Abstract:
An apparatus for a hearing device includes a first voltage regulator with an output terminal; a first voltage reference; a second voltage regulator with an output terminal; a switching element; and a decoupling element; wherein the switching element and the decoupling element are operatively between the first voltage reference and the first voltage regulator; wherein the output terminal of the first voltage regulator shares a same electrical node as the output terminal of the second voltage regulator; and wherein the first voltage regulator is configured to provide a first output voltage in response to applied battery power, the second voltage regulator is configured to provide a second output voltage if a certain condition is fulfilled, and the switching element is configured to disconnect the first voltage reference from the decoupling element if the condition is fulfilled.

Description:
FIELD 
       [0001]    This application relates to hearing aids. More specifically, it relates to battery-powered hearing aids comprising integrated electronic circuits. 
       BACKGROUND 
       [0002]    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. 
         [0003]    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. 
         [0004]    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. 
         [0005]    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. 
       SUMMARY 
       [0006]    A good choice of reference voltage is a band-gap reference due to the inherent high stability and temperature independence. The electronics in a microelectronic circuit in a hearing aid typically operates at voltages around one volt. However, since a band-gap voltage reference has a typical reference voltage of 1.25 volts, and a typical battery in a hearing aid is only capable of delivering 1.3 volts at the most, more typically 1.1 to 1.2 volts, a band-gap voltage reference cannot be fed directly from a hearing aid battery. A higher supply voltage could be provided, e.g. a double voltage provided by a voltage doubler circuit, but a double voltage generator would be dependent on a clock generator driven by the output voltage and running at a nominal frequency and output voltage swing from the moment the output voltage of the power supply was applied to the circuit. Such an oscillator is not feasible given the current state of technology and the power limitations of a hearing aid circuit. The oscillator would need at least 2-3 milliseconds to start up in order to be able to reach the required stability, frequency and voltage swing, and an associated voltage doubler circuit would require an additional period of 2-3 milliseconds in order to be capable of providing a sufficiently stable doubled battery voltage without drawing an inhibitory large amount of power. 
         [0007]    A doubled battery voltage would also be of benefit to the operational amplifier present in the linear voltage regulator, since this would allow for an amplifier design with a larger open loop gain, and thereby be able to provide a yet more stable voltage regulator circuit capable of powering a wider range of loads. As indicated, such a voltage regulator would need a start-up time of 4-6 milliseconds in order to provide the desired power and precision. This voltage regulator is therefore not capable of powering a hearing aid from the moment battery power is applied. 
         [0008]    A voltage regulator capable of delivering a desired output voltage immediately after being powered on would have to be a compromise on a number of features essential to the desired accuracy of the supply voltage due to the fact that no doubled voltage is available at that moment. For instance, the voltage reference could be a simple voltage reference such as a Zener diode or a current mirror. This choice would reduce the precision of the supplied voltage but would be capable of delivering a sufficient supply voltage immediately after applying power to the circuit, e.g. when the battery door is closed. Furthermore, the operational amplifier of this voltage regulator could be powered directly by the battery voltage at the cost of a lower open loop gain and a reduced power capability. 
         [0009]    A hearing aid having a power supply comprising two distinct voltage regulators, one featuring the required precision and one being capable of operating from the moment power is applied, is proposed. Such a design, however, presents the designer with a number of nontrivial problems. These problems are solved by the power supply of the disclosure. 
         [0010]    A power supply for a hearing device is provided, the power supply comprising a battery, a first linear voltage regulator, a first voltage reference, a second linear voltage regulator, a second voltage reference, wherein the output terminal of the first linear voltage regulator sharing the same electrical node as the output terminal of the second linear voltage regulator, a switching element and a decoupling element, said switching element and decoupling element being positioned between the first voltage reference and the first linear voltage regulator, and wherein the first linear voltage regulator is capable of providing a first output voltage when battery power is applied, the second voltage regulator is capable of providing a second output voltage on the fulfillment of a specific condition, and the switching element is capable of disconnecting the first voltage reference from the decoupling element if the specific condition is fulfilled. In this way a practical and reliable power supply for a hearing aid is realized. 
