Stimulation waveforms with high- and low-frequency aspects in an implantable stimulator device

Waveforms for a stimulator device, and methods and circuitry for generating them, are disclosed having high- and low-frequency aspects. The waveforms comprise a sequence of pulses issued at a low frequency which each pulse comprising first and second charge-balanced phases. One or both of the phases comprises a plurality a monophasic sub-phase pulses issued at a high frequency in which the sub-phase pulses are separated by gaps. The current during the gaps in a phase can be zero, or can comprise a non-zero current of the same polarity as the sub-phase pulses issued during that phase. The disclosed waveforms provide benefits of high frequency stimulation such as the promotion of paresthesia free, sub-threshold stimulation, but without drawbacks inherent in using high-frequency biphasic pulses.

FIELD OF THE INVENTION

This application relates to Implantable Medical Devices (IMDs), and more specifically to circuitry and methods to create high- and low-frequency multiplexed pulses in an implantable stimulator device.

INTRODUCTION

Implantable neurostimulator devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability with any implantable neurostimulator device system.

An SCS system typically includes an Implantable Pulse Generator (IPG)10shown inFIG. 1. The IPG10includes a biocompatible device case12that holds the circuitry and a battery14for providing power for the IPG to function. The IPG10is coupled to tissue-stimulating electrodes16via one or more electrode leads that form an electrode array17. For example, one or more percutaneous leads15can be used having ring-shaped or split-ring electrodes16carried on a flexible body18. In another example, a paddle lead19provides electrodes16positioned on one of its generally flat surfaces. Lead wires20within the leads are coupled to the electrodes16and to proximal contacts21insertable into lead connectors22fixed in a header23on the IPG10, which header can comprise an epoxy for example. Once inserted, the proximal contacts21connect to header contacts24within the lead connectors22, which are in turn coupled by feedthrough pins25through a case feedthrough26to stimulation circuitry28within the case12, which stimulation circuitry28is described below.

In the illustrated IPG10, there are thirty-two electrodes (E1-E32), split between four percutaneous leads15, or contained on a single paddle lead19, and thus the header23may include a 2×2 array of eight-electrode lead connectors22. However, the type and number of leads, and the number of electrodes, in an IPG is application specific and therefore can vary. The conductive case12can also comprise an electrode (Ec). In a SCS application, the electrode lead(s) are typically implanted in the spinal column proximate to the dura in a patient's spinal cord, preferably spanning left and right of the patient's spinal column. The proximal contacts21are tunneled through the patient's tissue to a distant location such as the buttocks where the IPG case12is implanted, at which point they are coupled to the lead connectors22. In other IPG examples designed for implantation directly at a site requiring stimulation, the IPG can be lead-less, having electrodes16instead appearing on the body of the IPG10. The IPG lead(s) can be integrated with and permanently connected to the IPG10in other solutions. The goal of SCS therapy is to provide electrical stimulation from the electrodes16to alleviate a patient's symptoms, such as chronic back pain.

IPG10can include an antenna27aallowing it to communicate bi-directionally with a number of external devices discussed subsequently. Antenna27aas shown comprises a conductive coil within the case12, although the coil antenna27acan also appear in the header23. When antenna27ais configured as a coil, communication with external devices preferably occurs using near-field magnetic induction. IPG10may also include a Radio-Frequency (RF) antenna27b. InFIG. 1, RF antenna27bis shown within the header23, but it may also be within the case12. RF antenna27bmay comprise a patch, slot, or wire, and may operate as a monopole or dipole. RF antenna27bpreferably communicates using far-field electromagnetic waves, and may operate in accordance with any number of known RF communication standards, such as Bluetooth, Zigbee, WiFi, MICS, and the like.

Stimulation in IPG10is typically provided by pulses each of which may include a number of phases such as30aand30b, as shown in the example ofFIG. 2A. Stimulation parameters typically include amplitude (current I, although a voltage amplitude V can also be used); frequency (f); pulse width (PW) of the pulses or of its individual phases such as30aand30b; the electrodes16selected to provide the stimulation; and the polarity of such selected electrodes, i.e., whether they act as anodes that source current to the tissue or cathodes that sink current from the tissue. These and possibly other stimulation parameters taken together comprise a stimulation program that the stimulation circuitry28in the IPG10can execute to provide therapeutic stimulation to a patient.

In the example ofFIG. 2A, electrode E1has been selected as an anode (during its first phase30a), and thus provides pulses which source a positive current of amplitude +I to the tissue. Electrode E2has been selected as a cathode (again during first phase30a), and thus provides pulses which sink a corresponding negative current of amplitude −I from the tissue. This is an example of bipolar stimulation, in which only two lead-based electrodes are used to provide stimulation to the tissue (one anode, one cathode). However, more than one electrode may be selected to act as an anode at a given time, and more than one electrode may be selected to act as a cathode at a given time. Note that at any time the current sourced to the tissue (e.g., +I at E1during phase30a) equals the current sunk from the tissue (e.g., −I at E2during phase30a) to ensure that the net current injected into the tissue is zero.

IPG10as mentioned includes stimulation circuitry28to form prescribed stimulation at a patient's tissue.FIG. 3shows an example of stimulation circuitry28, which includes one or more current sources40iand one or more current sinks42i. The sources and sinks40iand42ican comprise Digital-to-Analog converters (DACs), and may be referred to as PDACs40iand NDACs42iin accordance with the Positive (sourced, anodic) and Negative (sunk, cathodic) currents they respectively issue. In the example shown, a NDAC/PDAC40i/42ipair is dedicated (hardwired) to a particular electrode node ei39. Each electrode node ei39is connected to an electrode Ei16via a DC-blocking capacitor Ci38, for the reasons explained below. The stimulation circuitry28in this example also supports selection of the conductive case12as an electrode (Ec12), which case electrode is typically selected for monopolar stimulation. In some designs, the case electrode Ec12may not have a DC-blocking capacitor38, and therefore not all potential electrode nodes selected for stimulation may have a DC-blocking capacitor. PDACs40iand NDACs42ican also comprise voltage sources.

Proper control of the PDACs40iand NDACs42iallows any of the electrodes16to act as anodes or cathodes to create a current through a patient's tissue, R, hopefully with good therapeutic effect. In the example shown, electrode E1has been selected as an anode electrode to source current to the tissue R and E2as a cathode electrode to sink current from the tissue R. Thus PDAC401and NDAC422are activated and digitally programmed to produce the desired current, I, with the correct timing (e.g., in accordance with the prescribed frequency F and pulse widths PWa and PWb). Power for the stimulation circuitry28is provided by a compliance voltage VH, as described in further detail in U.S. Patent Application Publication 2013/0289665. More than one anode electrode and more than one cathode electrode may be selected at one time, and thus current can flow through the tissue R between two or more of the electrodes16.

