An Oscillator Based on LDMOS Capacitor

HUANG Wei-hai∗, DAI Yu-jie, ZHANG Xiao-xing, Lü Ying-jie Institute of Microelectronics, Nankai University, Tianjin 300457 P. R. China

nkhwh@hotmail.com

Abstract

The characteristic curve of the ordinary MOS

capacitor is not monotonic, whereas the Laterally Diffused MOS (LDMOS) capacitor has a nearly ideal monotonic property. An oscillator based on LDMOS capacitor is designed and implemented using the standard 0.5µm CMOS technology, and various essential factors have been considered, such as the layout size, the power consumption and the capacitance value, etc. The simulations and measurement results show that the oscillator has a small layout size, a low power consumption, a wide voltage range, and a high frequency accuracy. 1. Introduction

Oscillator plays an important role in the power management system chips. For example, if the battery load is accidentally shorted, the power management system chip has to turn off the discharging loop after a short delay about 400μs [1]. This requires that the oscillator module in the chip is able to provide accurate frequency under different practical situations [2].

More and more oscillators are using MOS capacitors instead of standard capacitors, aiming at decreasing the layout size and thus reducing the production cost [3]. We not only consider the production cost, but also deal with various factors such as the voltage varying scope, power consumption and accuracy. The oscillator designed in this paper is based on the Laterally Diffused MOS (LDMOS) capacitor. 2. Features of MOS capacitors

Capacitor, as the necessary component of time delay unit, is popularly used in the oscillator circuits. Neither the RC oscillator nor the LC oscillator can be implemented without capacitors. In previous studies, capacitors are usually implemented either by the Mental-Insulation-Mental (MIM) capacitor or the polycrystalline silicon (POLY) capacitor. Recently, in

order to reduce the production cost and to decrease the layout size, the industry has begun to design many power supply management chip (including battery protection chip and PWM DC/DC chip) by using MOS capacitors instead of original standard capacitors.

The ordinary MOS capacitor normally has a capacitance (Cmos) that is changing as the voltage difference between two plates of the capacitor (VGS) varies [3]. However such MOS capacitor does not have the same feature as the standard capacitor [4]. The characteristic curve of such an ordinary NMOS capacitor is not monotonic; instead, it has a concave shape. Such a non-monotonic curve, in turn, influences the performance of the circuit.

Obviously, if we want to improve the performance of the circuit, a MOS capacitor with a monotonic changing property is required. In this paper, considering the cause of the concave shape in the ordinary MOS capacitor, we select another MOS device from the library of the foundry, namely the Laterally Diffused MOS (LDMOS) device [5]. The characteristic curve of LDMOS capacitor has a nice monotonic property [5].

3. Structure and principle of circuit

The function of the oscillator is implemented based on continuous charging and discharging of an integrated capacitor from a constant current source. The oscillator designed in this paper will be used in the chip for the power management system. The power is supplied by a lithium ion battery. The system requires that the oscillator should be able to output a high-accuracy square wave under the large voltage range of 1.5V~5V and the large ambient temperature range of -40 ℃ ~85 ℃ . The oscillator circuit is based with enhancements on existing micropower designs [6].

Fig. 1 gives the simplified circuit structure diagram of the oscillator. There are three functional blocks: a block for charging and discharging the LDMOS capacitor (M1, M2 and C1), a comparator (C), and a waveform-conversion block (INV and D-flip-flop).

2009 Sixth International Conference on Information Technology: New Generations

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DOI 10.1109/ITNG.2009.149

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The oscillation frequency can be calculated as

ref

ref

VCI

f⋅

=12

, (1)

Fig. 1. Block diagram of the oscillator

where Vref is the reference voltage that is compared

with the voltage (VA) on the capacitor, Iref is the constant current charging the capacitor, and C1 denotes the capacitance of the LDMOS capacitor.

In this study, the reference voltage Vref=0.9V. In order to achieve the low power consumption of the whole system, we set Iref=40nA. The clock frequency required by the system is 2.5 KHz. From Formula (1), we can calculate C1=8.89pF. At this clock frequency, the current consumption of the whole oscillator circuit is designed to be 0.5μA.

According to the technical parameters provided by the foundry, we can calculate that the layout size of the LDMOS capacitor is 65µm×65µm. If we had not used LDMOS capacitor but the MIM capacitor in the same library, the size of the MIM capacitor in the layout would be 190µm×190µm in order to implement the same required oscillating frequency. In other words, by using LDMOS capacitor, the size of the capacitor in the layout is almost 1/9 of the original size. 4. Simulations and measurement results

We used the CADENCE software to conduct the simulations as well as the pre- and post-designs. It was observed that the output waveforms are totally consistent with our expectation.

After verifying the designed oscillator by simulations, the circuit was processed in a commercial p-well 0.5µm CMOS process. The main objects of interests were to measure the dependences of oscillation frequency on the supply voltage and the ambient temperature. When the power supply voltage is 3V, the average current consumption is 0.48μA, and the average oscillation frequency is 2.48 KHz.

We also measured the dependence of the oscillation frequency on the power supply voltage and the ambient temperature. We found that when the voltage gradually fluctuates from 1.5V to 5V, the accuracy of the output frequency is about 2.5KHz ± 8%. On average, the impact of the power supply voltage is -3.6 %/V. In addition, when the ambient temperature fluctuates with the range of -40℃ ~ +80℃ and the power supply voltage is 3V, the accuracy of the output frequency is about 2.5 KHz ± 5%.

The LDMOS capacitor features were also measured. The measurement results showed that under the working situation where VGS changes from 0V to 0.9V, the capacitance of the LDMOS capacitor is basically a gently-changed diagonal with a nice linearity, and the changing of the capacitance is fairly small. This is almost identical to the design purpose (8.89pF). 5. Conclusion

In this paper, we designed an oscillator with a high accuracy based on the LDMOS capacitor. The simulations and measurement results have shown that this oscillator significantly decreases the dependency of the oscillation frequency on the power supply voltage and the ambient temperature. In a word, Using LDMOS as the capacitor can significantly reduce the layout size, and simultaneously bring a higher stability than the ordinary MOS capacitor. 6. References [1] G. Smith, “Micro power protection chip for rechargeable lithium-ion batteries,” IEEE J. Proc. IEEE Custom IC Conf., 1996, pp. 7.6.1-7.6.4. [2] D. Salerno and R. KORSUNSKY, “Practical considerations in the design of lithium-ion battery protection system,” Proc. IEEE Applied Power Elec. Conf., 1998, pp. 700-707. [3] J.-H. DOU. Design of voltage-controlled oscillator based on MOS capacitor. Journal of Hefei University of Technology (Natural Science), 29(6), 2006, pp. 721 – 724. [4] S.-C. Qin and X.-L. Jia. Anolog Integrated Electronics. Tianjin Science and Technology Press, 1996. [5] J. Olsson, N. Rorsman, L. Vestling, C. Fager, J. Ankarcrona, H. Zirath, and K.-H. Eklund. 1 W/mm RF power density at 3.2 GHz for a dual-layer RESURF LDMOS transistor. IEEE Electron Device Letters, 23(4), 2002, pp. 206 – 208. [6] P. Kakela, T. Rahkonen, J. Kostamovaara, “A micropower RC oscillator chip for consumer ASIC applications,” Proceedings of the Melecon`91 Conf., 1991, pp. 278-281.

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