         [0011]    In one embodiment, the specific condition is that a predetermined time period has elapsed since battery power was applied to the circuit. The elapsed time period may be measured and conveyed in a number of ways known in the art, such as a delay circuit detecting the presence of the battery voltage, a digital counter starting together with the hearing aid processor, or a dedicated timing circuit. When the time period has elapsed, this circuit controls the switching element, thus disconnecting the first voltage reference from the decoupling element. 
         [0012]    In another embodiment, the specific condition is the presence of a predetermined voltage level at a specified node in the circuit. If the specified node is e.g. connected to a voltage doubler providing the double voltage of the battery the presence of this voltage could be detected, e.g. with a 1:2 voltage divider circuit and a comparator. When the voltage output from the voltage doubler equals the double battery voltage, the voltage output from the 1:2 voltage divider equals the battery voltage. This condition could then be tested by a comparator comparing the output voltage from the 1:2 voltage divider to the battery voltage. The output from the comparator could then be used to control the switching element directly, thus disconnecting the first voltage reference from the decoupling element when the voltage doubler is operating nominally, and the first linear voltage regulator therefore is no longer needed. 
         [0013]    In an embodiment, the timing constant of the decoupling element is larger than, or equal to, the timing constant of the control loop of the second linear voltage regulator. In this way, the second regulator may take over the supply of power gently from the first voltage regulator when the first voltage regulator is no longer needed by the circuit. 
         [0014]    The motivation behind the power supply according to the disclosure is born out of the desire to have both a fast power supply and a precise power supply in the hearing aid. This may be obtained by providing the hearing aid circuit with a power supply comprising two linear voltage regulators, where a first voltage regulator is capable of operating immediately after applying battery power to the hearing aid and a second voltage regulator is capable of providing a very precise supply voltage after a few milliseconds. The first voltage regulator thus has the advantage of being available immediately after powering on the hearing aid and the second voltage regulator has the advantage of delivering a voltage with an accuracy within 2% of the nominal supply voltage. Furthermore the first voltage regulator has the limitation of providing a voltage with an accuracy within 20% of the nominal supply voltage, and the second voltage regulator has the limitation of not being available immediately after powering up the hearing aid. 
         [0015]    If the two voltage regulators are designed to provide approximately the same voltage, they may operate concurrently for a short period of time, i.e. when the second voltage regulator is operating safely. In order to save power it is beneficial to shut down the first voltage regulator when the second voltage regulator is operating safely. This may be done by disconnecting the voltage reference from the first operational amplifier of the voltage regulator, effectively driving its output to zero volts. Such a disconnection may effectively be obtained by a transistor acting as a voltage-controlled switch. However, if the reference voltage is removed instantly whenever the first voltage regulator is no longer needed, the second voltage regulator will temporarily experience a big voltage drop due to the fact that the first voltage regulator suddenly does not supply current to the load anymore, and the second voltage regulator therefore has to deliver all current consumed by the load. Since this is a temporary situation, the second voltage regulator will eventually be able to deliver the extra load current. However, the control loop of the second voltage regulator cannot keep up with the sudden current demand. The cause is that the intrinsic slew rate of the second voltage regulator sets an upper limit to how fast the load current may change. 
         [0016]    In order to alleviate this problem, a discharging circuit is inserted between the voltage reference and the positive input of the operational amplifier of the voltage regulator. The purpose of this circuit is to provide a voltage decreasing with a lower speed than the highest possible regulation speed of the control loop of the second voltage regulator as defined by the slew rate. Preferably, the circuit comprises a capacitor in parallel with a semiconductor having a low leakage current, both connected to ground. 
         [0017]    Thus, the condition which must be satisfied by the discharging circuit is: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       ∂ 
                       
                         V 
                         C 
                       
                     
                     
                       ∂ 
                       t 
                     
                   
                   ≥ 
                   
                     
                       ∂ 
                       
                         V 
                         R 
                       
                     
                     