Other stimulation circuitries28can also be used in the IPG10. In an example not shown, a switching matrix can intervene between the one or more PDACs40iand the electrode nodes ei39, and between the one or more NDACs42iand the electrode nodes. Switching matrices allows one or more of the PDACs or one or more of the NDACs to be connected to one or more electrode nodes at a given time. Various examples of stimulation circuitries can be found in U.S. Pat. Nos. 6,181,969, 8,606,362, 8,620,436, and U.S. Patent Application Publications 2018/0071520 and 2019/0083796.

Much of the stimulation circuitry28ofFIG. 3, including the PDACs40iand NDACs42i, the switch matrices (if present), and the electrode nodes ei39can be integrated on one or more Application Specific Integrated Circuits (ASICs), as described in U.S. Patent Application Publications 2012/0095529, 2012/0092031, and 2012/0095519. As explained in these references, ASIC(s) may also contain other circuitry useful in the IPG10, such as telemetry circuitry (for interfacing off chip with telemetry antennas27aand/or27b), circuitry for generating the compliance voltage VH, various measurement circuits, etc.

Also shown inFIG. 3are DC-blocking capacitors Ci38placed in series in the electrode current paths between each of the electrode nodes ei39and the electrodes Ei16(including the case electrode Ec12). The DC-blocking capacitors38act as a safety measure to prevent DC current injection into the patient, as could occur for example if there is a circuit fault in the stimulation circuitry28. The DC-blocking capacitors38are typically provided off-chip (off of the ASIC(s)), and instead may be provided in or on a circuit board in the IPG10used to integrate its various components, as explained in U.S. Patent Application Publication 2015/0157861.

Referring again toFIG. 2A, the stimulation pulses as shown are biphasic, with each pulse comprising a first phase30afollowed thereafter by a second phase30bof opposite polarity. Biphasic pulses are useful to actively recover any charge that might be stored on capacitive elements in the electrode current paths, such as on the DC-blocking capacitors38. Charge recovery is shown with reference to bothFIGS. 2A and 2B. During the first pulse phase30a, charge will build up across the DC-blockings capacitors C1and C2associated with the electrodes E1and E2used to produce the current, giving rise to voltages Vc1and Vc2which increase in accordance with the magnitude of the current and the capacitance of the capacitors38(dV/dt=I/C). During the second pulse phase30b, when the polarity of the current I is reversed at the selected electrodes E1and E2, the stored charge on capacitors C1and C2is actively recovered, and thus voltages Vc1and Vc2fall and hopefully return to 0V at the end the second pulse phase30b.

To recover all charge by the end of the second pulse phase30bof each pulse (Vc1=Vc2=0V), the first and second phases30aand30bare charged balanced at each electrode, with the first pulse phase30aproviding a charge of +Q and the second pulse phase30bproviding a charge of −Q. In the example shown, such charge balancing is achieved by using the same pulse width (PW) and the same amplitude (|I|) for each of the opposite-polarity pulse phases30aand30b. However, the pulse phases30aand30bmay also be charged balance if the product of the amplitude and pulse widths of the two phases30aand30bare equal, or if the area under each of the phases is equal, as is known.

FIG. 3shows that stimulation circuitry28can include passive recovery switches41i, which are described further in U.S. Patent Application Publications 2018/0071527 and 2018/0140831. Passive recovery switches41imay be attached to each of the electrode nodes ei39, and are used to passively recover any charge remaining on the DC-blocking capacitors Ci38after issuance of the second pulse phase30b—i.e., to recover charge without actively driving a current using the DAC circuitry. Passive charge recovery can be prudent, because non-idealities in the stimulation circuitry28may lead to pulse phases30aand30bthat are not perfectly charge balanced. Therefore, and as shown inFIG. 2A, passive charge recovery typically occurs after the issuance of second pulse phases30b, for example during at least a portion30cof the quiet periods between the pulses, by closing passive recovery switches41i. As shown inFIG. 3, the other end of the switches41inot coupled to the electrode nodes ei39are connected to a common reference voltage, which in this example comprises the voltage of the battery14, Vbat, although another reference voltage could be used. As explained in the above-cited references, such passive charge recovery tends to equilibrate the charge on the DC-blocking capacitors38by placing the capacitors in parallel between the reference voltage (Vbat) and the patient's tissue.

FIG. 4shows an external trial stimulation environment that may precede implantation of an IPG10in a patient. During external trial stimulation, stimulation can be tried on a prospective implant patient without going so far as to implant the IPG10. Instead, one or more trial electrode arrays17′ (e.g., one or more trial percutaneous leads15or trial paddle leads19) are implanted in the patient's tissue at a target location52, such as within the spinal column as explained earlier. The proximal ends of the trial electrode array(s)17′ exit an incision54in the patient's tissue and are connected to an External Trial Stimulator (ETS)50. The ETS50generally mimics operation of the IPG10, and thus can provide stimulation to the patient's tissue as explained above. See, e.g., U.S. Pat. No. 9,259,574, disclosing a design for an ETS. The ETS50is generally worn externally by the patient for a short while (e.g., two weeks), which allows the patient and his clinician to experiment with different stimulation parameters to hopefully find a stimulation program that alleviates the patient's symptoms (e.g., pain). If external trial stimulation proves successful, the trial electrode array(s)17′ are explanted, and a full IPG10and a permanent electrode array17(e.g., one or more percutaneous15or paddle19leads) are implanted as described above; if unsuccessful, the trial electrode array(s)17′ are simply explanted.

Like the IPG10, the ETS50can include one or more antennas to enable bi-directional communications with external devices such as those shown inFIG. 5. Such antennas can include a near-field magnetic-induction coil antenna56a, and/or a far-field RF antenna56b, as described earlier. ETS50may also include stimulation circuitry58(FIG. 4) able to form stimulation in accordance with a stimulation program, which circuitry may be similar to or comprise the same stimulation circuitry28(FIG. 3) present in the IPG10. ETS50may also include a battery (not shown) for operational power.

FIG. 5shows various external devices that can wirelessly communicate data with the IPG10or ETS50, including a patient, hand-held external controller60, and a clinician programmer70. Both of devices60and70can be used to wirelessly transmit a stimulation program to the IPG10or ETS50—that is, to program their stimulation circuitries28and58to produce stimulation with a desired amplitude and timing described earlier. Both devices60and70may also be used to adjust one or more stimulation parameters of a stimulation program that the IPG10is currently executing. Devices60and70may also wirelessly receive information from the IPG10or ETS50, such as various status information, etc.

External controller60can be as described in U.S. Patent Application Publication 2015/0080982 for example, and may comprise a controller dedicated to work with the IPG10or ETS50. External controller60may also comprise a general purpose mobile electronics device such as a mobile phone which has been programmed with a Medical Device Application (MDA) allowing it to work as a wireless controller for the IPG10or ETS, as described in U.S. Patent Application Publication 2015/0231402. External controller60includes a user interface, preferably including means for entering commands (e.g., buttons or selectable graphical elements) and a display62. The external controller60's user interface enables a patient to adjust stimulation parameters, although it may have limited functionality when compared to the more-powerful clinician programmer70, described shortly.

