Low-Drop 5V Regulator


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.

Circuit diagram:

Low Drop 5V Regulator Circuit

Low-Drop 5V Regulator Circuit Diagram

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.


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