                       ∂ 
                       t 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where ∂V C  is the time constant of the capacitor discharge circuit and ∂V R  is the slew rate time constant of the operational amplifier. Equation (1) states that if the time constant of the capacitor discharge circuit is larger than the slew rate of the operational amplifier, the second voltage regulator will be able to maintain a stable output voltage when the first voltage regulator ceases to deliver current to the load. 
         [0018]    When the simple voltage reference is connected to the capacitor and the semiconductor of the discharging circuit and the input of the operational amplifier, the capacitor is charged to the same voltage as the voltage reference. The low leakage current of the semiconductor does not affect the reference voltage as long as the reference is connected, and the operational amplifier is designed in such a way that no significant current is flowing into the input node of the operational amplifier. When the voltage reference is disconnected by the voltage-controlled switch, the capacitor is discharged through the semiconductor, thus providing the slowly decreasing voltage needed in order to prevent the power surge which would otherwise result in a drop in the supply voltage from the second voltage regulator. 
         [0019]    An apparatus for a hearing device includes a first voltage regulator with an output terminal; a first voltage reference; a second voltage regulator with an output terminal; a switching element; and a decoupling element; wherein the switching element and the decoupling element are operatively between the first voltage reference and the first voltage regulator; wherein the output terminal of the first voltage regulator shares a same electrical node as the output terminal of the second voltage regulator; and wherein the first voltage regulator is configured to provide a first output voltage in response to applied battery power, the second voltage regulator is configured to provide a second output voltage if a certain condition is fulfilled, and the switching element is configured to disconnect the first voltage reference from the decoupling element if the condition is fulfilled. 
         [0020]    Optionally, the decoupling element comprises a capacitor and a semiconductor element. 
         [0021]    Optionally, the switching element is a semiconductor switching element. 
         [0022]    Optionally, the condition is that a predetermined time period has elapsed since an application of battery power. 
         [0023]    Optionally, the predetermined time period is anywhere from 3 ms to 10 ms. 
         [0024]    Optionally, the predetermined time period is anywhere from 4 ms to 6 ms. 
         [0025]    Optionally, the condition is a presence of a predetermined voltage level at a specified node in the apparatus. 
         [0026]    Optionally, the predetermined voltage level is a multiple of a nominal battery voltage. 
         [0027]    Optionally, the first voltage reference is a current mirror voltage reference. 
         [0028]    Optionally, the apparatus further includes a second voltage reference, wherein the second voltage reference is a band-gap voltage reference. 
         [0029]    Optionally, the second voltage regulator is configured to provide a more precise output voltage than the first voltage regulator. 
         [0030]    Optionally, a timing constant of the decoupling element is larger than, or equal to, a timing constant of a control loop of the second voltage regulator. 
         [0031]    Optionally, the second output voltage provided by the second voltage regulator deviates less than 2% from a nominal output voltage. 
         [0032]    Other and further aspects and features will be evident from reading the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0033]    The above and other features and advantages will become readily apparent to those skilled in the art by the following detailed description of exemplary embodiments thereof with reference to the attached drawings, in which: 
           [0034]      FIG. 1  is an exemplary schematic diagram of a prior art hearing aid power supply, 
           [0035]      FIG. 2  is an exemplary schematic diagram of a power supply comprising a dual linear voltage regulator according to the disclosure, 
           [0036]      FIG. 3  is an exemplary schematic diagram of a discharging circuit of the dual linear voltage regulator in  FIG. 2 , 
           [0037]      FIG. 4  is a timing diagram showing a startup sequence of the circuit shown in  FIG. 2 , 
           [0038]      FIG. 5  is an exemplary block schematic diagram of a hearing aid circuit incorporating the voltage regulator in  FIG. 2 , and 
           [0039]      FIG. 6  is an exemplary schematic diagram of an alternative embodiment of the discharging circuit in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    Various embodiments are described hereinafter with reference to the figures. It should also be noted that the figures are only intended to facilitate the description of the embodiments. They are not intended as an exhaustive description of the claimed invention or as a limitation on the scope of the claimed invention. In addition, an illustrated embodiment needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated, or if not so explicitly described. 
         [0041]      FIG. 1  shows a prior art hearing aid power supply  1 . The power supply  1  comprises a voltage reference  2 , an operational amplifier  3 , a MOSFET output transistor  4 , a first resistor  5  and a second resistor  6 . The voltage reference  2  provides the reference voltage V ref  and is connected to the positive input of the operational amplifier  3 , the output of the operational amplifier  3  is connected to the gate terminal of the output transistor  4 , the drain terminal of the output transistor  4  is connected to a battery voltage terminal V bat , the source terminal of the output transistor  4  is connected to an output terminal V out  of the power supply and the first terminal of the first resistor  5 , the second terminal of the first resistor  5  is connected to the first terminal of the second resistor  6  and the negative input terminal of the operational amplifier  3 , and the second terminal of the second resistor  6  is connected to ground. The operational amplifier  3  has a supply terminal connected to the battery voltage terminal V bat  and a biasing terminal V bias  connected to a biasing voltage source (not shown). 
         [0042]    As used in this specification, the term “voltage reference” refers to any component that is capable of providing a reference voltage (e.g., a stable reference voltage). In its simplest form, a voltage reference may be e.g. a Zener diode connected to a voltage source. A more advanced and precise voltage reference may be, e.g., a band-gap voltage reference connected to a voltage source. However, embodiments described herein are not limited to these examples of voltage reference. Different voltage references have various benefits and shortcomings, which are discussed in greater detail in the following. 
         [0043]    The operational amplifier is connected in a noninverting configuration, and the output voltage V out  of the operational amplifier  3  is: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     out 
                   
