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LT1073CN8-5 Datasheet(PDF) 6 Page - Linear Technology |
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LT1073CN8-5 Datasheet(HTML) 6 Page - Linear Technology |
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6 / 16 page  LT1073 6 APPLICATIO S I FOR ATIO Measuring Input Current at Zero or Light Load Obtaining meaningful numbers for quiescent current and efficiency at low output current involves understanding how the LT1073 operates. At very low or zero load current, the device is idling for seconds at a time. When the output voltage falls enough to trip the comparator, the power switch comes on for a few cycles until the output voltage rises sufficiently to overcome the comparator hysteresis. When the power switch is on, inductor current builds up to hundreds of milliamperes. Ordinary digital multimeters are not capable of measuring average current because of bandwidth and dynamic range limitations. A different approach is required to measure the 100 µA off-state and 500mA on-state currents of the circuit. Table 1. Component Selection for Step-Up Converters INPUT BATTERY OUTPUT OUTPUT INDUCTOR INDUCTOR CAPACITOR VOLTAGE (V) TYPE VOLTAGE (V) CURRENT (MIN) VALUE ( µH) PART NUMBER VALUE ( µF) NOTES 1.55-1.25 Single Alkaline 3 60mA 82 G GA10-822K, CB 7300-12 150 1.30-1.05 Single Ni-Cad 3 20mA 180 G GA10-183K, CB 7300-16 47 1.55-1.25 Single Alkaline 5 30mA 82 G GA10-822K, CB 7300-12 100 1.30-1.05 Single Ni-Cad 5 10mA 180 G GA10-183K, CB 7300-16 22 3.1-2.1 Two Alkaline 5 80mA 120 G GA10-123K, CB 7300-14 470 * 3.1-2.1 Two Alkaline 5 25mA 470 G GA10-473K, CB 7300-21 150 * 3.3-2.5 Lithium 5 100mA 150 G GA40-153K, CB 6860-15 470 * 3.1-2.1 Two Alkaline 12 25mA 120 G GA10-123K, CB 7300-14 220 3.1-2.1 Two Alkaline 12 5mA 470 G GA10-473K, CB 7300-21 100 3.3-2.5 Lithium 12 30mA 150 G GA10-153K, CB 7300-15 220 4.5-5.5 TTL Supply 12 90mA 220 G GA40-223K, CB 6860-17 470 * 4.5-5.5 TTL Supply 12 22mA 1000 G GA10-104K, CB 7300-25 100 * 4.5-5.5 TTL Supply 24 35mA 220 G GA40-223K, CB 6860-17 150 * G = GOWANDA CB = CADDELL-BURNS *Add 68 Ω from ILIM to VIN LT1073. The circuit must be “booted” by shorting V2 to VSET. After the LT1073 output voltage has settled, discon- nect the short. Input voltage is V2 and average input current can be calculated by this formula: I VV IN = Ω 21 100 – 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. 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 appro- priate switching topology. 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 LT1073 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 LT1073-based designs, small axial-lead units with Figure 1. Test Circuit Measures No-Load Quiescent Current of LT1073 Converter – + 1073 F01 LTC1050 LT1073 CIRCUIT + 12V 1M Ω 100 Ω VSET 1000 µF 1 µF* *NONPOLARIZED V1 V2 Quiescent current can be accurately measured using the circuit in Figure 1. VSET is set to the input voltage of the |
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