In shunt regulation a resistor is typically placed in series with the load and the unregulated voltage. The resistor is small enough so that the load could always receive somewhat more than the maximum current it would ever need. The shunt regulator is placed across the load and conducts excess current around the load such that the voltage across the load remains a constant as the load draws the actual current at any given time. A common shunt regulator is a Zener diode which is an example of an open loop system. Feedback control can also be used to drive the current through the control element (a transistor) across the load. Shunt regulators are generally only used for low power applications because they can be very inefficient. However, shunt regulators have an inherent fault current limiting feature and also can regulate even if the load is forcing current into the regulator rather than drawing current from it. Shunt regulators also have an interesting feature that the input current is constant – independent of load current (except if a load fault occurs – but that is a special case not in normal operation). Thus shunt regulators are very good at isolating a load with rapid and large current fluctuations.
A variation of the Standard regulator is the quasi-LDO, which uses an NPN and PNP transistor as the pass device. The dropout voltage for a quasi-LDO delivering rated current is usually specified at about 1.5V(max). The actual dropout voltage is temperature and load current dependent, but could never be expected to go lower than about 0.9V (25°C) at even the lightest load. The dropout voltage for the quasi-LDO is higher than the LDO, but lower than the Standard regulator.
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