A Mixed-Signal Programmable Time-Division Power-On-Reset and Volume Control Circuit for High-Resolution Hearing-Aid SoC Application

. A mixed-signal programmable Time-Division Power-On-Reset (TD-POR) circuit based on 8-bit Successive Approximation Analog-to-Digital Converter (SAR ADC) for accurate control in low-power hearing-aid System on Chip (SoC) is presented in this paper. The end-of-converter (EOC) signal of SAR ADC is used as the mode-change signal so that the circuit can detect the battery voltage and volume voltage alternately. And the TD-POR circuit also has brown-out reset (BOR) detection capability. Through digital logic circuit, the POR, BOR threshold, and delay time can be adjusted according to the system requirement. The circuit is implemented in SMIC 0.13 𝜇 m 1P8M CMOS process. The measurement results show that, in 1V power supply, the POR, BOR, and volume control function are accomplished. The detection resolution is the best among previous work. With 120 Hz input signal and 15kHz clock, the ADC shows that Signal to Noise plus Distortion Ratio (SNDR) is 46.5 dB and Effective Number Of Bits (ENOB) is 7.43bits. Total circuit power consumption is only 86 𝜇 w for low-power application.


Introduction
With the development of VLSI integrated circuits, SoC becomes more and more complex, which not only consists of transitional analog circuits, but also comprises registers, memory, and Digital Signal Processing (DSP) circuits.When SoC is powered up, some of their internal nodes are with intermediate metastable states.Therefore at the initial stage of operation, a Power-On-Reset (POR) circuit is needed.The POR signal should hold circuits in the reset state until the power supply reaches a steady-state level when all the circuits can operate correctly.And, in many applications, when supply voltage suddenly drops, the SoC may malfunction.The POR circuit also needs to generate a brown-out reset (BOR) signal for warning or reset.The parameter definition of POR and BOR is shown in Figure 1.
POR and  BOR are POR and BOR thresholds, respectively, which determine the system to enter or leave the stable state.DT POR and DT BOR are the delay time to generate the POR and BOR pulse when the supply voltage reaches  POR or drops under  BOR , respectively.DT POR and DT BOR must be set long enough to confirm all the modules have entered steady or reset state. BOR and  POR are the POR and BOR pulse width.As long as the DT POR and DT BOR meet the system specification,  POR and  BOR have no specific requirements.Transitional POR circuits are pure analog circuit that are based on delay cell and Schmitt Trigger [1][2][3][4].Delay time is set by resistor and capacitor of delay cell occupying large chip area.The resistance and capacitance devices are affected by temperature, voltage, and other parameters in the process of integrated circuit, and the uncertainty of the delay time can be easily caused.At the same time, the fixed POR and BOR threshold are generated by the bandgap-based reference, which has a large temperature drift.So it can greatly affect the detection accuracy of the POR and BOR voltage that can hardly meet the high-resolution requirement.
The zinc-air battery that supplies hearing-aid SoC has a large voltage variation from 1.4 V to 0.7 V.When supply voltage reaches or drops to the right threshold, to ensure the accurate power-on control and low-voltage warning, the POR with ADC must be needed.Moreover  POR and  BOR as well as DT POR and DT BOR should be programmable due to battery characteristic.Meanwhile the customers may adjust the volume of hearing-aid device according to the environment and their hearing condition.And the volume adjustment usually has 32 stages.For precision, an 8-bit ADC with 256 quantized steps is used to divide the adjustment stages.Since the power determines the operation duration of hearing-aid SoC and both supply voltage and volume voltage are 1 V, to minimize the power consumption and realize accurate control, a mixed-signal programmable TD-POR and volume control circuit based on SAR ADC for hearing-aid SoC is presented in this paper.The EOC signal of SAR ADC is used as the mode-change signal so that the circuit can detect the battery voltage and volume voltage alternately, which reduces both circuit size and total power consumption.

