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.
The function of current limiting circuitry is to prevent damage to the IC when an overload is placed on the output of the regulator (the load impedance is too low). Without current limiting, the regulator would source excessive load current and destroy the pass transistor inside the part.
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