                   = 
                   
                     
                       V 
                       ref 
                     
                      
                     
                       ( 
                       
                         1 
                         + 
                         
                           
                             R 
                              
                             
                                 
                             
                              
                             1 
                           
                           
                             R 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where V ref  is the voltage of the reference  2 . 
         [0044]    The load regulation, defined as the change in output voltage ΔV out  as a function of a static change in the current load ΔI load  may be written as: 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       V 
                       out 
                     
                   
                   = 
                   
                     - 
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           I 
                           load 
                         
                       
                       
                         
                           ( 
                           
                             
                               
                                 
                                   R 
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                                 
                                   
                                     R 
                                      
                                     
                                         
                                     
                                      
                                     1 
                                   
                                   + 
                                   
                                     R 
                                      
                                     
                                         
                                     
                                      
                                     2 
                                   
                                 
                               
                               · 
                               A 
                             
                             + 
                             1 
                           
                           ) 
                         
                          
                         
                           g 
                           M 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where g M  is the transconductance of the transistor  4  and A is the open-loop gain of the amplifier  3 . In other words, if the current load changes, the regulated voltage also changes. 
         [0045]    As indicated by equation (3), a change in the load current ΔI load  has a direct influence on the change of the output voltage ΔV out . If, e.g. the load current suddenly rises, this would result in a drop in the output voltage. The relative magnitude of the voltage drop is directly dependent on the value of the resistors  5  and  6 , the open-loop gain of the operational amplifier  3  and the transconductance of the transistor  4 . 
         [0046]      FIG. 2  illustrates a new exemplary dual linear voltage regulator power supply  10 . The power supply  10  comprises a first voltage regulator  11  and a second voltage regulator  12 , both delivering an output voltage to the terminal V out . The first voltage regulator  11  comprises a first operational amplifier  18 , a first output transistor  19 , a first resistor  20 , and a second resistor  21 . The operating point of the first operational amplifier  18  is controlled by a first bias voltage generator  29  delivering a first bias voltage to the terminal V bias1 . The first voltage regulator  11  is powered by the battery voltage and controlled by a simple voltage reference  13  connected to the non-inverting input of the first operational amplifier  18  via a discharging circuit  40 . The discharging circuit  40  comprises a voltage-controlled transistor switch  15 , a low-leakage current transistor  16  and a capacitor  17 . 
         [0047]    The second voltage regulator  12  comprises a second operational amplifier  22 , a second output transistor  23 , a third resistor  24  and a fourth resistor  25 . Also shown in  FIG. 2  is a slow-reacting subcircuit  30 . The subcircuit  30  comprises a master clock oscillator  26 , a voltage doubler circuit  27 , a band-gap voltage reference  14  and a second bias voltage generator  28 . The voltage doubler circuit  27  provides a doubled battery voltage to the terminal 2V bat , and the second bias voltage generator  28  controls the operating point of the second operational amplifier  22  by providing a second bias voltage to the terminal V bias2 . The doubled battery voltage from the terminal 2V bat  is used by the band-gap voltage reference  14  and the second operational amplifier  22 . 
         [0048]    The various parts of the subcircuit  30  have the inherent property of not being operational until a definite amount of time, e.g. 6-8 milliseconds, has elapsed from the moment when battery power is applied to the subcircuit  30 , the reasons for this being, among other things, that the master clock oscillator  26  has to reach a stable output frequency and output voltage swing. The master clock oscillator  26  is driven by V out  (at this moment in time delivered by the first voltage regulator  11 ) for producing an oscillating output voltage. Since the master clock oscillator  26  drives the voltage doubler circuit  27 , and the voltage doubler circuit  27  in turn drives the band-gap voltage reference  14  and the second bias voltage generator  28 , the subcircuit  30  needs to have power applied for a period of about 6-8 milliseconds in order to be fully functional. 
