Novel Single-Phase Stimulus Driver with Voltage-Mode Adaptive Loading Consideration

Electrical stimulation therapies are new generations of medical science. A number of diseases that lack of properly treatment in the past are curable by functional electrical stimulation (FES) or therapeuticelectrical stimulation (TES) such as retinitis pigmentosa (RP) [2]. Thus, stimulus drivers for medical treatment become a prospective research topic currently, and a variety of design of stimulus driver have been researched and pronounced; according to these researches and applications, the mainly considerations of device for medical treatment have been thoroughly investigated too [35]. By medical prospective, safety and reliability of embedded device are prerequisite, and it must be able to tolerant variation of the operating

environment, the human body, such as moisture. By electrical engineering prospective, in order to prolong the operating time, power consumption is a vital issue for implantable device. However, there is no implantable stimulus driver take both issues into consideration.

This chapter, a new implantable stimulus driver for epileptic seizures control with adaptive loading consideration of safety, power consumption, and reliability of variety of tissue impedance is proposed.

3.3.1 Implementation

The proposed stimulus driver consists of output stage with block-capacitors, voltage-mode adaptor, and control block, as shown in Fig. 3.4. In practice, depending on different kind of stimulus sites, therapeutic requirement, and implanted time, tissue impedance varies from tens of kΩ to hundreds of kΩ. The experimental required stimulus current is 40 μA in this work. The effective impedance varies from 20 kΩ to 200 kΩ. That is to say, output voltage and required power supply are much higher than operating voltage (V_DD) that in common use. And it is obvious that variation of required operating voltage (Vcp) of output stage depends on tissue impedance correspondingly. Thus, output stage of stimulus driver adopts 0.35-μm 3.3-V/24-V BCD process which is able to tolerate 24 V at most; therefore, the proposed stimulus driver won’t suffer from problems such as gate-oxide reliability and hot-carrier degradation that might lead to failure of implantable stimulus driver. In order to reduce size of the device and power consumption, the voltage-mode adaptor and the control block adopt the 3.3-V devices with fixed operating voltage (VDD).

For safety consideration, the use of blocking-capacitors in series with the output stage of stimulus driver serves an important purpose: to prevent prolonged DC current

from flowing into the tissue in the event of semiconductor failure [36]. The current passes through the blocking-capacitors can be explained by Eq. (3-1).

Istim = C * dV / dt (3-1)

However, in order to correspond to requirement of stimulation which includes stimulus duration and amplitude of stimulus current, the size of blocking-capacitors are too large to integrate into a chip. For example, for stimulation with current amplitude of 100 μA during 1 ms, minimum capacitance requires at least 0.1 μF with 1 V voltage-drop.

Fig. 3.4. The proposed stimulus driver consists of output stage with block-capacitors, voltage-mode adaptor, and control block.

In order to resolve problem of blocking-capacitors, a number of circuit design have been studied. High-frequency current-switching (HFCS) blocking-capacitors are

utilized to reduce size of blocking-capacitors [28]. For a given stimulus current (I_stim), both reduction of pulse width (dt) or increase of voltage drop (dV) across the blocking-capacitors can lessen required size of capacitor. HFCS capacitors utilize two complementary current sources with blocking-capacitors and turn on current sources out-of-phase. Complementary stimulus current outputs (I_source1 and I_source2) converge and form a complete stimulus current (Istim) as shown in Fig. 3.5. Thus, each stimulus period is reducing, while current amplitude is unchanging, reduction ratio of blocking-capacitors is directly proportional to switching frequency. That is to say, as above example, by adopting HFCS and delivering stimulus current of 100 nS each,

Fig. 3.5. Complementary stimulus current outputs (Isource1 and Isource2) converge and form a complete stimulus current (Istim).

For the purpose of reducing size of blocking-capacitors and minimizing voltage drop on blocking-capacitors, switching frequency is set up at high frequency, 2.5

MHz. As shown in Fig. 3.4, output stage of the proposed stimulus driver adopts HFCS, during the stimulus driver “turn-on” interval, the control signal of Trigger, Tri_up, and Tri_down switch complementary. During upper stimulating phase, Tri_up is low (0 V) and Tri_down is high (3.3 V). For upper current source, Mn2 is biased through Mp1, and Mn3 is switched-off. The stimulus current is delivered by current mirror Mp2 and Mp3, and passes through C1 and D2. Meanwhile, down current source is discharging phase, Mn5 is switched-off, and Mn6 is switched-on. The charged C2 can discharge by D3, C2, and Mn6.

During down stimulating phase, Tri_up is high and Tri_down is low. For upper current source is discharging phase, Mn2 is switched-off, and Mn3 is switch-on. The charged C1 can discharge by D1, C1, and Mn3. Meanwhile, down current source is stimulating phase, stimulus current is delivered by current mirror Mp5 and Mp6, and passes through C2 and D4. During the stimulus driver “turn-off” interval, both Tri_up and Tri_down are high (3.3 V), gate of Mn2 and Mn5 are grounded, no stimulus current is delivered. According to structure of the proposed stimulus driver, equation of the operating voltage (Vcp) that output stage required is shown by Eq. (3-2).

