the description that follows relates to use of the invention within a spinal cord stimulation ( scs ) system . however , it is to be understood that the invention is not so limited . rather , the invention may be used with any type of implantable medical device system that could benefit from improved communications between an external controller and the device . for example , the present invention may be used as part of a system employing an implantable sensor , an implantable pump , a pacemaker , a defibrillator , a cochlear stimulator , a retinal stimulator , a stimulator configured to produce coordinated limb movement , a cortical and deep brain stimulator , or in any other neural stimulator configured to treat any of a variety of conditions . one embodiment of the improved driving circuit 200 is shown in fig5 , which offers significant improvement in power efficiency while at the same time being easy to implement . as will be discussed further below , the driving circuit 200 allows some of the excess current produced during resonance of the coil l to be shunted back to the rechargeable battery in the transmitter ( e . g ., the external controller ). in so doing , the driving circuit 200 is naturally dampened , but in a way that is considerate of battery capacity . the result is a driving circuit which has a relatively broad frequency response profile . this allows frequencies f0 and f1 to be adjusted by the transmitter to match receiver characteristics without using additional circuits or hardware ; is relatively responsive to allow for fast transitions between logic states and fast data rates ; and is energy efficient . the driving circuit 200 of fig5 uses a transformer 202 with a primary winding l 1 ( which comprises the coil 17 in the external controller 12 for example ) and a secondary winding l 2 in a transformer feedback configuration . as shown , no discrete resistors ( such as used in the approach of fig3 ) are used in the resonant circuit , which comprises the resonance capacitor , c and the equivalent inductance l of the primary coil of the transformer 202 . the circuit 200 produces a center resonance frequency , fc , in a medical implantable system application of about 125 khz , where fc =( 2π * sqrt ( lc )) − 1 . the primary winding l 1 ( 17 ) in conjunction with capacitor c , are made to resonate by toggling switches 160 and 160 ′. because the switches 160 and 160 ′ are controlled with complementary clock signals , clk and clk ′, vbat is applied to the resonant circuit with alternating polarities . driving the resonant circuit from both of its ends by the two switches 160 and 160 ′ has the benefit of doubling the voltage across coil l 1 and thus doubling magnetic field 29 . however , the use of two switches 160 and 160 ′ is not strictly necessary , and instead a single switch can be used in driving circuit as well ( such as will be illustrated in the alternative embodiment of fig6 below ). the windings or coils l 1 and l 2 in the transformer 202 have n 1 and n 2 turns respectively , which set the relation of the voltages across them : vl 1 = vl 2 * n 1 / n 2 . the voltage produced on the secondary winding l 2 , vl 2 , is connected to the battery 76 via a diode d . this has the effect of limiting vl 2 to the battery voltage , vbat . ( this assumes that the threshold voltage of the diode is negligible , i . e ., vt ≈ 0 ). should vl 2 try to exceed vbat during resonance , the diode d becomes forward biased ( again , assuming that the diode has a threshold voltage of zero , i . e ., vt ≈ 0 ), and a battery recovery current , i e , flows to the battery 76 . such current flow limits the potential of vl 2 to vbat , which in turn clamps the voltage across the coil 17 l 1 to a maximum value : vl 1 max = vbat *( n 1 / n 2 ). ( should a significant diode threshold voltage vt be present , this equation is modified as follows : vl 1 max =( vbat + vt )*( n 1 / n 2 )). by contrast , when vl 2 is less than vbat during resonance , the diode d prevents the flow of current out of the battery 76 ( i e ≈ 0 ) to prevent discharging . the bypass capacitor cbp reduces the peak current flowing in / out of the rechargeable battery 76 to provide for stability . by shunting the recovery current i e to the rechargeable battery 76 , the battery 76 is recharged . such recharging occurs during a portion of the time that the circuit is resonating , i . e ., when the resonance produces high voltages across the coil 17 that ( absent clamping ) would exceed vbat *( n 1 / n 2 ). the result is an energy efficient solution rivaling that of the solution depicted in fig4 . this is not without its drawbacks : because vl 1 is clamped to vl 1 max , the magnetic field of the wireless modulated data signal 29 produced by the coil 17 is also limited , and thus the driving circuit 200 effectively acts as a low - q - factor circuit . however , and unlike the solution of fig3 , such magnetic field reduction does not result from energy dissipation caused through the use of a discrete resistor . moreover , shunting the recovery current i e assists in dampening the resonance , which allows the driving circuit 200 to switch frequencies , and hence data states , more quickly , enabling the transmission of higher data rates . such active dampening also tends to spread the width of the resonance of the driving circuit ( i . e ., at 121 khz and 129 khz ). this eases the need to precisely match hardware components of the driving circuit 200 to that of the receiver , and hence allows the driving circuit to be constructed of components of lesser accuracy . the various values for the components used in the improved driving circuit 200 of fig5 may be as follows for the medical implantable device system for which the circuit was originally developed . n 1 =˜ 22 turns ; n 2 =˜ 3 turns ; l 1 = 34 μh ; c = 0 . 