Datasheet LT1073 (Analog Devices) - 7

LT1073
APPLICATIONS INFORMATION
saturation current ratings in the 300mA to 1A range (de- 1200 pending on application) are adequate. Lastly, the inductor must have sufficiently low DC resistance so that excessive 1000 power is not lost as heat in the windings. An additional 800 consideration is electro-magnetic interference (EMI). (mA) Toroid and pot core type inductors are recommended in 600 applications where EMI must be kept to a minimum; for I SWITCH 400 example, where there are sensitive analog circuitry or transducers nearby. Rod core types are a less expensive 200 choice where EMI is not a problem. 0 0 1 2 3 4 5 Specifying a proper inductor for an application requires VIN (V) first establishing minimum and maximum input voltage, 1073 F02 output voltage and output current. In a step-up converter,
Figure 2. Maximum Switch Current vs Input Voltage
the inductive events add to the input voltage to produce the output voltage. Power required from the inductor is
Capacitor Selection
determined by: Selecting the right output capacitor is almost as important P as selecting the right inductor. A poor choice for a filter L = (VOUT + VD – VIN)(IOUT) capacitor can result in poor efficiency and/or high output where VD is the diode drop (0.5V for a 1N5818 Schottky). ripple. Ordinary aluminum electrolytics, while inexpensive Maximum power in the inductor is and readily available, may have unacceptably poor equiva- P lent series resistance (ESR) and ESL (inductance). There L= EL • fOSC are low-ESR aluminum capacitors on the market specifically designed for switch-mode DC/DC converters which work = 1L i 2 PEAK • fOSC much better than general purpose units. Tantalum capaci- 2 tors provide still better performance at more expense. We where recommend OS-CON capacitors from Sanyo Corporation (San Diego, CA). These units are physically quite small and    –Rt  i ON have extremely low ESR. To illustrate, Figures 3, 4, and 5 PEAK = VIN   1– e   R   L  show the output voltage of an LT1073 based converter with three 100µF capacitors. The peak switch current is 500mA R = Switch equivalent resistance (1Ω maximum) in all cases. Figure 3 shows a Sprague 501D aluminum added to the DC resistance of the inductor and tON = ON capacitor. V time of the switch. OUT jumps by over 150mV when the switch turns off, followed by a drop in voltage as the inductor At maximum VIN and ON time, iPEAK should not be al- dumps into the capacitor. This works out to be an ESR of lowed to exceed the maximum switch current shown in over 300mΩ. Figure 4 shows the same circuit, but with a Figure 2. Some input/output voltage combinations will Sprague 150D tantalum capacitor replacing the aluminum cause continuous1 mode operation. In these cases a unit. Output jump is now about 30mV, corresponding to resistor is needed between ILIM (Pin 1) and VIN (Pin 2) an ESR of 60mΩ. Figure 5 shows the circuit with an OS- to keep switch current under control. See the “Using the CON unit. ESR is now only 30mΩ. ILIM Pin” section for details. 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 cur-
NOTE 1:
i.e., inductor current does not go to zero when the switch is off. rent in the 5µA to 10µA range. If the load is also in the Rev B 7 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagrams Operation Applications Information Typical Applications Package Description Typical Application Related Parts