The external controller60can have one or more antennas capable of communicating with the IPG10. For example, the external controller60can have a near-field magnetic-induction coil antenna64acapable of wirelessly communicating with the coil antenna27aor56ain the IPG10or ETS50. The external controller60can also have a far-field RF antenna64bcapable of wirelessly communicating with the RF antenna27bor56bin the IPG10or ETS50.

Clinician programmer70is described further in U.S. Patent Application Publication 2015/0360038, and can comprise a computing device72, such as a desktop, laptop, or notebook computer, a tablet, a mobile smart phone, a Personal Data Assistant (PDA)-type mobile computing device, etc. InFIG. 5, computing device72is shown as a laptop computer that includes typical computer user interface means such as a screen74, a mouse, a keyboard, speakers, a stylus, a printer, etc., not all of which are shown for convenience. Also shown inFIG. 5are accessory devices for the clinician programmer70that are usually specific to its operation as a stimulation controller, such as a communication “wand”76coupleable to suitable ports on the computing device72, such as USB ports79for example.

The antenna used in the clinician programmer70to communicate with the IPG10or ETS50can depend on the type of antennas included in those devices. If the patient's IPG10or ETS50includes a coil antenna27aor56a, wand76can likewise include a coil antenna80ato establish near-filed magnetic-induction communications at small distances. In this instance, the wand76may be affixed in close proximity to the patient, such as by placing the wand76in a belt or holster wearable by the patient and proximate to the patient's IPG10or ETS50. If the IPG10or ETS50includes an RF antenna27bor56b, the wand76, the computing device72, or both, can likewise include an RF antenna80bto establish communication at larger distances. The clinician programmer70can also communicate with other devices and networks, such as the Internet, either wirelessly or via a wired link provided at an Ethernet or network port.

To program stimulation programs or parameters for the IPG10or ETS50, the clinician interfaces with a clinician programmer graphical user interface (GUI)82provided on the display74of the computing device72. As one skilled in the art understands, the GUI82can be rendered by execution of clinician programmer software84stored in the computing device72, which software may be stored in the device's non-volatile memory86. Execution of the clinician programmer software84in the computing device72can be facilitated by control circuitry88such as one or more microprocessors, microcomputers, FPGAs, DSPs, other digital logic structures, etc., which are capable of executing programs in a computing device, and which may comprise their own memories. For example, control circuitry88can comprise an i5 processor manufactured by Intel Corp, as described at https://www.intel.com/content/www/us/en/products/processors/core/i5-processors.html. Such control circuitry88, in addition to executing the clinician programmer software84and rendering the GUI82, can also enable communications via antennas80aor80bto communicate stimulation parameters chosen through the GUI82to the patient's IPG10.

The user interface of the external controller60may provide similar functionality because the external controller60can include the same hardware and software programming as the clinician programmer. For example, the external controller60includes control circuitry66similar to the control circuitry88in the clinician programmer70, and may similarly be programmed with external controller software stored in device memory.

SUMMARY

In a first example, a stimulator device is disclosed which may comprise: a plurality of electrode nodes, each electrode node configured to be coupled to one of a plurality of electrodes configured to contact a patient's tissue; and stimulation circuitry configured to provide in a single timing channel a sequence of pulses at at least two of the electrode nodes selected to create a stimulation current through the patient's tissue, wherein the stimulation circuitry is configured to form each pulse at the selected electrode nodes with a first phase and a second phase, wherein the first phase at each selected electrode node comprises a plurality of first monophasic sub-phase pulses of a first polarity and separated by gaps during which no current is issued to the tissue, wherein the second phase at each selected electrode node comprises a plurality of second monophasic sub-phase pulses of a second polarity opposite the first polarity and separated by gaps during which no current is issued to the tissue, and wherein at each selected electrode node a first total charge of the plurality of first monophasic sub-phase pulses is equal but opposite a second total charge of the plurality of second monophasic sub-phase pulses.

The stimulator device may further comprise a case for housing the stimulation circuitry, wherein the case is conductive, and wherein the conductive case comprises one of the plurality of electrodes. At least one selected electrode node may be coupled to its associated electrode through a DC-blocking capacitor. The stimulator device may further comprise at least one implantable lead, wherein the electrodes are located on the lead.

Each first and second monophasic sub-phase pulse may be of a constant amplitude, which may comprises a constant current. An amplitude of the first monophasic sub-phase pulses may vary during the first pulse phase, or an amplitude of the second monophasic sub-phase pulses may vary during the second pulse phase. The amplitude of the first monophasic sub-phase pulses may vary during the first pulse phase and the amplitude of the second monophasic sub-phase pulses may vary during the second pulse phase. A pulse width of the first monophasic sub-phase pulses may vary during the first pulse phase, or a pulse width of the second monophasic sub-phase pulses may vary during the second pulse phase. The pulse width of the first monophasic sub-phase pulses may varies during the first pulse phase and the pulse width of the second monophasic sub-phase pulses may vary during the second pulse phase. A frequency of the first monophasic sub-phase pulses may vary during the first pulse phase, or a frequency of the second monophasic sub-phase pulses may vary during the second pulse phase. The frequency of the first monophasic sub-phase pulses may vary during the first pulse phase and the frequency of the second monophasic sub-phase pulses may vary during the second pulse phase.

The stimulator device may further comprise control circuitry, wherein the control circuitry is configured to receive a plurality of stimulation parameters including a first frequency of the pulses, a second frequency of the first and second monophasic sub-phase pulses, a pulse width of at least one of the first and second phases, and a pulse width of the first and second monophasic pulses, and the control circuitry may be configured to use the stimulation parameters to provide a plurality of control signals to the stimulation circuitry to cause the stimulation circuitry to form the sequence of pulses at the selected electrode nodes. The stimulator device may further comprise an antenna, wherein the control circuitry is configured to receive the stimulation parameters from the antenna. The control circuitry may be configured to produce a first digital signal at the second frequency, a second digital signal at the first frequency and corresponding to a timing of the first phase, and a third digital signal at the first frequency and corresponding to a timing of the second phase. The stimulation circuitry may comprise a plurality of switches, wherein the plurality of switches are controlled by a mixture of the first and second digital signals during the first phase, and wherein the plurality of switches are controlled by a mixture of the first and third digital signals during the second phase.

The stimulator device may comprise an implantable pulse generator or an external stimulator.

The stimulation circuitry may be configured to form an interphase period at the selected electrode nodes between the first phase and the second phase, wherein no current is issued to the tissue during the interphase period.

The first monophasic sub-phase pulses may be positive at at least one of the selected electrode nodes and negative at at least one other of the selected electrode nodes such that the net current injected into the tissue at any time is zero during the first phase, and the second monophasic sub-phase pulses may be negative at the at least one of the selected electrode nodes and positive at the at least one other of the selected electrode nodes such that the net current injected into the tissue at any time is zero during the second phase.