System Design Considerations
Figure 2 shows the architecture of the proposed TD-POR and volume control circuit, which consists of Time-Division input stage, 8-bit SAR ADC, clock generator, LDO, and digital logic circuit.The input stage comprises a MUX, two resistors (1, 2), and a MOS capacitor.MUX controlled by EOC signal inputs battery voltage (bat level) and volume voltage (vc level) alternately.The ratio of 1 and 2 is 2 : 3. Since in 1 V power supply the voltage range of battery and volume is 0-1 V, the two resistors' mechanism reduces the voltage detection range to 0-0.6 V to minimize the detection error.And the MOS capacitor in input stage is used to provide a channel for high frequency power supply ripple to ground and relieve the noise effect.
A complete SAR ADC conversion cycle requires 12 clock cycles, where three clock cycles are for sampling, eight cycles are for a binary searching, and half cycle is for EOC signal.Common mode voltage 0.5 V and reference voltage 0.6 V are provided by the LDO circuit.Clock generator provides 15 kHz frequency signal for ADC.EOC is divided into two outputs: one is input to the clock port of a  flip-flop.And the output of the  flip-flop controls input stage of MUX which completes the input switching.At the same time the output of the  flip-flop goes through an inverter and then generates Time-Division multiplex flag signal Vc bat mux.And Vc bat mux is output to the digital logic circuit (when Vc bat mux is high, the input voltage signal is the volume signal; when Vc bat mux is low, the input voltage signal is the battery signal).The second output of EOC goes through a delay buffer to clock ports of eight  flip-flops, which maintains the ADC 8-bit output data for a conversion cycle.Meanwhile, the output of the delay buffer is delayed by seven clock cycles and generates a digital logic read flag signal Data en, to ensure the output can be read by digital logic circuit correctly.Finally the 8-bit ADC output codes Out⟨7:0⟩ are maintained by  flip-flops for a SAR ADC conversion cycle until the next EOC signal arrives.
The inputs of digital logic circuit are 8-bit ADC output code, Data en, Vc bat mux, 4 bit  POR , and 2 bit  BOR .The final outputs of digital logic circuits are POR counting signal cnt en and POR (BOR) signal Soft rst.Firstly  POR is set after initialization.When the battery voltage starts to rise and exceeds  POR , cnt en starts counting.If the voltage is greater than  POR and lasts for 30 ms, Soft rst turns to high value; when the battery voltage drops to  BOR and also lasts for 30 ms, Soft rst turns to low value and makes system enter the reset state.According to signal Vc bat mux, TD-POR circuit determines whether the battery voltage or volume voltage is read at this time.When Vc bat mux is high, the 8-bit ADC output codes will be output to the DSP for volume control.And when Vc bat mux is low, the circuit runs into the POR mode.Timing relationships of Data en, Out⟨7:0⟩, and Vc bat mux are shown in Figure 3, Vc0, Vc1, . . .represented the volume voltage, and bat0, bat1, . . .indicated the battery voltage.
Figure 5 shows the circuit of 8-bit charge redistribution DAC.To meet the requirement of full-swing input, only one sampling capacitor (s) of 32 times unit capacitor is adopted [10].
The DAC has two phases as sampling phase and charge redistribution phase.When DAC is in sampling phase, the switches 0∼S7 are connected with GND.The switch sample is connected with IN.And Vcm is also on, so the   ( When DAC is in charge redistribution phase, the Most Significant Bit (MSB/7th bit) is initialized as "1" and the switch 7 is connected to reference voltage REF.At this time if the input voltage (IN) is greater than half of REF, the comparator output is "0" and the MSB stays as "1"; otherwise the MSB changes to "0."After that the 6th bit is initialized as "1" and 6 is connected to REF.Then the 6th bit can be "1" or "0" which depends on the output of comparator.The DAC needs eight cycles until the Least Significant Bit (LSB) is determined.And the DAC output is where   is the  bit output of DAC.
The comparator is shown in Figure 6, which comprises three preamplifiers and an output latch.The three preamplifiers with input offset reduction and output offset reduction techniques can relieve the effect of offset voltage efficiently.The output latch is adopted to change the differential output into single output and hold the output [11,12].SAR logic circuit is the most important part of SAR ADC.It realizes the feedback control of DAC.A complete ADC conversion requires twelve clock cycles, including two sampling cycles, nine successive approximation cycles, and one endsignal generation cycle.The SAR logic circuit comprises  flip-flops, inverters, AND gates, and  flip-flops, as shown in Figure 7. 0∼F7 are 8-bit SAR logic registers made up of  flip-flops., , and  constitute the start-up circuit, and timing generator consists of FA∼FJ.EN is start-up signal lasting for two clock cycles.c is the comparator output.EOC is the end-of-converter signal.7∼D0 are DAC input signal and 7∼b0 are the digital output of ADC [13].
The timing of POR signal is shown in Figure 8.The signal sample is a three-cycle sampling signal.EN is the start-up signal of SAR logic circuit.And reset and lat are the reset signal of preamplifier and latch, respectively.

