The electronic circuits in contemporary hearing aids are usually powered by batteries, e.g. rechargeable batteries of the lithium-ion or lithium-polymer variety, or non-rechargeable zinc-air batteries. A typical hearing aid circuit operates at a voltage of about one volt and draws a current of between 1 mA and 10 mA. A hearing aid user would want to change the batteries in his or her hearing aids as rarely as possible, e.g. one to three times a week. In order to prolong battery life, hearing aid designers therefore strive to reduce current consumption as much as possible when devising new hearing aids. The supply voltage in a hearing aid has to be maintained within narrow limits in order to ensure stable and proper operation of the hearing aid signal processing circuit, while the current consumption is kept at a minimum.
Prior art hearing aids are powered by a switching or linear voltage regulator providing a stable and accurate voltage to the electronic circuit in the hearing aid. In hearing aids comprising radio receivers, linear voltage regulators are generally preferred for power supplies over switching voltage regulators because they emit much less high-frequency electromagnetic noise. In this context, a linear voltage regulator is considered as an electronic circuit comprising a voltage reference, an operational amplifier, an amplifying element such as a transistor, and a voltage divider circuit. The voltage regulator is powered by a voltage source such as a battery, and a biasing voltage generator is providing a proper operating point for the operational amplifier.
Proper and stable operation of the signal processing circuit in a contemporary, digital hearing aid is highly dependent on a stable and reliable power supply. A deviation of more than 5% from the nominal supply voltage may easily present a problem to e.g. the digital-to-analog converters present in the hearing aid, since the conversion of an input voltage to a digital number may go astray if e.g. the internal voltage reference of the analog-to-digital converter or the input voltage deviates as a result of an unstable supply voltage. An unstable supply voltage may also introduce noise and distortion into the analog parts of the signal processor due to changes in the operating points of the amplifying semiconductor elements. Even worse, it may cause the program execution of the digital signal processor to crash or fail. In order for the power supply to be stable within 2-5% of the nominal supply voltage, a very stable voltage reference circuit must be provided.
Dual voltage regulator circuits are known from the prior art, e.g. from the article “Dual-voltage regulator meets USB-power needs”, by Wayne Rewinkel of National Semiconductor, published in EDN online magazine, August 2004. The dual voltage regulator disclosed by Rewinkel does not provide an output voltage to a common output node, and does not teach a handover procedure between the two regulators.