 04 ELI L PEAK = () 1 2 05 2 It V R e L IN Rt L () = () ' – –' 103 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 in order for the converter to regulate the output. When the switch is closed, current in the inductor builds according to where R' is the sum of the switch equivalent resistance (0.8 typical at 25 °C) and the inductor DC resistance. When the drop across the switch is small compared to VIN, the simple lossless equation 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 LT1109 speci- fication table (typically 4.2 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 EL must be greater than PL/FOSC for the converter to deliver the required power. For best efficiency IPEAK should be |
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