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The Linear-Switcher Regulator

A.K.A the 7800-Series Switching Regulator

IC switching regulator designs trace their history to the introduction of linear regulators in the late 1960s. National’s AN1 from 1967 included a switch-mode application of the LM100 (Widlar included a slightly simpler design in AN2). A switcher based on Fairchild’s uA723 appears in a 1968 app note. The LM109 (the first monolithic 3-terminal regulator) was introduced two years later but it apparently wasn’t until Fairchild released the uA78XX series that a 3-terminal switcher design was published in their 1973 datasheet.

Widlar’s LM100 Switcher from National’s AN1
Simpler LM100 Switcher from National’s AN2
uA723 Switcher from a 1968 Fairchild App Note
78XX Switcher from a 1973 Fairchild Datasheet
78XX Switcher Susanne Nell EDN 2002

I built the circuit below, which is essentially Nell’s design. It worked surprisingly well with the 7805, 78M05, 7807, 7810, and 7812 regulators I had on hand. Efficiency was similar across all parts. At 500mA load, efficiency was 55%-75%, increasing as Vin-Vout decreased. Output ripple was generally not good: 100mV-800mV, greater with larger Vin-Vout.

The name for this design is “hysteretic buck regulator” and its switching frequency depends on line, load, temperature, and pretty-much-everything-else. Modern switching regulator ICs are typically not hysteretic; they use PWM control on a fixed switching frequency. Such modern ICs (some of which date back to the 80s!) provide a more refined and controlled solution, and many require only 3 external parts.