Vcp = VDSp + ΔVC + ΔVDiode + RTissue Istim (3-2)

where VDSp denotes the voltage between drain and source terminals of Mp3 or Mp6, ΔVC denotes the voltage of C1 or C2, and ΔV_Diode is the voltage of D2 or D4. It is obvious that Vcp depends on tissue impedance (RTissue) and stimulus current (RTissue).

In addition, the last item of Eq. (3-2) usually dominates the required Vcp. Thus, if tissue impedance varies by factors mentioned above such as implanted time, the required Vcp changes largely, too.

Tissue impedance of experiment results verses implanted time in different stimulus

site of right-side Zona Incerta (ZI) of Long-Evans rats is shown in Fig. 3.6 [37]. A 4-microwire bundle, each made of Teflon-insulated stainless steel microwires, was used to stimulate the right-side ZI (posterior 4.0 mm, lateral 2.5 mm, and depth 6.7-7.2 mm). A ground electrode was implanted 2 mm caudal to the lambda. After two hours current stimulation test, the tissue impedance between arbitrarily two microwires varies from 50 kΩ to 170 kΩ.

Fig. 3.6. Tissue impedance of experiment results verses implanted time in different stimulus site (ZI) [37].

Under the condition that required seizure suppressing stimulus current is 40 μA, the difference of varying required operating voltage is 4 V. Conventional stimulus devices are used to set the operating voltage at highest requirement and enlarge voltage compliance as large as possible [31]; however, it enlarges power consumption in great amount. The tissue impedance of specification of proposed design ranges from 25 kΩ to 200 kΩ. While tissue impedance is varying with fixed stimulus current, output

voltage is changing correspondingly. Assuming headroom of stimulus driver is 1 V, the highest required operating voltage is 9 V. By detecting tissue impedance by 1-bit ADC, stimulus driver is able to provide two levels operating voltage, 5 V while tissue impedance ranges from 25 kΩ to 100 kΩ and 9V while tissue impedance ranges from 100 kΩ to 200 kΩ; therefore, power consumption of stimulus driver is 78% of conventional works. While stimulus driver adopts 2-bit ADC, the power consumption is 66% of conventional works. While stimulus driver adopts 3-bit ADC, the power consumption is 61%. Power consumption is fewer while tissue impedance is classified into more sub-groups; however, bit number of ADC is proportional to chip area and power consumption of ADC. Thus, this work adopts 3-bit ADC with consideration of chip area and power consumption. Voltage-mode adaptor as illustrated in Fig. 3.7 is used to detect output voltage every cycle of stimulation and classifies tissue impedance into 8 sub-groups. Thus, stimulus driver can adjust Vcp for each group of tissue impedance in the most power-saving way. Voltage-mode adaptor that composes of a voltage capacitance divider and a 3-bit ADC as shown in Fig. 3.8, the adapter adopts 3.3-V devices for saving chips area.

Fig. 3.7 The voltage-mode adaptor is used to detect output voltage every cycle of stimulation and classifies tissue impedance into 8 sub-groups.

Logic gate

Enable

Vin

Bit2 Bit1 Bit0 VDD

Mn

Fig. 3.8. The 3-bit ADC of voltage-mode adaptor is used to detect the voltage at output electrode.

The 3-bit ADC of adaptor adopts comparator as shown in Fig. 3.9 [38]. The comparator without DC current no matter in compare phase or hold phase is power-saving. For the purpose of converting output voltage larger than V_DD (3.3 V) into digital code, the capacitance divider is used to scale down the voltage of output stage. In accordance with product of the largest tissue impedance of 200 kΩ and stimulus current of 40 μA, the maximum convertible output voltage is 8 V, and it can be scaled down to 3.3 V by voltage divider. The relationship between input and output of capacitance voltage divider can be shown as Eq. (3-3)

Vout = Vin C3 * (C3 / C4) (3-3)

The capacitance of C3 is 1 pF, and capacitance of C4 is 1.42 pF.

Fig. 3.9. The comparator of 3-bit ADC of voltage-mode adaptor which without DC current no matter in compare phase or hold phase [37].

The Vcp is conditioned in accordance with digital output of 3-bit ADC of voltage-mode adaptor. “111” indicates the highest output voltage results from fixed stimulus current and the highest tissue impedance of specification, the highest Vcp is given. “000” indicates the lowest output voltage, the lowest Vcp is given. Assuming headroom of current source of stimulus driver is 1 V, operating voltage of conventional design and Vcp of proposed driver verses tissue impedance and corresponding digital code is shown in Fig. 3.10. While stimulation is requested, stimulus driver presets Vcp at the highest operating voltage, and delivers the first stimulus pulse. Therefore, output voltage and tissue impedance is known, according to digital output of adaptor. Namely, variety of tissue impedance is monitered. The stimulus driver can adjust Vcp before next stimulus pulse. The new proposed stimulus driver is provided with variable operating voltage, and is more power efficient.

The proposed single-phase stimulus driver to suppress epileptic seizure with

110 111

Fig. 3.10. Assuming headroom of current source of stimulus driver is 1 V, operating voltage of conventional design and Vcp of proposed driver verses tissue impedance and corresponding digital code.