CMOS Temperature Sensor with Ring Oscillator for Mobile DRAM Selfrefresh Control

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CMOS Temperature Sensor with Ring Oscillatorfor
Mobile DRAM Self-refresh Control

• IEEE International Symposium on Circuits and
  Systems, 2008.
• Chan-Kyung Kim Bai-Sun Kong Chil-Gee Lee
  Young-Hyun Jun
• ???? ??? ??
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• ?? 96662010
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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 P4P0 and N4N0 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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PROPOSED TEMPERATURE SENSOR WITH RING OSCILLATORS

• 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.

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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 !