In a second example, a stimulator device is disclosed which may comprise: a plurality of electrode nodes, each electrode node configured to be coupled to one of a plurality of electrodes configured to contact a patient's tissue; and stimulation circuitry configured to provide a sequence of pulses at at least two of the electrode nodes selected to create a stimulation current through the patient's tissue, wherein the stimulation circuitry is configured to form each pulse at the selected electrode nodes with a first phase and a second phase, wherein one of the first or second phases at each selected electrode node comprises a plurality of monophasic sub-phase pulses of a first polarity and separated by first gaps, wherein a non-zero current of the first polarity is provided during the first gaps, wherein at each selected electrode node a first total charge of the plurality of monophasic sub-phase pulses plus the non-zero current is equal but opposite a second total charge of the second phase.

The stimulator device may further comprising a case for housing the stimulation circuitry, wherein the case is conductive, and wherein the conductive case comprises one of the plurality of electrodes. At least one selected electrode node may be coupled to its associated electrode through a DC-blocking capacitor. The stimulator device may further comprise at least one implantable lead, wherein the electrodes are located on the lead.

Each monophasic sub-phase pulses may be of a constant amplitude, which may comprise a constant current. The other of the first or second phases at each selected electrode node may comprise a plurality of monophasic sub-phase pulses of a second polarity opposite the first polarity and separated by second gaps, wherein a non-zero current of the second polarity is provided during the second gaps.

The other of the first or second phases at each selected electrode node may comprise a constant pulse.

An amplitude of the monophasic sub-phase pulses may vary during the one of the first or second phases. The non-zero current provided during the first gaps may be constant during the one of the first or second phases. The non-zero current provided during the first gaps may also vary during the one of the first or second phases. A pulse width of the monophasic sub-phase pulses may vary during the one of the first or second phases. A frequency of the monophasic sub-phase pulses may vary during the one of the first or second phases.

The stimulator device may further comprise control circuitry, wherein the control circuitry is configured to receive stimulation parameters including a first frequency of the pulses, a second frequency of the monophasic sub-phase pulses, a pulse width of at least one of the first and second phases, and a pulse width of the monophasic pulses, wherein the control circuitry is configured to use the stimulation parameters to provide a plurality of control signals to the stimulation circuitry to cause the stimulation circuitry to form the sequence of pulses at the selected electrode nodes. The stimulator device may further comprise an antenna, wherein the control circuitry is configured to receive the stimulation parameters from the antenna.

The stimulator device may comprises an implantable pulse generator or an external stimulator.

The stimulation circuitry may be configured to form an interphase period at the selected electrode nodes between the first phase and the second phase, wherein no current is issued to the tissue during the interphase period.

The monophasic sub-phase pulses may be positive at at least one of the selected electrode nodes and negative at at least one other of the selected electrode nodes such that the net current injected into the tissue at any time is zero during the one of the first or second phases.

The stimulation circuitry may be configured to provide the sequence of pulses in a single timing channel.

In a third example, a stimulator device is disclosed, which may comprise: a plurality of electrode nodes, each electrode node configured to be coupled to one of a plurality of electrodes configured to contact a patient's tissue; and stimulation circuitry configured to provide in a single timing channel a sequence of pulses at at least two of the electrode nodes selected to create a stimulation current through the patient's tissue, wherein the stimulation circuitry is configured to form each pulse at the selected electrode nodes with a first phase and a second phase, wherein the first phase at each selected electrode node comprises a plurality of first monophasic sub-phase pulses of a first polarity and separated by first gaps, wherein the second phase at each selected electrode node comprises a plurality of second monophasic sub-phase pulses of a second polarity opposite the first polarity and separated by second gaps, and wherein at each selected electrode node a first total charge of the first phase is equal but opposite a second total charge of the second phase.

No current may be issued to the current during the first and second gaps. A non-zero current of the first polarity may be provided during the first gaps, and a non-zero current of the second polarity may be provided during the second gaps. Alternatively, a non-zero current of the same polarity may be provided during the first and second gaps, and the non-zero current may have the same amplitude during the first and second gaps.

DETAILED DESCRIPTION

Traditionally, pain relief in Spinal Cord Stimulation (SCS) systems was achieved by using a sequence of pulses operating at a relatively low frequency, fL, as shown inFIG. 6A.FIG. 6Ashows a stimulation program essentially similar to that illustrated inFIG. 2A, in which biphasic pulses with phases100aand100bare used to create a current I between any electrodes (e.g., E1and E2) selected from the plurality of electrodes. Here it is assumed that the duration PWa of both phases100aand100bof the pulses are the same as well as their amplitudes I for balanced active charge recovery, although these pulse widths/amplitudes can differ while still achieving charge-balanced phases, as explained earlier. Also shown is an interphase period (IP) between the phases100aand100bin each pulse, during which no current is provided; this is useful to provide time to allow switching in the stimulation circuitry28/58in the IPG or ETS to stabilize after exiting a first pulse phase100aand before entering the second pulse phase100b. Further shown are the passive charge recovery periods100cthat can occur during quiet periods100dwhen passive recovery switches41i(FIG. 3) can be closed, again as explained earlier. Generally speaking, the low frequency fLof the pulses inFIG. 6Acan comprise 200 Hz or less.

The use of lower-frequency pulses for spinal cord stimulation has been reported by patients, in addition to pain relief, to sometimes cause paresthesia, i.e., a tickling, prickling, or temperature-change sensation. While some patients don't mind, or may actually enjoy, the feeling of paresthesia, other patients would prefer to not feel paresthesia. High-frequency stimulation has been reported as helpful in reducing paresthesia perhaps to sub-threshold levels, as shown inFIG. 6B. Here, biphasic pulses with charge-balanced phases102aand102bare issued at a higher frequency fH, which can comprise 2000 Hz or greater. Because the pulses are issued at a high frequency, their pulse widths are generally smaller as well, and inFIG. 6Ba single pulse width PWb is shown for both of the phases102aand102b, although again this is not strictly necessary as charge balance between the phases can be achieved in different manners.

While high-frequency pulses such as those shown inFIG. 6Bmay be helpful in reducing or eliminating paresthesia, such pulses also present implementation challenges in an IPG10or ETS50. First, the quiet period102dbetween the pulses—i.e., after a second pulse phase102band before a next first pulse phase102a—is short. This can make use of passive charge recovery during periods102cdifficult or ineffective, either because there is not enough time to close the passive recovery switches41i(FIG. 3), or because the time period during which those switches can be closed is not long enough to allow the DC-blocking capacitors38(FIG. 3) to equilibrate charge that may be stored on them. See, e.g., U.S. patent application Ser. No. 15/799,499, filed Oct. 31, 2017, discussing this problem in further detail.

Second, it can also be difficult to accommodate a significant interphase period (IP) between each of pulses phases102aand102b, because again there may not be sufficient time to implement such a period. This can make switching in the stimulation circuitry28/58difficult, and can lead to unwanted ringing in the pulse phases.

