INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 1, 2011

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1 Current Mode PWM generator based on Active Inductor Saberkari Alireza, Panahdar Mohammadreza, Niaraki Rahebeh Department of Electrical Engineering, University of Guilan, Rasht, Iran ABSTRACT A current mode pulse width modulator (PWM) based on active floating inductance is presented. The modulator consists of a cascade structure of a current conveyor (CCII) based active inductor as a triangular current wave generator (TCWG) and a current comparator to generate the output PWM by comparing the triangular and reference currents. The proposed current conveyor and PWM circuit are designed and the obtained performance metrics corresponding to HSPICE simulations in 0.18 μm CMOS process and ±1.5 V supply voltage reveal the performance of the modulator. Keywords: Active inductor, Current comparator, Current conveyor, Pulse width modulation. 1. Introduction Power management is very essential approach in battery powered electronic applications which often need multiple voltages. Portable electronic devices like digital cameras, cellular phones, and PDAs need efficient power management in order to reduce the standby power and increase the battery life. To satisfy the demand of battery based systems, a high efficiency DC DC converter is needed as an interface between the battery source and load circuit. In addition, the DC DC Converters can be used to supply the circuit with a dynamic range of voltage and perform more efficient power amplifier for wireless communication transceivers (Hanington et al., 1999). The control mechanism of the DC DC converter is done by sigma delta or pulse width modulation (PWM). The PWM is widely used in many fields such as communications, electronic and power electronic control, and measurement circuits. The most common implementation of PWM is comparing the input signal with a triangular (double sided) or saw tooth (single sided) signal (Siripruchyanun et al., 2000). The saw tooth or triangular signal can be generated either in analog or digital domain. Therefore, there are two possibilities: continues time or natural sampling PWM (NPWM) and discrete time or uniform sampling PWM (UPWM) (Syed et al., 2004). UPWM has not only switching aliases but also harmonic distortion due to the time quantization effects in the compared signal (Pascual et al., 2003), (Song and Sarwate, 2003). For this reason, the NPWM is widely used and multiple implementations have been proposed. But, most of the realizations are based on the voltagemode operational amplifiers which suffer from fixed gain bandwidth product and limited slew rate. These limitations affect the switching frequency of the converter. This paper presents a current mode NPWM as shown in Figure 1, in which an active inductor based on current conveyor (CCII) is used as a triangular current wave generator (TCWG) and a current comparator is cascaded to the TCWG to compare the reference and triangular currents and generate the output PWM signal. 234

2 Figure 1: Block diagram of the proposed PWM This paper is organized as follows; in Section 2, the utilized class AB CCII is introduced and the floating inductance generator based on the CCII is presented. Section 3 describes the design of the PWM controller and its operation. HSPICE simulation results of the proposed PWM in 0.18 µm CMOS process and conclusion are in sections 4 and 5, respectively. 2 Current Conveyor and Inductance Generator CCII is a useful, flexible, and basic current mode building block proposed by Sedra and Smith which is demonstrated its versatility in CMOS analog circuit design (Smith and Sedra, 1968), (Sedra and Smith, 1970). An ideal CCII is a three port device and its voltage and current characteristics are given by the following matrix; I y V y V x = I x I z V ± z (1) where the nodes (X, Y) are input nodes, the node (Z) is the output node, and the signs + and are used for positive (CCII+) and negative (CCII ) conveyors. The resistance at the node Y is ideally infinite, while that at node X is low. 2.1 Class AB low voltage low power CCII The transistor level schematic of the utilized current conveyor is shown in Figure 2 (Ferri and Guerrini, 2001). The class AB CCII is basically consists of a differential input stage (MN1, MN2) loaded by an active load (MP1, MP2) and a feedback stage. MP3 and MN3 allow for having the voltage follower action between the Y and X nodes, reduce the resistance at the node X, and supply the required current to the load eventually connected to X. The current flowing from X node is sensed through MP4 and MN4 transistors and mirrored to the high impedance Z terminal. The great advantage of the proposed solution is that the presented circuit can operate with low supply voltage. Compensation networks (Cc&Rc) are necessary to reduce the phase delay between Iz and Ix. 235

3 2.2 Floating inductance generator Figure 2: Class AB CMOS current conveyor In Figure 3, a generalized impedance converter is presented (Kiranon and Pawarangkoon, 1997) which creates a floating inductance generator circuit by using four current conveyors shown in Figure 2, two resistors, and a grounded capacitor. The expected value for the equivalent impedance shown by the floating inductance generator circuit is as follow; Z eq ( R1 + 2 R X )( R R X )(1 + sct R Z ) = R Z (2) where C T is equal to C + C Y + C Z, and R X and R Z are the resistance at nodes X and Z, respectively. In ideal case, R X is zero and R Z is very high and so the value of ideal inductance between the input and output nodes is equal to R 1 R 2 C. 3. The Proposed Pulse Width Modulator Figure 3: Floating inductance generator As mentioned before, the goal of this paper is to introduce a current mode NPWM, as shown in Figure 1. In the proposed PWM, an active inductor based on the CCII is used as a 236

