Low Drop 5V Regulator
Monday, August 5, 2013
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A 4-cell pack is a convenient, popular battery size. Alkaline manganese batteries are sold in retail stores in packs of four, which usually provide sufficient energy to keep battery replacement frequency at a reasonable level. Generating 5 V from four batteries is, however, a bit tricky. A fresh set of four batteries has a terminal voltage of 6.4 V, but at the end of their life, this voltage is down to 3.2 V. Therefore, the voltage needs to be stepped up or down, depending on the state of the batteries. A flyback topology with a costly, custom designed transformer could be used, but the circuit in the diagram gets around the problem by using a flying capacitor together with a second inductor.
The circuit also isolates the input from the output, allowing the output to go to 0 V during shutdown. The circuit can be divided conceptually into boost and buck sections. Inductor L1 and switch IC1 comprise the boost or step-up section, and inductor L2, diode D1 and capacitor C3 form the buck or step-down section. Capacitor C2 is charged to the input voltage, Vin, and acts as a level shift between the two sections. The switch toggles between ground and Vin+Vout , while the junction of L2, C2 and D1 toggles between –Vin and Vout +Vd1. Efficiency is directly related to the quality of the capacitors and inductors used.
Better quality capacitors are more expensive. Better quality inductors need not cost more, but normally take up more space. The Sanyo capacitors used in the prototype (C1–C3) specify a maximum ESR (effective series resistance) of 0.045 ½ and a maximum ripple current rating of 2.1 A. The inductors used specify a maximum DCR (direct current resistance) of 0.058 ½. Worst-case r.m.s. current through capacitor C2 occurs at minimum input voltage, that is, 400 mA at full load with an input voltage of 3 V.
The circuit also isolates the input from the output, allowing the output to go to 0 V during shutdown. The circuit can be divided conceptually into boost and buck sections. Inductor L1 and switch IC1 comprise the boost or step-up section, and inductor L2, diode D1 and capacitor C3 form the buck or step-down section. Capacitor C2 is charged to the input voltage, Vin, and acts as a level shift between the two sections. The switch toggles between ground and Vin+Vout , while the junction of L2, C2 and D1 toggles between –Vin and Vout +Vd1. Efficiency is directly related to the quality of the capacitors and inductors used.
Better quality capacitors are more expensive. Better quality inductors need not cost more, but normally take up more space. The Sanyo capacitors used in the prototype (C1–C3) specify a maximum ESR (effective series resistance) of 0.045 ½ and a maximum ripple current rating of 2.1 A. The inductors used specify a maximum DCR (direct current resistance) of 0.058 ½. Worst-case r.m.s. current through capacitor C2 occurs at minimum input voltage, that is, 400 mA at full load with an input voltage of 3 V.
MIC2941 regulator has guaranteed 1.25A output
Parts:
For the diode radio (any germanium diode is suitable in this design) the key to success is correct impedance matching so that none of the received signal energy is lost. The antenna coil on the 10 mm diameter by 100 mm long ferrite rod is made up of 60 turns with a tap point at every 10 turns; this is suitable for medium wave reception. If a long external aerial is used it should be connected to a lower tap point to reduce its damping effect on the circuit. You can experiment with all the available tapping points to find the best reception. With such a simple radio design, the external aerial will have a big influence on its performance.
