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LT1073CS8-12 Datasheet(PDF) 7 Page - Linear Technology |
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LT1073CS8-12 Datasheet(HTML) 7 Page - Linear Technology |
7 / 16 page LT1073 7 saturation current ratings in the 300mA to 1A range (depending on application) are adequate. Lastly, the in- ductor must have sufficiently low DC resistance so that 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 recom- mended 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) where VD is the diode drop (0.5V for a 1N5818 Schottky). Maximum power in the inductor is PE f Li f L L OSC PEAK OSC = = • • 1 2 2 where i V R e Rt L PEAK IN ON = 1– – R = Switch equivalent resistance (1 Ω maximum) added to the DC resistance of the inductor and tON = ON time of the switch. At maximum VIN and ON time, iPEAK should not be allowed to exceed the maximum switch current shown in Figure 2. Some input/output voltage combinations will cause con- tinuous1 mode operation. In these cases a resistor is needed between ILIM (Pin 1) and VIN (Pin 2) to keep switch current under control. See the “Using the ILIM Pin” section for details. Capacitor Selection Selecting the right output capacitor is almost as important as selecting the right inductor. A poor choice for a filter capacitor can result in poor efficiency and/or high output ripple. Ordinary aluminum electrolytics, while inexpensive and readily available, may have unacceptably poor equiva- lent series resistance (ESR) and ESL (inductance). There are low-ESR aluminum capacitors on the market specifi- cally designed for switch-mode DC/DC converters which work much better than general purpose units. Tantalum capacitors provide still better performance at more ex- pense. We recommend OS-CON capacitors from Sanyo Corporation (San Diego, CA). These units are physically quite small and have extremely low ESR. To illustrate, Figures 3, 4, and 5 show the output voltage of an LT1073 based converter with three 100 µF capacitors. The peak switch current is 500mA in all cases. Figure 3 shows a Sprague 501D aluminum capacitor. VOUT jumps by over 150mV when the switch turns off, followed by a drop in voltage as the inductor dumps into the capacitor. This works out to be an ESR of over 300m Ω.Figure4showsthe same circuit, but with a Sprague 150D tantalum capacitor replacing the aluminum unit. Output jump is now about 30mV, corresponding to an ESR of 60m Ω. Figure 5 shows the circuit with an OS-CON unit. ESR is now only 30m Ω. In very low power applications where every microampere is important, leakage current of the capacitor must be considered. The OS-CON units do have leakage current in the 5 µA to 10µA range. If the load is also in the APPLICATIO S I FOR ATIO VIN (V) 0 1200 1000 800 600 400 200 0 123 4 1073 F02 5 Figure 2. Maximum Switch Current vs Input Voltage NOTE 1: i.e., inductor current does not go to zero when the switch is off. |
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