For systems with miniscule power budgets, implementing a low-power voltage reference involves many compromises. One design option is the low-voltage shunt reference. Available since the birth of the bandgap cell, this device traditionally operates with less current than a series reference. The shunt reference offers flexibility in its bias and application. Yet it can sink or source only modest currents. Using a series reference can provide improved sink/source capability, but this choice would incur the penalty of a tenfold increase in supply current.
This circuit combines the low power consumption of a shunt reference with series-reference performance (see the figure). As a result, it yields an ultra-low-power series reference with excellent line and load regulation. At the heart of this circuit is a 1-µA, 1.25-V shunt reference (U1). Bias current for this shunt reference comes di-rectly from the amplified reference voltage, virtually eliminating line-regulation error. Be-cause the op amp (U2) supplies the load current, load regulation is greatly improved compared to designs using resistively biased bandgap references.
A low supply current is the most important attribute of this op-amp/shunt-reference configuration. The total supply current for U1 and U2 is 2.4 µA, including the current in the feedback network. While this network sets the output to 2.0 V, it can easily be adjusted for other voltages.
The circuit sinks and sources current quite well. Line regulation is 90 dB from VIN = 2.2 V to 5.5 V. Load regulation (with VIN = 2.5 V) is 1 µV per 10 µA of output current. The total supply current is 2.4 µA, independent of the supply voltage. This performance greatly exceeds that of existing series references, whose minimum supply currents are 10 µA and above.
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