High-frequency pulses as shown inFIG. 6Balso require more power to produce. As one skilled in the art will appreciate, higher-frequency pulses require more frequent switching in the stimulation circuitry28/58used to form the pulses. This leads to higher power consumption compared to lower-frequency pulses that require less frequent switching, even if the time-average of the current is the same for the lower and higher frequency pulses (e.g., if they have the same on/off duty cycle). This is especially problematic for an IPG10powered by a battery14, which is implanted and cannot be changed. High-frequency pulses will cause a permanent battery14to deplete more quickly, which could require explantation of the IPG10before its useful life is otherwise spent. If the battery14is rechargeable, high-frequency pulses will require more frequent external (wireless) charging of the battery, possibly to a point where such charging becomes inconvenient or is impractical. The need to more frequently externally charge a rechargeable battery14also accelerates the battery's degradation, again running the risk of the need for a premature explantation.

InFIG. 6B, the high-frequency biphasic pulses are free running, effectively issuing without cessation over some therapeutic time period. One manner of mitigating increased power consumption when using high-frequency pulses is to issue such pulses in packets103, as shown inFIG. 6C. As shown, a number of biphasic pulses pulse of high frequency fHare issued in each packet103, with each packet103being followed by a period104of no current (stimulation). This reduces power consumption, both because the time-averaged current is reduced (no current issues during periods104), and because the number of times the stimulation circuitry28/58must be switched is reduced. However, it can still be difficult to provide passive charge recovery102cduring the quiet102dperiods between the biphasic pulses in each packet103, and again it is difficult to provide a significant interphase period (IP) to assist with switching between pulse phases102aand102b.

To remedy these problems while still providing reduced paresthesia (sub-threshold) stimulation to a patient, new stimulation waveforms, and methods and circuitry for producing them, are disclosed having both low-frequency (fL) and high-frequency (fH) features. A first example of such a waveform is shown inFIG. 7.FIG. 7shows a sequence of pulses, with each pulse comprising a first phase106afollowed by a second phase106b. These pulses are issued at a low frequency fL, similar to the pulses ofFIG. 6A. The duration PWLof each of pulse phases106aand106bmay equal the duration PWa of the pulse phases100aand100bof the low-frequency biphasic pulses ofFIG. 6A, but this is not strictly necessary.

Each pulse phase106aand106bis fractionalized into sub-phase pulses107aand107b. Each of the sub-phase pulses107aand107bare issued at a high frequency fH, similar to the high frequency at which the biphasic pulses are issued inFIGS. 6B and 6C. Notice however that the sub-phase pulses107aand107bare monophasic, not biphasic. Thus, at electrode E1, each sub-phase pulse107ais only positive during first pulse phases106a, and each sub-phase pulse107bis only negative during second pulse phases106b. At electrode E2the polarity is reversed, such that each sub-phase pulse107ais only negative during first pulse phases106a, and each sub-phase pulse107bis only positive during second pulse phases106b.

In the illustrated example, each of the sub-phase pulses107aand107bhas the same amplitude +I or −I and the same pulse width PWH. Note that at any time the current sourced to the tissue (e.g., +I at E1during sub-phase pulses107a) equals the current sunk from the tissue (e.g., −I at E2during sub-phase pulses107a) to ensure that the net current injected into the tissue at any time is zero.

The pulse width PWHof the sub-phase pulses107aand107bmay equal the duration of either of the pulse phases102aor102b(e.g., PWb), or of the total duration of both phases (e.g., 2 PWb), of the high-frequency biphasic pulses shown inFIGS. 6B and 6C. However, this is not strictly necessary.

The sub-phase pulses107aand107bare separated by gaps108of duration tH, during which, in one example, no stimulation current occurs. The duration tHof gaps108may equal the duration of the quiet periods102dbetween the high-frequency biphasic pulses shown inFIGS. 6B and 6C, but again this is not strictly necessary. The duration of the sub-phase pulses107aand107b(PWH) may equal the duration of the gaps108(tH) as shown, but again this is not necessary.

In a preferred example, the total charge provided by first pulse phase106aat each electrode equals the opposite total charge provided by the second pulse phase106b. In other words, the total charge +Q provided by the sub-phase pulses107aof the first pulse phase106aat electrode E1equals the opposite total charge −Q provided by the sub-phase pulses107bof the second pulse phase106bat electrode E1, and likewise for electrode E2but with the charge polarity reversed. This provides charge balance to the pulses at each electrode.

Notice that while the sub-phase pulses107aand107bare issued at a high frequency fH, the fact that they are not biphasic means that the number of times the stimulation circuitry28/58must be switched is reduced. For example, and by comparison to the high frequency biphasic pulses ofFIGS. 6B and 6C, the frequency of switching can be reduced by half: whereas each high-frequency biphasic pulse at an electrode must switch on, then off (assuming an interphase period is used), then to the opposite polarity, then off, each sub-phase pulse107aor107bwill only switch on and then off during the same duration. This saves power, and is thus more considerate of the IPG or ETS battery.

As shown inFIG. 7, an interphase period (IP) may intervene between the first and second pulse phases106aand106b, and passive recovery may (closing of switches41i;FIG. 3) occur during periods106cfollowing the second pulse phases106b, which periods106ccan occur for at least a portion of quiet periods106dbetween the pulses. The duration of the passive recovery periods106cmay equal the duration of similar periods100cin the low-frequency pulses ofFIG. 6A, although this isn't necessary. Likewise, the quiet periods106dbetween the pulses may equal the duration of the similar periods106din the low-frequency pulses ofFIG. 6A, or may equal the duration104between the packets103of high-frequency pulses inFIG. 6C, although again this isn't necessary. Unlike the high-frequency biphasic pulses used inFIGS. 6B and 6C, the low-frequency, longer-time-scale aspects of the waveforms ofFIG. 7provide ample time to accommodate the interphase period (IP) and passive recovery during periods106c.

While the pulses ofFIG. 7desirably provide aspects of high-frequency paresthesia-free stimulation, they behave like low frequency pulses from a charge injection standpoint. This is illustrated by reviewing the manner in which charge accumulates on the DC-blocking capacitors38(FIG. 3) during each of the pulses (Vc1, Vc2). For the low-frequency pulses ofFIG. 6A, and as already explained (FIG. 2A), charge accumulates on the capacitors during the first pulse phases100a(Vc1and Vc2increase) and is actively recovered during the second pulse phases100b(as Vc1and Vc2decrease back to zero). For the high-frequency pulses ofFIGS. 6B and 6C, essentially the same process occurs, but on a faster time scale due to the higher-frequency bi-phasic pulses.