Experiment Result
The proposed TD-POR circuit has been implemented in 0.13 m CMOS technology with 1 V supply.The chip is shown in Figure 9.The power consumption and the core chip area are 86 W (28 W @ LDO, 25 W @ SAR ADC, 20 W @ oscillator, and 13 W @ others) and 0.175 mm 2 (0.675 mm 2 including IO cells), respectively.Figures 10 and 11 show the measurement results.When the input signal p-p is 800 mV, the input frequency is 120 Hz and the clock frequency is 15 kHz, the FFT spectrum of ADC output signal is analyzed, the SNDR is 46.5 dB, and ENOB reaches 7.43 bits.As shown in Figure 11(a), when the voltage exceeds  POR (800 mV), cnt en turns to high value and starts counting.When its counting exceeds 30 ms, which meets the POR condition, Soft rst turns to high value and completes the power-on process.In Figure 11(b) when the battery voltage drops below  BOR (910 mV), and cnt en counts for 30 ms; Soft rst turns to low value to reset the system.The performance summary of the proposed Time-Division Power-On-Reset and volume control circuit is given in Table 1, and the performance comparisons with published POR circuits are also listed.The circuit designed in this paper is the only one POR circuit, which can be configured with  POR / BOR and delay time, and has high flexibility.It also has the highest detection accuracy.But due to the ADC-based structure the chip area is larger than other circuits.In order to optimize power consumption, because the frequency of both POR and volume control signal is low, a suitable clock signal frequency is used to reduce DAC activity, thus decreasing the dynamic switching power.On the other hand, near-threshold transistors are adopted to design preamplifiers and comparators that save much power than their saturation opponents.Since the power dissipation of traditional analog volume control circuit is in hundreds of microwatts, our work puts forward a Time-Division multiplexing way, which completes the two functions of POR/BOR  and volume control through a single circuit, and it also has excellent power consumption and detection performance.

Conclusion
A mixed-signal programmable TD-POR and volume control circuit for hearing-aid SoC is proposed in this paper.It uses the EOC signal of SAR ADC as the mode-change signal that makes circuit detect the battery voltage and volume voltage alternately.And the POR and BOR function are both included.The mixed-signal structure ensures flexible configuration of  POR / BOR and delay time, which greatly improve the stability of the application system.The circuit is implemented in SMIC 0.13 m 1P8M CMOS process.The measurement results show that, in 1 V power supply, the POR, BOR, and volume control function are accomplished.With 120 Hz input frequency and 15 kHz clock frequency, the ADC shows that SNDR is 46.5 dB and ENOB is 7.43 bits.Total circuit power consumption is 86 w.And the circuit shows the best detection resolution among all references.

Figure 1 :
Figure 1: Parameter definition of POR and BOR.

Figure 2 :
Figure 2: Block diagram of Time-Division Power-On-Reset and volume control circuit.

Figure 11 :
Figure 11: Function of measurement: (a) Soft rst for a POR event; (b) Soft rst for a BOR event.

Table 1 :
Comparison with previous work.