An apparatus (200, 200a, 200b) for controlling a low voltage digital circuit with a voltage source having a wide range of voltage values, said apparatus comprising: an adjustable shunt regulator having an anode, a cathode a reference input; a resistor network (214, 216) for providing a reference voltage to the reference input of the adjustable shunt regulator, wherein the reference voltage is representative of a current through the resistor network (214, 216); characterized in that the apparatus (200, 200a, 200b) further comprises: a depletion-mode field effect transistor, FET, having a drain, a gate and a source, wherein the drain thereof is adapted to be coupled to the voltage source; an isolation circuit having an isolated input and an isolated output; wherein the isolated input of the isolation circuit is coupled between the source of the depletion-mode FET and the resistor network (214, 216), the cathode of the adjustable shunt regulator is coupled to the gate of the depletion-mode FET, the anode of the adjustable shunt regulator and the resistor network (214, 216) are configured to be coupled to a common node of the voltage source, the resistor network comprises a pair of series connected resistors (214, 216) coupled between an input return of the isolation circuit and the common node, the pair of series connected resistors (214, 216) forming a voltage divider having a junction therebetween coupled to the reference input of the adjustable shunt regulator, and the resistor network comprising a third resistor having a high resistance value, the third resistor connected between the gate of the depletion-mode FET and the input return of the isolation circuit; whereby the adjustable shunt regulator is configured to cause the depletion-mode FET to maintain a substantially constant current drawn from the voltage source over a wide range of input voltages therefrom. an adjustable shunt regulator having an anode, a cathode a reference input; a resistor network (214, 216) for providing a reference voltage to the reference input of the adjustable shunt regulator, wherein the reference voltage is representative of a current through the resistor network (214, 216); characterized in that the apparatus (200, 200a, 200b) further comprises: a depletion-mode field effect transistor, FET, having a drain, a gate and a source, wherein the drain thereof is adapted to be coupled to the voltage source; an isolation circuit having an isolated input and an isolated output; wherein the isolated input of the isolation circuit is coupled between the source of the depletion-mode FET and the resistor network (214, 216), the cathode of the adjustable shunt regulator is coupled to the gate of the depletion-mode FET, the anode of the adjustable shunt regulator and the resistor network (214, 216) are configured to be coupled to a common node of the voltage source, the resistor network comprises a pair of series connected resistors (214, 216) coupled between an input return of the isolation circuit and the common node, the pair of series connected resistors (214, 216) forming a voltage divider having a junction therebetween coupled to the reference input of the adjustable shunt regulator, and the resistor network comprising a third resistor having a high resistance value, the third resistor connected between the gate of the depletion-mode FET and the input return of the isolation circuit; whereby the adjustable shunt regulator is configured to cause the depletion-mode FET a depletion-mode field effect transistor, FET, having a drain, a gate and a source, wherein the drain thereof is adapted to be coupled to the voltage source; an isolation circuit having an isolated input and an isolated output; wherein the isolated input of the isolation circuit is coupled between the source of the depletion-mode FET and the resistor network (214, 216), the cathode of the adjustable shunt regulator is coupled to the gate of the depletion-mode FET, the anode of the adjustable shunt regulator and the resistor network (214, 216) are configured to be coupled to a common node of the voltage source, the resistor network comprises a pair of series connected resistors (214, 216) coupled between an input return of the isolation circuit and the common node, the pair of series connected resistors (214, 216) forming a voltage divider the isolated input of the isolation circuit is coupled between the source of the depletion-mode FET and the resistor network (214, 216), the cathode of the adjustable shunt regulator is coupled to the gate of the depletion-mode FET, the anode of the adjustable shunt regulator and the resistor network (214, 216) are configured to be coupled to a common node of the voltage source, the resistor network comprises a pair of series connected resistors (214, 216) coupled between an input return of the isolation circuit and the common node, the pair of series connected resistors (214, 216) forming a voltage divider having a junction therebetween coupled to the reference input of the adjustable shunt regulator, and the resistor network comprising a third resistor having a high resistance value, the third resistor connected between the gate of the depletion-mode FET and the input return of the isolation circuit; whereby the adjustable shunt regulator is configured to cause the depletion-mode FET