A DIGITAL HEARING AID WITH A VOLTAGE CONVERTER

Abstract: In a digital hearing aid including one or more integrated circuit signal processing parts, to which operation voltages are supplied by a power source including a standard hearing aid battery, at least one of said integrated circuit signal processing parts is designed to operate at a reduced unstabilized operation voltage substantially below a nominal voltage of the battery and a switched step-down voltage converter is connected between the power source and such signal processing parts for lowering the battery voltage to provide the reduced operation voltage.

Description: A digital hearing aid with a voltage converter The present invention relates to a digital hearing aid comprising a microphone, an output transducer, a digital signal processor interconnected between the microphone and the output transducer and including one or more integrated circuit signal processing parts, and a power source including a standard hearing aid battery for the supply of operation voltage for each of said signal processing parts . In its fresh condition a normal hearing aid battery supplies a voltage of about 1.3 V and during its active life the battery can supply current sufficient for the operation of the hearing aid down to a voltage of about. 1 V, below which the power supplying capacity of the battery drops rapidly. In prior art hearing aid technology it is well known, e.g. from EP-A-0 335 542, US-A-4 , 539, 440 and US-A-5, 581, 455 to provide operation voltages higher than the nominal battery voltage for certain processing circuits or components, e.g. EEPROM memories and microphone circuits, by means of voltage step-up converters, mostly in the form of switched capacitor networks designed e.g. as socalled charge pump voltage multipliers. Further examples of use of voltage regulators in hearing aids have been disclosed e.g. in DE-A-27 38 339, DE-C-31 34 888, DE-A-197 02 151 and O-A- 96/03848. Outside the hearing aid field a voltage dropping circuit in MOSFET technology with reduced power consumption has been disclosed in US-A- 4,205,369. Whereas in conventional hearing aid technology the major power supply requirement has been to provide operation voltages sufficiently high for the operation of signal processing circuits and reduction from the nominal battery voltage has only been used for voltage stabilization or provision of reference voltages, further lowering of the operation voltage has been considered inconvenient, since it would result in loss of processing speed. In certain parts of digital hearing aids such as a D/D output converter, which is responsible for a main part of the power consumption, reduction of the operation voltage would only result in a current increase and provide no saving in power consumption for the same output power from the D/D converter . Moreover, in small size hearing aids with a low voltage drop and a current drain of a few mA or even a fraction of mA only, power saving by reduction of the operation voltage has been considered outside interest due to the complex circuitry of a low loss stabilized series voltage regulator. The invention is based on the recognition of the fact that, as long as the operation voltage is kept above a defined minimum voltage, some integrated circuit signal processing parts of a digital hearing aid, like e.g. digital filters, are less sensitive to variations in the operation voltage in the sense that such variations would not result in any significant change of performance. It is therefore the object of the invention to provide a digital hearing aid having a longer active battery life and a reduced power consumption. According to the invention this and other objects are accomplished in that at least one of said integrated circuit signal processing parts is designed to operate at a reduced operation voltage substan- tially below a nominal voltage of said battery and that a switched step-down voltage converter is connected between the power source and said at least one signal processing part for lowering the battery voltage to provide said reduced operation voltage. By lowering of the operation voltage requirement for parts of the integrated signal processing circuits the total current drain and power consumption of the hearing aid is reduced. In particular, this brings substantial benefits in terms of power consumption, when the digital signal processing is operated by large hardware programmed programs, which would otherwise result in significant power consumption. Preferably, the digital signal processing parts having reduced operation voltage requirements will be designed in MOS or CMOS technology using transistors having a low operating voltage, e.g. a low threshold or pinch-off voltage, compared to bipolar processing circuits as normally used in hearing aids. Typically, such signal processing parts will comprise circuits that are not stressed with respect to processing speed or output power demand, such as digital filter circuits, whereas more stressed circuits such as an output D/D converter or output amplifier may still be supplied with a higher operation voltage. By suitable design of such signal processing blocks, which are stressed in processing speed involving e.g. a split-up in more parallel or serial processing blocks, the requirements to processing speed and consequently the operation voltage requirement may be lowered even for such circuit blocks . Thus, in a preferred embodiment of the hearing aid according to the invention one or more signal processing parts are implemented as parallel signal processing blocks each operating at a reduced operation voltage. The reduced operation voltage for the signal processing parts in question would preferably be equal to or below 0.8 V, e.g. in a voltage range of half the nominal battery voltage such as 0.7 down to 0.4 V or preferably 0.65 down to 0.5 V. In a preferred embodiment the switched step-down voltage converter providing the reduced operation voltage or voltages would be a capacitive charge pump converter, which may advantageously be designed to deliver two or more output voltages. However, alternatively also a switched inductor type converter could be envisaged. In the following the invention will be explained in further detail with reference to the accompanying drawings, of which fig. 1 is a schematical block diagram of an embodiment of a digital hearing according to the invention, and fig. 2 shows a first configuration of a switched capacitor voltage step-down converter of the charge pump type for use in the hearing aid shown n fig. 1, figs. 3 and 4 are simplified diagrams illustrating charge situations in the converter configuration in fig. 2, fig. 5 shows a second configuration of a switched capacitor voltage step-down converter of the charge pump type for use in the hearing aid shown in fig. 1, and figs. 6 and 7 are simplified diagrams illustrating charge situations in the converter configuration in fig. 2. The hearing aid schematically illustrated in fig. 1 comprises electric circuits 1 interconnected between a microphone 2 and an output transducer or receiver 3. The electric circuits 1 include a signal processing part 5, a control part 6 and a power supply part 7. In a digital hearing aid the signal processing parts 5 will at least comprise an A/D converter for conversion of the analog signal from the microphone 2 into digital form, digital signal processing circuits including filters and amplifiers and an output converter supplying to the output transducer 3 a digital or analog output signal compensating for the users hearing impairment. The switched capacitor voltage step down converter of the charge pump type illustrated in fig. 2 is of a type generally known from US-A-4 , 205, 369 and comprises in series connection with a voltage source DC such as a hearing aid battery supplying a nominal voltage U cc of about 1.3 V a converter configuration supplying an output voltage xU out which is about half the nominal battery voltage. The converter circuit comprises a pair of transistors Tl and T2 shown as p- and n-type MOSFET transistors, respectively, which are controlled by a control voltage v and connected with switch circuits SI and S2, respectively, which may each be implemented as a pair of n- and p-type MOSFET transistors, respectively, controlled by opposite clock phases. Transistors Tl and T2 and switch circuits SI and S2 control charging and discharging of two capacitors C f and C 3 as follows. When the control voltage v is non-active or "low", transistor Tl is on and transistor T2 off and switch circuit SI is inactive and switch circuit S2 active, so that capacitors C f and C s are charged in series as shown in the equivalent diagram in fig. 3. When the control voltage v is active or "high", transistor Tl is off and transistor T2 on and switch circuits SI is active and switch circuit S2 inactive, so that capacitors C f and C s are discharged in parallel to the load as shown in the equivalent diagram in fig. 4. In the diagrams in figs. 3 and 4 a load is represented by a resistor Rl . If capacitors C f and C s are of equal capacitance the battery voltage U cc is divided into a half and the reduced supply voltage xU out will be about the half of the battery voltage. In the configuration shown in fig. 5 three MOSFET transistors Tl, T2 and T3 and four switch circuits SI, S2, S3 and S4 are connected to control the charging and discharging of three capacitors C f , C f2 and C s in the same way as described above. When the control voltage v is "low" transistors Tl and T3 are on and transistor T2 off, while switch circuits S2 and S4 are active and switch circuits SI and S3 inactive, so that capacitor C s is charged in series with the parallel connection of capacitors C f and C f2 as shown in the equivalent diagram in fig. 6. When the control voltage v is "high" transistors Tl and T3 are off and transistor T2 on, while switch circuits S2 and S4 are inactive and switch circuits SI and S3 active, so that the series connection of capacitors C f and C f2 is discharged in parallel to the capacitor C s and the resistor Rl in parallel therewith as shown in the equivalent diagram in fig. 7. If capacitors C f , C f2 and C s are of equal capacitance the battery voltage U cc is divided into thirds and the reduced supply voltage xU out will be about the two thirds of the battery voltage. The configurations shown in fig.s 3 and 6 are only examples of preferred embodiments of switched capacitor charge pump converters for use in digital hearing aids according to the invention. Within the scope of the invention one or more reduced operation voltages for different signal processing parts of the hearing aid can be obtained as fractions of the battery voltage. As will follow from the description above the reduced operation voltage supplied by the voltage step-down converter of the invention will initially not be stabilized and will thus follow fluctuations of the battery voltage. It would obvious for an expert, however, to generate also a stabilized lower voltage, when needed, by means of a conventional stabilizing voltage regulator, while maintaining the benefit of a lower power consumption resulting from the invention.