         [0049]    When in use, the exemplary power supply in  FIG. 2  works in the following way; When battery power is applied to the circuit, the voltage-controlled transistor switch  15  of the discharging circuit  40  is closed, allowing the simple voltage reference  13  to provide a first reference voltage V ref1  to the first voltage regulator  11 , which then delivers a regulated voltage to the output terminal V out . The first voltage regulator  11  is not very accurate. In one embodiment, it delivers a regulated supply voltage level of approximately 1100 mV±230 mV, i.e. with a long-term accuracy of about 20%. The exact magnitude of this output voltage is dependent on a number of factors such as the ambient temperature, the condition of the battery, the amount of power initially drawn from the hearing aid circuit and chip fabrication tolerances. However, for the purpose of starting up the hearing aid circuit and initially providing it with power, it is considered sufficiently adequate. 
         [0050]    In order to regulate the supply voltage more accurately, the second voltage regulator  12  is supposed to take over from the first voltage regulator  11  when the aforementioned subcircuit  30  is considered to be operating nominally. In the embodiment shown in  FIG. 2 , this is accomplished by determining if a predetermined period of time has elapsed since the hearing aid was powered up. Typically, this occurs within 5 ms from the moment battery power is applied. At this point in time, the band-gap voltage reference delivers a second reference voltage to the terminal V ref2  connected to the noninverting input of the second operational amplifier  22 . The second voltage regulator  12  is designed to provide a regulated supply voltage level of about 900 mV±20 mV, i.e. an accuracy of about 2%, or approximately ten times better than the accuracy of the output voltage from the first voltage regulator  11 . When the second voltage regulator  12  is operative, the voltage output from the first voltage regulator  11  is no longer needed, and the first voltage regulator  11  may be turned off in order to conserve battery power. 
         [0051]    Obviously, the first voltage regulator  12  could be turned off simply by disconnecting the first voltage reference  13  from the noninverting input of the operational amplifier  18 . This would, however, present the output of the second voltage regulator  12  with a sudden rise in required output current, which again would lead to a big drop in the voltage supplied by the second voltage regulator  12 , the supply voltage at the output terminal V out  only rising back to the nominal voltage level again as fast as the control loop of the second voltage regulator  12  would permit. This would leave the parts of the hearing aid circuit supplied by this lower voltage in a potentially hazardous situation, since e.g. the signal processing circuits of the hearing aid are very susceptible to dropouts in the supply voltage, as stated in the foregoing. 
         [0052]    In order to prevent this problem, the discharging circuit  40  of the power supply  10  is placed between the simple voltage reference  13  and the noninverting input of the first operational amplifier  18 . The output terminal of the simple voltage reference  13  of the discharging circuit  40  is connected on the input side of the voltage-controlled transistor switch  15 . The low-leakage current transistor  16  has its gate and source terminals connected to ground and its drain terminal connected on the output side of the voltage controlled transistor switch  15 , and the capacitor  17  is connected between the drain terminal of the low-leakage current transistor  16  and ground. The voltage-controlled switch  15  controls the connection between the simple voltage reference  13  and the noninverting input of the first operational amplifier  18 , and the low-leakage current transistor  16  in parallel with the capacitor  17  performs a discharging function when the voltage-controlled switch  15  is opened. 
         [0053]    When a sufficient time period has elapsed from the moment of applying battery power to the hearing aid circuit to the moment in time where the slow-reacting subcircuit  30  is considered to be operating nominally, a signal to the input gate of the voltage-controlled transistor switch  15  disconnects the first reference voltage V ref1  of the simple voltage reference  13  from the noninverting input of the first operational amplifier  18 . The capacitor  17  will leak its charge slowly through the low-leakage current transistor  16  as a discharge current I off , resulting in the reference voltage V ref1  decreasing over time. The voltage contribution from the first voltage regulator  11  is thus reduced gradually when the first voltage reference  13  is disconnected. 
         [0054]    The reduction rate of the voltage contribution from the first voltage regulator  11  has to be sufficiently slow for the control loop of the second voltage regulator  12  to be able to compensate, the second voltage regulator  12  thereby being capable of maintaining the required stable supply voltage for powering the rest of the hearing aid circuit. This is achieved by optimizing the transistor  16  for having a low, but well-defined leakage current I leak . If the first operational amplifier  18  and the voltage-controlled switch  15  are considered ideal, the capacitor will only leak its charge through the transistor  16 , thus: 
         [0000]        I   leak   =I   off   (4)
 