For the pulses ofFIG. 7, charge is injected during each sub-phase pulse107aduring the first pulse phases106a, causing Vc1and Vc2to increase. Vc1and Vc2remain constant (or may slightly decay) during the gaps108when no current is provided. Charge is then actively recovered during each sub-phase pulse107bduring each second pulse phase106b, causing Vc1and Vc2to decrease (except during the gaps108) and return to zero at the end of the second pulse phase106b. Notice that the overall charge injection profile inFIG. 7as evidenced by Vc1and Vc2is similar in shape to, and occurs on the same time scale as, the low-frequency pulses ofFIG. 6A. This is preferable to the higher-frequency charge injection that occurs in the high-frequency pulses ofFIGS. 6B and 6C. As explained in the '499 Application referenced above, the short time scales of high-frequency biphasic pulses can make charge recovery difficult.

Preferably the waveforms ofFIG. 7are formed in a single timing channel, i.e., with each of the sub-phase pulses107aand107bat an electrode being defined and formed in a single timing channel. This is different, and more convenient, than forming some of the sub-phase pulses107aand107bin different timing channels and combining them at the electrode, as described in U.S. Pat. No. 9,358,391 for example. Plus, forming the sub-phase pulses in a single timing channel frees the other timing channels in the IPG or ETS, which may now be used to provide pulses at different electrodes, therefore allowing more complex therapies to be provided to the patient. Use of timing channels in an IPG is discussed further in U.S. Pat. Nos. 6,516,227 and 9,656,081, which are incorporated herein by reference.

FIG. 8shows a Graphical User Interface (GUI)120which can be used to program an IPG115to provide the pulses ofFIG. 7. GUI120may be provided on an external device, such as the external controller60or clinician programmer70ofFIG. 5. One skilled in the art will understand that the particulars of the GUI120will depend on where the external device's software is in its execution, which may depend on the GUI selections the clinician or patient has previously made. The instructions for GUI120can be stored on a non-transitory computer readable media, such as a solid state, optical, or magnetic memory, and loaded into the relevant external device.

FIG. 8shows the GUI120at a point allowing for the manual setting of stimulation parameters for the patient. A stimulation parameters interface122is provided in which specific stimulation parameters can be defined for a stimulation program. Adjustable settings for stimulation parameters are shown, including the amplitude I of the stimulation pulses, and, as particularly relevant to the pulses ofFIG. 7, settings to adjust the low (fL) and high (fH) frequency aspects of the pulses. Pulse widths PWLand PWHare also provided to set the duration of the pulse phases106a/106band the duration of each of the sub-pulse phases107a/107brespectively. GUI120assumes for simplicity that PWLwill be the same for each of the pulse phases106aand106band that and PWHwill be the same for the sub-phase pulses107aand107b, but this isn't necessary, and instead means can be provided to allow these parameters to be set separately. The duration tHof the gaps108could also be made adjustable in the stimulation parameters interface122, but this isn't shown for simplicity.

FIG. 9shows an illustration of an IPG or ETS115capable of forming the pulses ofFIG. 7. As discussed in the Introduction, the stimulation parameters entered from the GUI120ofFIG. 8can be wirelessly transmitted by the external device60or70to an antenna in the IPG or ETS115, including the amplitude I, low frequency fL, high frequency fH, low-frequency pulse width PWL, high-frequency pulse width PWH, the anode and cathode electrodes (A and C) selected to receive the stimulation pulses, and the relative percentage X of amplitude each anode and cathode is to receive (not shown inFIG. 9for simplicity).

InFIG. 9, the stimulation parameters, once wirelessly received, are provided to control circuitry140. Control circuitry140may comprise a microcontroller for example, such as Part Number MSP430, manufactured by Texas Instruments, which is described in data sheets at http://www.ti.com/lsds/ti/microcontroller/16-bit_msp430/overview.page? DCMP=MCU_other& HQS=msp430. The control circuitry140more generally can comprise a microprocessor, Field Programmable Grid Array, Programmable Logic Device, Digital Signal Processor or like devices. Control circuitry140may also be based on well-known ARM microcontroller technology. Control circuitry140may include a central processing unit capable of executing instructions, with such instructions stored in volatile or non-volatile memory within or associated with the control circuitry. Control circuitry140may also include, operate in conjunction with, or be embedded within an Application Specific Integrated Circuit (ASIC), such as described in U.S. Patent Application Publications 2008/0319497, 2012/0095529, 2018/0071513, or 2018/0071520, which are incorporated herein by reference. The control circuitry140may comprise an integrated circuit with a monocrystalline substrate, or may comprise any number of such integrated circuits operating as a system. Control circuitry may also be included as part of a System-on-Chip (SoC) or a System-on-Module (SoM) which may incorporate memory devices and other digital interfaces.

InFIG. 9, the control circuitry140includes pulse logic142, which receives the stimulation parameters and forms various control signals144for the stimulation circuitry28/58. Such control signals144specify the timing and polarity of the stimulation pulses appearing at each of the selected electrodes, as well as the amplitude of the current each selected electrode will provide. As relevant to forming the waveforms ofFIG. 7, the pulse logic142will in particular receive the information relevant to the timing of the waveforms, e.g., fL, fH, PWL, and PWH, and use this information to form the waveforms with the prescribed timing.

FIG. 10shows another example of circuitry that can be used to form the pulses ofFIG. 7in IPG or ETS115. In this example, the control circuitry140outputs a high-frequency digital signal H and low-frequency digital signals La and Lb with the appropriate timings and mixes them. The mixed signals are then used to control switches146iand148iadded to the stimulation circuitry28/58.

Before describing the digital signals, the stimulation circuitry28/58as modified is described. As just mentioned, the stimulation circuitry28/58includes a switch146iin the current path between a given PDAC40iand a given DC-blocking capacitor Ci, and a switch148iin the current path between a given NDAC42iand the DC-blocking capacitor Ci. This establishes different electrode nodes39pfor each PDAC output (eip) and different electrodes nodes39nfor each NDAC output (ein). Passive recovery switches41iare connected between the switches146iand148iand as before are coupled to the inside plate of the DC-blocking capacitors38.

Control circuitry140forms a high-frequency digital signal H, which is shown for simplicity as a free running signal. However, this is not strictly necessary, as H may instead only be issued at times that the stimulation circuitry28/58is scheduled to issue pulses—i.e., during low-frequency signals La and Lb, as explained further below. High-frequency signal H is formed at high frequency fHwith pulse width PWH, leaving gaps of duration tHas explained previously.

Control circuitry140also forms low-frequency digital signals La and Lb, at a low frequency fLwith pulse width PWL. The timing of La and Lb correspond to the timing of first pulse phase106aand second pulse phase106brespectively.

Logic circuitry149receives H, La and Lb, and forms control signals for the switches146icoupled to the PDACs and switches148icoupled to the NDACs. (Logic circuitry149may be implemented and comprise part of control circuitry140). Specifically, switch1461is controlled by control signal1P; switch1462is controlled by control signal2P; etc. Switch1481is controlled by control signal1N; switch1482is controlled by control signal2N; etc. Digital signals H and La are mixed to form a digital signal Ma having both the high- and low-frequency timing information, which mixing can be achieved using an AND gate150a. Likewise, digital signals H and Lb are mixed to form digital signal Mb using AND gate150b.