047 μf ; vbat = 4 . 1v ; cbp = 100 μf . however , in a different application , different component values could be chosen . fig6 depicts an alternative driving circuit 200 ′. like the driving circuit 200 of fig5 , the driving circuit 200 ′ of fig6 also lacks a discrete resistor , but also allows for excess resonance energy to be shunted back to the battery 76 for the purpose of dampening and recharging . driving circuit 200 ′ employs a single inductor l ( i . e ., the coil of the external controller 12 for example ). a tap point 82 is added to the transmit coil 17 ( in what is known in the art as an autotransformer configuration ) to connect the coil 17 to the rechargeable battery 76 via a diode , d . the number of turns in the coil above ( n 1 ) and below ( n 2 ) the tap point 82 relates the voltage across the coil ( vl ) to the voltage at the tap point 82 ( vtap ). specifically , vl = vtap *( n 2 + n 1 )/ n 2 . as in the transformer feedback configuration of fig5 , the diode d has the effect of clamping the voltage across the coil 17 to a maximum , vlmax . this is because the voltage at tap point 82 ( vtap ) cannot exceed the battery voltage ( vbat ). should vtap try to exceed vbat during resonance , the diode d becomes forward biased ( assuming that the diode has a threshold voltage of zero , i . e ., vt ≈ 0 ), and a battery recovery current , i e , flows to the battery 76 . such current flow limits the potential of vtap to vbat , which in turn clamps the voltage across the coil 17 to a maximum value : vlmax = vbat *( n 2 + n 1 )/ n 2 . ( should a significant diode threshold voltage vt be present , this equation is modified as follows : vlmax =( vbat + vt )*( n 2 + n 1 )/ n 2 ). by contrast , when vtap is less than vbat during resonance , the diode d prevents the flow of current out of the battery 76 ( i e ≈ 0 ) to prevent discharging . as with the driving circuit 200 of fig5 , the center frequency of resonance is fc =( 2π * sqrt ( lc )) − 1 , where l comprises the inductance of the coil l 17 . the effect , as with the driving circuit 200 of fig5 , is that recovery current i e is shunted to the rechargeable battery 76 during high voltage portions of the resonance , providing a very energy efficient solution . although vl is clamped to vlmax , which limits the magnetic field producible by the coil 17 , such magnetic field reduction does not result from energy dissipation caused through the use of a discrete resistor . and once again , shunting the recovery current i e to the battery assists in dampening the resonance , which allows for the transmission of higher data rates . additionally , and as before , such active dampening also tends to widen the resonant response of the resonant circuit , easing reception without the need for hardware trimming . the various values for the components used in the improved driving circuit 200 ′ of fig6 may be as follows for the medical implantable device system for which the circuit was originally developed . n 1 =˜ 20 turns ; n 2 =˜ 4 turns ; l = 34 μh ; c = 0 . 047 μf ; vbat = 4 . 1v ; cbp = 100 μf . however , in a different application , different component values could be chosen . the disclosed driving circuits can be used with any switching type amplifier ( class c , d , e , h , etc . ), and for both the transformer feedback configuration ( fig5 ) and the autotransformer ( fig6 ) configuration . while disclosed in the context of a medical implantable device system for which the invention was originally contemplated , it should be recognized that the improved driving circuitry disclosed herein is not so limited , and can be used in other contexts employing communications via magnetic inductive coupling , such as in radio - frequency identification ( rfid ) systems , etc . the disclosed circuitry can further be used in any context in which magnetic inductive coupling could be used as a means of communication , even if not so used before . although particular embodiments of the present invention have been shown and described , it should be understood that the above discussion is not intended to limit the present invention to these embodiments . 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 present invention . thus , the present invention is intended to cover alternatives , modifications , and equivalents that may fall within the spirit and scope of the present invention as defined by the claims .
a driving circuit useful in a magnetic inductive coupling wireless communication system is disclosed . the circuit includes an inductor and capacitor in series selectively coupled to a power source such as a rechargeable battery . the lc circuit is made to resonate in accordance with a frequency shift keying or other protocol . such resonance produces a voltage across the inductor . this voltage is used to create a first voltage either by tapping into the coil , or by providing a transformer . the first voltage is coupled to the rechargeable battery by a diode . when the circuit resonates , and when the first voltage exceeds the voltage of the power source , the diode turns on , thus shunting excess current back to recharge the rechargeable battery . by use of this circuit , energy is conserved . additionally , oscillations can be quickly dampened so as to allow the circuit to transmit at high data rates .