Claims: P A T E N T C L A I M S 1. A digital hearing aid comprising a microphone (2), an output transducer (3), a digital signal processor (5) interconnected between the microphone (2) and the output transducer (3) and including one or more integrated circuit signal processing parts, and a power source (7) including a standard hearing aid battery for the supply of operation voltage for each of said signal processing parts, c h a r a c t e r i z e d in that at least one of said integrated circuit signal processing parts is designed to operate at a reduced operation voltage substantially below a nominal voltage of said battery and that a switched step-down voltage converter is connected between the power source and said at least one signal processing part for lowering the battery voltage to provide said reduced operation voltage. 2. A digital hearing aid as claimed in claim 1, c h a r a c t e r i z e d in that said at least one signal processing part is designed in CMOS technology. 3. A digital hearing aid as claimed in claim 1 or 2, c h a r a c t e r i z e d in that one or more signal processing parts are implemented as parallel signal processing blocks each operating at a reduced operation voltage. 4. A digital hearing aid as claimed in claim 1, 2 or 3, c h a r a c t e r i z e d in that said reduced operation voltage is equal to or lower than 0.8 volt. 5. A digital hearing aid as claimed in any of the preceding claims, c h a r a c t e r i z e d in that said converter is a capacitive charge pump converter. 6. A digital hearing aid as claimed in claim 5, c h a r a c t e r i z e d in that said charge pump converter is designed to deliver two or more output voltages, at least one of which is said reduced operation voltage. 7. A digital hearing aid as claimed in any of claims 1 to 4, c h a r a c t e r i z e d in that said voltage converter is a switched inductor network converter . 8. A digital hearing aid as claimed in any of the preceding claims, c h a r a c t e r i z e d in that said reduced operation voltage is an unstabilized voltage .

Inventors: ANDERSEN HENNING HAUGAARD, ANDERSEN, HENNING, HAUGAARD
Applicants: ANDERSEN HENNING HAUGAARD, TOEPHOLM & WESTERMANN, ANDERSEN, HENNING, HAUGAARD, TOEPHOLM & WESTERMANN APS
Application year: 2001
Main classification: H04R25
IPC codes: H04R25