Mixed signal Ma is used to control switches146iand/or148iduring the first pulse phase106a, while mixed signal Mb is used to control these switches during the second pulse phase106b. To send Ma and Mb to the correct switches, multiplexers (MUXes)152aand152bare used. Both MUXes152aand152bare controlled in accordance with the electrodes selected to act as anodes or cathodes during the pulses phases106aand106b(A,C).

Thus, to form the pulses ofFIG. 7, MUX152ais informed that electrode E1will act as an anode during the first pulse phase106a, and that electrode E2will act as a cathode during the first pulse phase106a. This will cause MUX152ato pass Ma to outputs control signals1P and2N, which will open and close switches1461and1482during the first pulse phase106aand creating the monophasic sub-phase pulses107aduring this period. Note that control circuitry140has programmed PDAC401associated with switch1461and NDAC422associated with switch1482with the prescribed amplitude I so that the sub-phase pulses107awill form at electrodes E1and E2with the correct amplitude.

Similarly, MUX152bis informed that electrode E1will act as a cathode during the second pulse phase106b, and that electrode E2will act as an anode during the second pulse phase106a. This will cause MUX152bto pass Mb to outputs control signals1N and2P, which will open and close switches1462and1481during the second pulse phase106b, thus creating the monophasic sub-phase pulses107bduring this period. Again, control circuitry140has programmed PDAC402associated with switch1462and NDAC421associated with switch1481with the prescribed amplitude I.

FIGS. 11A-11Cshow other examples of stimulation waveforms having both low frequency (fL) and high frequency (fH) features. For simplicity, the waveform at only a single selected electrode (e.g., E1) is shown, although it should be understood that one or more electrodes (e.g., E2) would also be active and of the opposite polarity to ensure that the net current injected into the tissue at any time is zero, as occurred inFIG. 7.

The waveforms ofFIGS. 11A and 11Bare similar to those ofFIG. 7in that each pulse is issued at a low frequency fLand comprises a first phase160afollowed by a second phase160b. Each pulse phase160aand160bis also fractionalized into monophasic sub-phase pulses161aand161bissued at a high frequency fH, which sub-phase pulses are separated by gaps162. Although not shown, the same durations described earlier with reference toFIG. 7(PWL, PWH, tH, etc.) can apply to the phases and sub-phases shown inFIG. 11.

InFIG. 11A, the current amplitudes of the sub-phase pulses161aand161bare not constant over the duration of the first and second pulse phases160aand160b, but instead vary. The variation in the amplitudes is similar during phases160aand160b(with each ramping up and the down), but this isn't required. For example, the amplitudes of sub-phase pulses161aduring the first pulse phase160acould vary, while the amplitudes of sub-phase pulses161bduring the second pulse phase160bcould vary differently or be constant. Nonetheless, the two phases160aand160bare charge balanced at each electrode, i.e., with the total charge provided by the sub-phase pulses161aequaling +Q during the first pulse phase160aand the total charge provided by the sub-phase pulses161bequaling −Q during the second pulse phase160b.

InFIG. 11B, the current amplitudes of the sub-pulses161aand161bare constant over the duration of the first and second pulse phases160aand160b, but the high-frequency pulse widths PWHof the sub-phase pulses161aand161bvary during each of the pulse phases. The variation in the pulse width PWHis similar during phases160aand160b, but again this isn't required. For example, the pulse width PWHof sub-phase pulses161aduring the first pulse phase160acould vary, while the pulse width PWHof sub-phase pulses161bduring the second pulse phase160bcould vary differently or be constant. Nonetheless, the two phases160aand160bare charge balanced at each electrode, i.e., with the charge provided by the sub-phase pulses161aequaling +Q during the first pulse phase160aand the charge provided by the sub-phase pulses161bequaling −Q during the second pulse phase160b. Note also that the high frequency of the sub-phase pulses161aand161bmay vary within each of the pulse phases160aand160b.

InFIG. 11C, charge balancing at each electrode is provided even though sub-phase pulses are provided during only one of the pulse phases160aor160b. For example, inFIG. 11C, only first pulse phase160ahas monophasic sub-phase pulses161a(of total charge +Q); the second pulse phase160bcomprises a constant pulse110aor110b(of charge −Q). The duration or amplitude of the constant pulse110aor110bcan vary. For example, constant pulse110ahas a pulse width equal to the duration of the first pulse phase160a, but an amplitude −I′ that is smaller than the amplitude +I of the sub-phase pulses161a. Constant pulse110bhas an amplitude −I equal to the amplitude +I of the sub-phase pulses161a, but has a pulse width that is smaller than the duration of the first pulse phase160a. In either case, the charged is balanced during both pulses phases160aand160b(+Q=|−Q|).

FIG. 12Ashows another example of stimulation waveforms having both low frequency (fL) and high frequency (fH) features. Different in the waveforms ofFIG. 12Aare current levels provided during the gaps172between the sub-phase pulses171aand171b. InFIG. 7, the current returned to zero during the gaps108between the sub-phase pulses107aand107b. InFIG. 12Ahowever, the current does not return to zero during the gaps172, but instead returns to a smaller magnitude current of the same polarity as the sub-phase pulses171aand171b. Thus, during first phase170a, electrode E1's sub-phase pulses171aare positive, and have a magnitude of +11. During the gaps172, the current returns to a smaller positive value of +I2. Electrode E2's waveform during the first phase170ais similar, but of opposite polarity to ensure that a net amount of current is not delivered to the patient's tissue. E2's sub-phase pulses171aare thus negative with a magnitude of −I1, with the current returning to a smaller negative value of −I2. During the second pulse phase170b, electrode E1's sub-phase pulses171bare negative with a magnitude of −I1, and returning to −I2during the gaps172; electrode E2's sub-phase pulses171bare positive with a magnitude of +I1, and returning to +I2during the gaps172. A passive recovery period170ccan occur during quiet periods between the pulses as before. Preferably the waveforms ofFIG. 12Aare formed in a single timing channel as discussed earlier.

The waveforms ofFIG. 12Acan be beneficial, because a small non-zero return current (I2) during gaps172can enhance polarization of neural fibers. The combination of the polarizing return current (I2) with superimposed higher-intensity sub-phase pulses171aand171b(I1) can create changes in excitability and recruitment order. For example, large fibers can be excited once, such as during a first sub-phase pulse171alin first pulse phase170a, while smaller fibers are excited multiple times by all of the sub-phase pulses171ain the first pulse phase170a, or at least during subsequent sub-phase pulses171a2.

FIG. 12Aalso shows how the GUI120(FIG. 8) can be modified to form the waveforms ofFIG. 12A. Specifically, the stimulation parameters interface122′ of GUI120′ has been changed to add for the setting or adjustment of the two current levels I1and I2used to define the pulses. Settings to adjust the low (fL) and high (fH) frequency aspects of the pulses, and pulse widths PWLand PWH, can be included as before, as can other aspects not shown inFIG. 12A.

