low drop out regulator

That answers my question. Thanks John.

John Fields wrote:

drop

"linear

"LDO"

With "normal" three-terminal voltage regulators, you need the input voltage to be 2 volts or more above the regulated voltage - otherwise the regulator doesn't have room to work correctly, and the output voltage will drop.

A low dropout regulator will continue to operate with as little as 0.5 volts (or so) difference between input and output.

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Peter Bennett VE7CEI 
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It _is_ a linear voltage regulator, the difference being that an "LDO"
regulator uses a PNP or a PNP-like pass transistor in order to
minimize the input-to output differential with low input voltages.

Low drop out regulators are a variant of linear regulators. Most linears are based on an emitter follower output stage, and usually an emitter follower driving a bigger emitter follower. This implies a significant voltage across the regulator to make it function (usually

1.5 to 2 volts). If the pass transistor is connected as a common emitter (output on the collector) the device may operate below one diode drop (less than .6 volts) across the regulator. This configuration is harder to stabilize against oscillations under varied conditions than the emitter follower configuration is. The base current for the pass transistor must also go through the reference pin of the regulator, which makes the current in that pin related to the load current. The follower configuration has a more stable reference pin current, making it easier to connect that kind of regulator in an adjustable output voltage arrangement.

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John Popelish