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CMOS Temperature Sensor with Ring CMOS Temperature Sensor with Ring OscillatorOscillator

for Mobile DRAM Self-refresh Controlfor Mobile DRAM Self-refresh Control

IEEE International Symposium on Circuits and Systems, 2008.IEEE International Symposium on Circuits and Systems, 2008.

Chan-Kyung Kim; Bai-Sun Kong; Chil-Gee Lee; Young-Hyun JuChan-Kyung Kim; Bai-Sun Kong; Chil-Gee Lee; Young-Hyun Jun;n;

指導老師 指導老師 : : 易昶霈 老師易昶霈 老師班級 ：積體碩二班級 ：積體碩二

學號 ：學號 ： 9666201096662010姓名 ：陳鴻鑫姓名 ：陳鴻鑫

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outline

INTRODUCTION

PROPOSED TEMPERATURE SENSOR WITH RING OSCILLATORS

MEASUREMENT RESULTS

CONCLUSION

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INTRODUCTION

As mobile devices are required to provide very low power consumption, schemes such as monitoring the internal temperature of a chip and adjusting its power consumption based on this temperature are popularly used.

A low-power mobile DRAM can adjust its self-refresh period according to internal temperature to minimize data retention current during power-down mode . Usually, the leakage characteristic of a DRAM cell becomes worse at high temperature than at low temperature.

If a local clock signal to determine the self-refresh interval is generated by a ring oscillator as conventional DRAMs do, the wasted data retention current tends to be further increased at low temperature because the oscillation frequency of the oscillator increases as temperature decreases.

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On the other hand, if we can measure the temperature using a temperature sensor, the self-refresh period can be adjusted adaptively based on this measured temperature. That is, the period can be set long at low temperature and short at high temperature.

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After input signal Refresh_EN becomes active, oscillator output SELF_OSC starts oscillation with a period decided by the latencies of inverter stages.

For setting a proper oscillation frequency of the oscillator under a given temperature, control signals P4~P0 and N4~N0 from the temperature sensor adjust the conductive distance between the power supply and the active devices. SELF_OSC is then fed into the counter to generate command signal SELF_REF for periodically invoking self-refresh operations in the DRAM core.

With this configuration, the circuit can effectively change the timing period between consecutive

self-refresh operations to reliably retain DRAM cell data based on on-chip temperature.

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Usually, the oscillation frequency of a ring oscillator controlled by a temperature sensitive bias current can be effectively utilized as a means to monitor the temperature of a chip.

The CMOS temperature sensor proposed in this paper utilizes the temperature dependency of poly resistance to generate a temperature dependent bias current, and a set of ring oscillators to convert this bias current to a digital code.

PROPOSED TEMPERATURE SENSOR WITH RING OSCILLATORS

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The bias current generator in the upper part provides the ring oscillator with a proper bias current to allow its oscillation period to be proportional to temperature.

The bias current generator in the lower part provides its oscillator with a bias current to make the oscillation period relatively constant regardless of temperature variation.

The divide-by-4 circuit is used to decrease the frequency of the temperature -dependent clock by 4 times, and allows a low frequency clock to be generated by the ring oscillator having a small number of inverter stages.

The pulse generator selects one-cycle pulse from the low-frequency temperature dependent clock, while the 10-bit counter counts the number of high-frequency temperature-independent clock cycles during the period of the one-cycle pulse to generate an equivalent digital code.

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The rise and fall delays of a single inverter stage in the oscillator is determined by the inverter bias current Isource (Isink), the load capacitance Cload, and the inverter trip voltage Vtrp.

The above equation indicates that the oscillation frequency of the ring oscillator is linearly dependent on the inverter bias current.

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The temperature-dependent bias current is generated by the circuit shown in Fig. 5(a).Because the temperature coefficient of poly resistor Rs is positive, the voltage drop across the resistor gets larger as the temperature goes up. Then, the bias current through transistor P1 becomes smaller since the gate-source voltage of P1 is decreased.

A temperature-independent bias current is generated by the circuit shown in Fig. 5(b). In the circuit, transistors N1 and N2 are operated in the weak inversion region, while transistor N4 is operated in the linear region.

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When the start signal triggers the measurement process, the temperature related oscillator is activated. After enlarging the period of the clock and selecting a single pulse, the width of the resulting single pulse takes on the temperature of the chip.

This ring oscillator is only activated during the high period of the pulse, and the 10-bit counter counts the number of clock cycles during this period. Once the width of the pulse is measured, the counter keeps the measured digital value with both ring oscillators deactivated until a new start signal is entered.

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MEASUREMENT RESULTS

The bias current generators are placed at the vicinity of the associated ring oscillators to minimize device mismatches.

The oscillation period of the upper ring oscillator varies about 2.20uS. The variation of the oscillation period of the lower ring oscillator for the same temperature

range is less than 0.04uS, which is very small compared to that of the upper ring oscillator.

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The proposed temperature sensor achieves area reduction of 73% with improved resolution.

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CONCLUSION

A new CMOS temperature sensor with ring oscillators is proposed and implemented in a DRAM process.

The proposed temperature sensor occupies smaller silicon area with higher resolution than the conventional temperature sensor based on bandgap reference.

With the proposed temperature sensor, the self-refresh period of a mobile DRAM can be effectively controlled to improve power efficiency during power-down mode.

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Thank you for your attention !