FIG. 12Bshows a modification to the waveforms ofFIG. 12A. In this example, the non-zero return current (I2) is established at the beginning and end of each of the pulses phases170aand170b. That is, the pulse phases170aand170bstart and end with the non-zero return current, rather than starting and ending with the sub-phase pulses171aand171b(I1), as occurred inFIG. 12A. Particularly at the start of the phases170aand170b, providing the non-zero return current before the sub-phase pulses can assist in neural polarization, and hence selective recruitment of different neural fibers as described earlier.

FIGS. 13A-13Fshow other examples of stimulation waveforms having both low frequency (fL) and high frequency (fH) features, and similar toFIGS. 12A and 12Bin that the current does not return to zero in gaps182between the sub-phase pulses181aand181b. Again, the waveform at only a single selected electrode (e.g., E1) is shown.

InFIG. 13A, the current amplitudes of the sub-phase pulses181aand181b, the non-zero return current during gaps182, or both, are not constant over the duration of the first and second pulse phases180aand180b, but instead vary. Thus, during the first pulse phase180a, the amplitude of the sub-phase pulses181avaries as value +I1, while the amplitude of return current varies as value +I2, both shown generally with dotted lines. During the second pulse phase180b, the amplitude of the sub-phase pulses181band the non-zero return current vary as values −I1and −I2. Nonetheless, the two phases180aand180bare preferably charge balanced at each electrode, i.e., with the charge provided by the sub-phase pulses181aand the non-zero return current equaling +Q during the first pulse phase180aand the charge provided by the sub-phase pulses181band the non-zero return current equaling −Q during the second pulse phase180b.

FIG. 13Bshows another example in which only the amplitude of the non-zero return current (I2) during gaps182varies, and specifically is ramped; the amplitude of the sub-phase pulses181aand181bare held constant (I1). The variation in the amplitudes inFIGS. 13A and 13Bare shown to vary similarly during phases180aand180b, but as before this isn't required. For example, inFIG. 13C, the amplitude of the non-zero return current and the sub-phase pulses181aare constant during the first pulse phase180a; however, the amplitude of the non-zero return current varies during the second pulse phases180b. The amplitude of the sub-phase pulses181bcould also vary during the second pulse phase180b, although this isn't shown inFIG. 13C. Even if the pulse phases180aand180bare not symmetrical as shown inFIG. 13C, they can still be charge balanced (+Q and −Q).

InFIG. 13D, the current amplitudes of the sub-pulses181aand181b(+I1or −I1) and the return currents (+I2and −I2) are constant over the duration of the first and second pulse phases180aand180b, but the high-frequency pulse widths PWHof the sub-phase pulses181aand181bvary during each of the pulse phases. The variation in the pulse width PWHis again shown to vary similarly during phases180aand180b, but this isn't required. Again, the two phases180aand180bare charge balanced at each electrode (+Q=|−Q|).

InFIG. 13E, charge balancing at each electrode is provided even though sub-phase pulses are provided during only one of the pulse phases180aor180b. For example, inFIG. 13E, only first pulse phase180ahas monophasic sub-phase pulses181aand a non-zero return current (of total charge +Q); the second pulse phase180bcomprises a constant pulse112aor112b(of charge −Q). The duration or amplitude of the constant pulse112aor112bcan vary. For example, constant pulse112ahas a pulse width equal to the duration of the first pulse phase180a, but an amplitude −I3that is smaller than the amplitude +I1of the sub-phase pulses181aand larger than the amplitude +I2of the non-zero return current. Constant pulse112bhas an amplitude −I1equal to the amplitude +I1of the sub-phase pulses181a, but has a pulse width that is smaller than the duration of the first pulse phase180a. In either case, the charged is balanced during both pulses phases180aand180b(+Q=|−Q|).

FIG. 13Fshows a different example of waveforms in which the non-zero return current is equal and of the same polarity in both of the pulse phases180aand180b. In the first pulse phase180a, sub-phase pulses181aare provided having a positive amplitude of +I1. However, in the gaps182between the sub-phase pulses181a, the non-zero return current is negative, having an amplitude of −I2. In total, the first pulse phase180acan have a net charge of +Q, with the positive sub-phase pulses181aadding to this net value and the negative return current subtracting from this net value. In the second pulse phase180b, the sub-phase pulses181bhave a negative amplitude of −I3. In the gaps182between the sub-phase pulses181b, the non-zero return current has the same negative amplitude of −I2that occurred in the first pulse phases180a. In total, the second pulse phase180bcan have a net charge of −Q, with the negative sub-phase pulses181band the negative return current contributing to this net charge. To achieve charge balance between the first and second pulses phases180aand180b(+Q and −Q), the absolute value of −I3would be smaller than +I1. The waveform ofFIG. 13Fcan be beneficial because it provides the same degree of neural polarization during the gaps182in both of the pulse phases180aand180b.

The waveforms illustrated can also be used with passive charge recovery. This is shown inFIGS. 14A and 14Bfor a waveform having a zero return current during gaps192(FIG. 14A), and a waveform having a non-zero return current during gaps192(FIG. 14B). In these examples, sub-phase pulses191aand the non-zero return current (FIG. 14Bonly) are actively driven by the stimulation circuitry28(FIG. 3) only during a first pulse phase190a. There is no actively-driven second pulse phase. Instead, a passive recovery phase190cis provided after the first pulse phase190a. As explained earlier, passive recovery can recover charge stored on capacitive elements in the current path (e.g., between electrodes E1and E2), such as the DC-blocking capacitors38. This occurs using passive recovery switches41i, as shown in the circuit diagram ofFIG. 14C. After the first pulse phase190a, capacitors C1and C238coupled to the electrodes E1and E2would be charged (Vc1, Vc2) with the polarities as shown. When the passive recovery switches411and412connected to electrode nodes e1and e239are closed during the passive recovery phase190c, the electrode nodes are shorted to a reference voltage (e.g., Vbat). This causes the charge on the capacitors to equilibrate, causing a current193flow from E2to E1through the patient's tissue, R. Given the R-C nature of this circuit, this current193will exponentially decay, and assuming the passive recovery switches41iare closed for a long enough duration, the voltages across the capacitors and the resulting current193will decay to zero. Thus, the charge of the first pulse phases (+Q) is passively recovered during the passive recovery phase190c(−Q).

To this point, the waveforms have been shown as having pulses with only two phases, such as the first and second pulse phases106aand106bofFIG. 7, the first and second pulse phases170aand170bofFIG. 12A, or the first and passive recovery pulse phases190aand190bofFIGS. 14A and 14B. However, each pulse could have more than two pulse phases, and all pulse phases can be charged balanced with a pulse, although this is not illustrated for simplicity.

The modifications to the various waveforms illustrated to this point can be combined in different manners, even if such combinations are not illustrated in the figures. It is not practical to illustrate all of these possible combinations, but it should be understood that any combination of the various modifications can be used in a practical implementation and are within the scope of this disclosure.