         [0055]    The voltage level presented to the noninverting output of the operational amplifier  18  is thus defined by: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       
                         ref 
                          
                         
                             
                         
                          
                         1 
                       
                     
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         1 
                         C 
                       
                        
                       
                         
                           ∫ 
                           
                             t 
                             off 
                           
                           ∞ 
                         
                          
                         
                           
                             
                               I 
                               off 
                             
                              
                             
                               ( 
                               t 
                               ) 
                             
                           
                            
                           
                               
                           
                            
                           
                              
                             t 
                           
                         
                       
                     
                     + 
                     
                       
                         V 
                         
                           ref 
                            
                           
                               
                           
                            
                           1 
                         
                       
                        
                       
                         ( 
                         
                           t 
                           off 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0056]    Where t off  is the time when the voltage-controlled switch  15  is opened. In a practical circuit, the discharge process will end when V ref1  reaches the pinch-off level of the transistor  16 . However, this level is sufficiently low for the resulting contribution from the first voltage regulator  11  to be negligible. 
         [0057]      FIG. 3  shows an exemplified discharging circuit  40  of the double voltage regulator  10  shown in  FIG. 2 . The discharging circuit  40  comprises the voltage-controlled transistor switch  15 , the low-leakage current transistor  16  and the capacitor  17 . The source terminal of the voltage-controlled switch  15  is connected to the output of the simple voltage reference  13  (see  FIG. 2 ) providing the reference voltage to the input terminal V ref1 . The drain of the voltage-controlled transistor switch  15  is connected to the noninverting input of the first operational amplifier  18  (see  FIG. 2 ) and providing the reference voltage to the output terminal V in . The drain of the low-leakage current transistor  16 , a first terminal of the capacitor  21  and the gate of the low-leakage current transistor  16  are sharing the same node as the drain of the voltage-controlled transistor switch  15 . The gate of the voltage-controlled transistor switch  15  is connected to the output of a timing circuit  54 . The gate and the source of the low-leakage current transistor  16  are connected to ground, and a second terminal of the capacitor  17  is also connected to ground. For the sake of simplicity, the voltage-controlled transistor switch  15  is considered to be an ideal switch, i.e. providing no resistance when closed and infinite resistance when open. 
         [0058]    When power is applied to the hearing aid circuit, e.g. by applying a battery voltage to the circuit by closing a battery door of the hearing aid, the timing circuit  54  simultaneously applies a control voltage (denoted Ctrl) to the gate of the voltage-controlled transistor switch  15 , effectively connecting the input terminal V ref1  to the terminal V in . The reference voltage at the terminal V ref1  is thus applied to the noninverting input of the first operational amplifier  18 , the drain of the low-leakage current transistor  16  and the first terminal of the capacitor  17 , respectively, and the capacitor  17  is thus charged with the reference voltage present at the terminal V ref . 