4 triangular current wave generator (TCWG) which converts the input signal consisting of the amplitude summation of the input modulating signal and the carrier pulse signal to the triangular current. This triangular current is then compared with the reference current through a current comparator cascaded to the TCWG and finally the output PWM signal in produced. Duty cycle of the output signal is controlled by the reference current level. 3.1 Triangular current wave generator Since the current of an inductor is the integral of its voltage, by applying a pulse voltage across the inductor a triangular current can be achieved. This issue is applied for the TCWG in which an active inductor is utilized to generate the needed triangular current. The operation of the circuit can be described by the following equations; = + V = + V When signal is high (3) B B IOUT t t L R1R 2 C = V = V When signal is low (4) B B IOUT t t L R1R 2 C where V B is the amplitude level of the pulse signal applied to the IN terminal of the floating inductor. 3.2 Current comparator Figure 4: Block diagram of the TCWG circuit There are many current comparators given in the references (Ziabakhsh et al., 2008), (Tang and Pun, 2009), (Kong et al., 2005). The current comparator used in the proposed PWM is shown in Figure 5 (Ziabakhsh et al., 2008). When the input current is positive the output voltage is high and when the input current is negative the output voltage is low. The comparator is controlled by the reference current I ref. When the reference current is zero, the positive and negative currents injected to the comparator are equal and so the duty cycle of the output PWM signal is 50%. When I ref is positive, the triangular wave is shifted up by the positive value of the I ref and the input positive current increases which causes to increase the on time duration and hence the duty cycle of the comparator output signal. By applying the negative I ref, the triangular wave is shifted down and so the on time duration and duty cycle will be decreased. 237

5 4. Simulation Results Figure 5: The utilized current comparator To evaluate the performance of the proposed PWM circuit, the CCII and PWM are verified by performing HSPICE simulation in 0.18 μm CMOS process and ±1.5 V supply voltages. Figure 6 shows the X terminal voltage of the CCII versus its Y terminal voltage for 10 KΩ load at X and Z ports. The Z terminal output current swing versus the X terminal input current is shown in Figure 7. The CCII has achieved up to 40 MHz bandwidth for voltage follower Vx/Vy and 380 MHz bandwidth for current follower Iz/Ix as shown in Figures 8 and 9, respectively while consumes 53 μw power. Figure 6: The X terminal voltage versus Y terminal 238

6 Figure 7: The Z terminal current versus X terminal Figure 8: Frequency response of the voltage follower Vx/Vy Figure 9: Frequency response of the current follower Iz/Ix 239

7 Figure 10 shows the output current of the TCWG with zero reference current and the corresponding output voltage of the PWM which indicates a 50% duty cycle. Figures 11 and 12 show the output current of the TCWG with ±23 μa reference current and in each case the output voltage of the PWM indicates 24% and 76% duty cycle, respectively. Figure 10: The output current of the TCWG with zero reference current and its proportional output PWM Waveform with 50% duty cycle 240

8 Figure 11: The output current of the TCWG with 23 μa reference current and its proportional output PWM Waveform with 24% duty cycle Figure 12: The output current of the TCWG with +23 μa reference current and its proportional output PWM Waveform with 76% duty cycle 5. Conclusion In this paper, a current mode NPWM is introduced in which a floating active inductor based on current conveyor (CCII) is utilized as a triangular current wave generator (TCWG) and a 241

9 current comparator is cascaded to the TCWG to compare the reference and triangular currents and generate the output PWM signal. The obtained performance metrics corresponding to HSPICE simulations in 0.18 μm CMOS process and ±1.5 V supply voltage indicate the performance of the proposed PWM modulator. References 1. Hanington G., Chen P.F., Asbeck P.M., and Larson L.E., (1999), High Efficiency Power Amplifier using Dynamic Power Supply Voltage for CDMA Applications, IEEE Transactions on Microwave Theory and Techniques, 47(8), pp Siripruchyanun M., Wardkein P., and Sangpisit W., (2000), A Simple Pulse Width Modulator using Current Conveyor, IEEE TENCON Conference, pp Syed A., Ahmed E., Maksimovic D., and Alarcon E., (2004), Digital Pulse Width Modulator Architectures, 35 th IEEE Annual Power Electronics Specialists Conference (PESC 04), pp Pascual C., Song Z., Krein P.T., Sarwate D.V., Midya P., and Roeckner W.J., (2003), High Fidelity PWM Inverter for Digital Audio Amplification: Spectral Analysis, Real Time DSP Implementation, and Results, IEEE Transactions on Power Electronics, 18(1), pp Song Z., and Sarwate D.V., (2003), The Frequency Spectrum of Pulse Width Modulated Signals, Signal Processing, Elsevier North Holland, 83, pp Smith K.C., and Sedra A., (1968), The Current Conveyor A New Circuit Building Block, Proceedings of the IEEE, 56(8), pp Sedra A., and Smith K.C., (1970), A Second Generation Current Conveyor and its Applications, IEEE Transactions on Circuit Theory, 17(1), pp Ferri G., and Guerrini N., (2001), High Valued Passive Element Simulation using Low Voltage Low Power Current Conveyors for Fully Integrated Applications, IEEE Transactions on Circuits and Systems II: Analog and Digital Signal Processing, 48(4), pp Kiranon W., and Pawarangkoon P., (1997), Floating Inductance Simulation based on Current Conveyors, Electronics Letters, 33(21), pp Ziabakhsh S., Rad H.A., Saberkari A., and Shokouhi Sh. B., (2008), An Ultra High Speed Low Power CMOS Integrated Current Comparator, 3 rd IEEE International Design and Test Workshop (IDT 08), pp Tang X., and Pun K.P., (2009), High Performance CMOS Current Comparator, Electronics Letters, 45(20), pp

10 12. Kong Z.H., Yeo K.S., and Chang C.H., (2005), Design of an Area Efficient CMOS Multiple Valued Current Comparator Circuit, IEE Proceedings Circuits, Devices and Systems, 152(2), pp