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5-to-1.8V converter works without magnetics

Dan Christman, Maxim Integrated Products, Sunnyvale, CA -- EDN, 4/13/2000

To derive 1.8V from 5V, you might think of using a switch-mode regulator. Switchers are highly efficient but also complicated and expensive. Linear regulators, too, are out of the question unless your design can tolerate 40% efficiencies. The circuit in Figure 1, on the other hand, is more than 70% efficient (Figure 2a), sources as much as 100 mA, costs less than a switch-mode regulator, and requires less space.

IC₁ is a CMOS charge-pump voltage converter that the circuit configures as a voltage inverter. With its output grounded and 5V at its V+ pin, IC₁ generates V+/2, or approximately 2.5V at Pin 3. This nominal 2.5V output, which sags as the device sources current, drives linear-regulator IC₂, which regulates the 2.5V input to 1.8V. IC₂ is can source 100 mA before its sagging input voltage falls below the dropout level (Figure 2b). Using larger values for C₁ and C₂ enables IC₁ to maintain its output voltage with heavier load currents. (DI #2511)

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