         [0059]    When the timing circuit  54  times out, the control voltage Ctrl is removed from the gate of the voltage-controlled transistor switch  15 , effectively disconnecting the terminal V ref1  from the terminal V in . The charge voltage present on the plate of the capacitor  17  is now used for reference voltage. The capacitor  17  is discharged in a controlled manner through the low-leakage current transistor  16 , slowly reducing this reference voltage towards zero while discharging the current I off . The low-leakage current transistor  16  is selected so as to have a very low leakage current, e.g. 10% of the leakage current of the second output transistor  23  (see  FIG. 2 ), in order to draw as small a current as possible, thus reducing the load on the discharging circuit on the current mirror voltage reference. The capacitance of the capacitor  17  and the characteristics of the low-leakage current transistor  16  is selected in order to discharge the capacitor  17  with a velocity smaller than, or equal to, the velocity of the control loop of the second regulator  12  (see  FIG. 2 ). 
         [0060]      FIG. 4  is a timing diagram showing a startup sequence of an exemplary hearing aid power supply circuit of the type shown in  FIG. 2 . The startup sequence shows the operation of the power supply circuit posterior to the application of battery power. The curve segment marked A in  FIG. 4  illustrates the output voltage over time of the first linear voltage regulator  11  in  FIG. 2 . The output voltage starts at zero and rises within 500 μs to a voltage level of about 1100 mV. The slow-reacting subcircuit  30  is starting to operate after approximately 4 ms, illustrated by the point E in  FIG. 4 , while the voltage level of 1100 mV is maintained by the first linear voltage regulator  11 . The second linear voltage regulator  12  is operating at nominal level after about 5.5 ms, illustrated by the point D in  FIG. 4 . The nominal voltage level output by the second linear voltage regulator  12  is approximately 900 mV, as illustrated by the curve below the point D in  FIG. 4 . At this time, the voltage contributed by the first linear voltage regulator  11  may be safely turned off. 
         [0061]    The simple voltage reference  13  is disconnected from the first linear voltage regulator in  FIG. 2  about 6 ms after battery power is applied, illustrated by the point F in  FIG. 4 . At this point in time, the output voltage from the dual linear voltage regulator  10  will begin to drop slowly from the 1100 mV provided by the first linear voltage regulator  11  to the 900 mV provided by the second linear voltage regulator  12 , illustrated by the curve segment B in  FIG. 4 . After a period of approximately 11.5 ms has elapsed, the second linear voltage regulator  12  has taken over completely from the first linear voltage regulator  11 , which has shut down completely. The open-circuit voltage contribution from the first linear voltage regulator  11  over time is illustrated by the curve segment C in  FIG. 4 . 
         [0062]    All the voltage levels and timings shown in  FIG. 4  are exemplary. The accuracy of the output voltage from the first linear voltage regulator  11  is about 20%, the accuracy of the output voltage from the second linear voltage regulator  12  is about 2%, and the timing values may also vary, e.g. with different loads being presented to the power supply circuit  10 . Different loads may, for instance, be the result of various parts of the hearing aid circuit being turned on or off. If a power-consuming subcircuit, e.g. an acoustic feedback cancellation circuit or a radio transceiver, are turned on or off in the hearing aid, this may have a significant impact on the timing values shown in  FIG. 4 . 
         [0063]      FIG. 5  is a simplified block schematic of an exemplified hearing aid  50  comprising a power supply  10  of the type shown in  FIG. 2 . The hearing aid  50  comprises a digital signal processor  43 , a microphone  41 , an analog-to-digital converter  42 , a digital-to-analog converter  44 , an acoustic output converter or loudspeaker  47 , a memory bank  45 , a telecoil  46 , a battery  60 , a master clock oscillator  26 , a voltage doubler  27 , a band-gap voltage reference  14 , a simple voltage reference  13 , a discharge circuit  40 , a first linear voltage regulator  11 , a second linear voltage regulator  12 , a wireless radio transceiver  48 , and an antenna  49 . 
         [0064]    The digital signal processor  43  is the main functional block in the hearing aid  50 , providing amplification, compression, acoustic feedback suppression and source selection of a range of input signals for the benefit of a hearing aid user, including a digitized signal from the microphone  41  via the analog-to-digital converter  42 , a signal from the telecoil  46  and an audio stream received by the wireless radio transceiver  48 . The processed signals are fed to the digital-to-analog converter  44  feeding an analog signal to the loudspeaker  47  for acoustic reproduction by the hearing aid  50 . 
