Online proceedings - EDA Publishing Association

7-9 October 2009, Leuven, Belgium
0.8
0.75
Voltage [V]
0.7
0.65
0.6
Sensor
Localization
0.55
0.5
0.45
0 20 40 60 80 100 120 140 160
Temp [C]
Fig. 3. Simulations of the output signal from the sensor.
III.
SIMULATIONS
The whole system containing the PTAT sensor, as already
mentioned in previous section, was simulated in the
CADENCE environment. In order to simulate this circuit,
the transistor models from the austriamicrosystems ® (AMS)
0.35 μm high voltage technology were used. This technology
was chosen for the practical realization of the ASIC. The
simulations confirmed that the behavior of the sensor
conforms to the design specifications. With increasing
temperature, the sensor output voltage decreases with the
negative slope of –1.72 mV/K. When temperature reaches
150 °C, the additional current mirror is switched off and the
current delivered to the bipolar transistor is twice smaller.
This means that the output voltage rapidly drops down by
some 25 mV what allows the generation of logic one at the
overheat warning pin, which is clearly visible in Fig. 3.
When temperature decreases, the output voltage increases
again. When the temperature reaches 130 °C, the output
voltage rapidly grows and the warning signal is switched off,
which can be observed in Fig. 4.
Fig. 5. Photo of a die.
IV. ASIC REALIZATION
The presented overheat protection circuit containing the
PTAT sensor, as already mentioned, was manufactured in
the thermal test ASIC designed in the Department of Microelectronics
& Computer Science at the Technical University
of Lodz, Poland. The location of the sensor in the circuit
layout is shown in Fig. 5. For full description of the test
ASIC, refer to [6]-[7].
The sensor layout, presented in Fig. 6, consists of 9 PMOS
transistors which are connected as current mirrors. Besides,
there are 3 bipolar transistors and 1 polysilicon resistor. All
the devices are placed so as to obtain the minimal area in the
ASIC. Because the circuit contains 9 large power transistors
acting as heat sources, the total area of the whole ASIC
amounts to almost 20 mm 2 . The dimensions of the PTAT
sensor alone are 125 μm × 90 μm, so it is a very small sensor
which can be used in different integrated circuits, where
precise temperature measurements are necessary. Obviously,
the PTAT sensor together with the overheat warning system
occupies slightly bigger area.
Voltage [V]
3
2.5
2
1.5
1
0.5
MOS
Transistors
Bipolar
Transistors
Resistor
0
0 20 40 60 80 100 120 140 160
Temp [C]
Fig. 4. Simulations of the overheat warning signal.
Fig. 6. Layout of PTAT sensor.
©EDA Publishing/THERMINIC 2009 82
ISBN: 978-2-35500-010-2