         [0065]    During use, the digital signal processor  43  may operate in a number of different modes or programs according to the requirements of a hearing aid user. The digital signal processor DSP may provide a selection of signal processing algorithms for performing alleviating amplification in order to compensate for a hearing loss. One program may incorporate several different signal processing algorithms operating simultaneously in order to perform a desired function. The various programs may be stored in the memory bank  45  for later retrieval by the hearing aid user. The wireless radio transceiver  48  may be used for receiving programming information, e.g. user specific parameter settings tailored by a hearing aid professional in order to compensate an individual hearing loss, it may receive remote control commands from a remote control (not shown), e.g. for volume changes or program selection in the hearing aid  50 , or it may be used for receiving an audio stream from an external source for acoustic reproduction by the hearing aid  50  to the benefit of the hearing aid user. All electronic subcircuits of the hearing aid draw their power from the power supply  10 . In turn, the power supply  10  draws its power from the hearing aid battery  60 , said battery being e.g. of the zinc-air variety or the lithium-polymer variety according to the requirements of the hearing aid  50 . 
         [0066]    When the hearing aid  50  is powered on by closing the on/off switch  51 , e.g. by closing the door of the hearing aid battery compartment, the battery  60  immediately provides a battery voltage Vbat to the power supply  10 . However, the slow-reacting subcircuit  30  is not considered operational until a predetermined condition is fulfilled, such as the condition that a period of time has elapsed, e.g. 5 milliseconds, since the moment the on/off switch  51  has been closed. During that period of time, power is delivered by the first linear voltage regulator  11 , the voltage being regulated based on the simple voltage reference  13  via the discharging circuit  40 . 
         [0067]    When an appropriate period of time has elapsed, e.g. 5 milliseconds, the discharge circuit  40  disconnects the simple voltage reference  13  from the first linear voltage regulator  11 , thus causing its voltage contribution to drop gradually to 0 volts over a period of a couple of milliseconds. Then, the slow-reacting subcircuit  30  is considered to having reached its nominal operating level, and the second linear voltage regulator  12  is now capable of providing the supply voltage for the hearing aid subcircuits based on the voltage level of the band-gap voltage reference  14 . The discharge circuit  40  may therefore disconnect the simple voltage reference  13  in order to save battery power, and thanks to the constructional details discussed in conjunction with  FIGS. 3 and 4  be capable of reducing the contribution from the first linear voltage regulator  11  sufficiently slowly for the control loop of the second linear voltage regulator  12  to be able to compensate, thus maintaining the supply voltage level within 2% during normal operation of the hearing aid  50 . 
         [0068]      FIG. 6  shows an exemplary embodiment of a discharging circuit  40  similar to the circuit shown in  FIG. 3 . Like the embodiment shown in  FIG. 3 , this embodiment comprises the voltage-controlled transistor switch  15 , the low-leakage current transistor  16  and the capacitor  17 . In this embodiment, a voltage sensor  70  and a comparator  71  provides the input to the voltage-controlled transistor switch  15 . The voltage sensor is fed the output voltage 2*V bat  from the voltage doubler  27  (not shown in  FIG. 6 ), and provides a detectable, proportional voltage, e.g. V bat , to the comparator  71 . When the output voltage from the voltage doubler  27  has reached 2*V bat , the output voltage from the voltage sensor  70  will have reached V bat , and the comparator will output a control voltage to the input of the voltage-controlled transistor switch  15 , which will turn off, thus disconnecting V ref1  from V in . This will start the discharging of the charge present in the capacitor  17  through the low-leakage current transistor  16 , slowly reducing V in  to zero. The circuit shown in  FIG. 6  is thus capable of turning off the first voltage regulator in the controlled manner described in the foregoing when the voltage doubler  27  is providing a properly doubled voltage. 
         [0069]    Although the above embodiments have been described with reference to the voltage regulators being linear voltage regulators, in other embodiments, the voltage regulators may be non-linear voltage regulators, or other types of voltage regulators. 
         [0070]    The skilled person will appreciate that the design of the hearing aid power supply may be varied in several ways without leaving the scope of the disclosed power supply as defined by the claims. 
         [0071]    Although particular exemplary earmolds have been shown and described, it will be understood that it is not intended to limit the claimed inventions to the exemplary earmolds, and it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the claimed inventions. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense. The claimed inventions are intended to cover alternatives, modifications, and equivalents.