Temperature sensing apparatus and methods

Details Temperature sensing apparatus and methods Temperature sensing apparatus and methods

2002-10-07

2004-10-05

Gutierrez, Diego (Department: 2859)

Thermal measuring and testing

Temperature measurement

By electrical or magnetic heat sensor

C374S183000, C327S513000, C327S538000, C323S312000, C323S315000

Reexamination Certificate

active

06799889

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to temperature sensing apparatus, and in particular to circuits and methods for temperature sensing.
BACKGROUND TO THE INVENTION
For high power circuits such as power amplifiers for audio speakers and linear power supply regulators, there is a possibility of fault conditions such as external short circuits causing high on-chip currents. The on-chip power dissipation caused by these currents can result in excessive temperatures which can degrade the characteristics of circuits on the silicon chip and, in extreme cases, may even constitute a fire hazard. For this reason such power circuits are often provided with a thermal shutdown function where power outputs are disabled if the chip temperature exceeds a predetermined limit, for example 150° C. To implement such a function an on-chip circuit is needed to detect and flag when such a predetermined temperature threshold is exceeded. There is also a need for a temperature detector in some microprocessor systems, for example where the microprocessor is clocked at a high speed. In such a system if a temperature limit is reached the clock may be slowed down to reduce the supply current drawn by the microprocessor and/or an output signal may be provided to turn on a fan.
In the early days a Zener diode voltage would be resistively divided and applied to the base of a common-emitter bipolar transistor. The base-emitter voltage (V
be
) to turn on a bipolar transistor decreases by approximately 2 mV per ° C. so that as the temperature increased with a constant voltage applied (or even a rising voltage if the Zener had a positive temperature coefficient or tempco) a temperature would be reached where the bipolar transistor turned on and its collector current could then be used as an output.
As supply voltages have reduced this method has become impracticable as typical Zener voltages, which are difficult to achieve reliably below 5 to 7V, are too large. Instead it has become conventional to use a bandgap voltage instead of a Zener voltage, as described for example in U.S. Pat. No. 3,959,713, U.S. Pat. No. 4,692,688, U.S. Pat. No. 4,574,205 and U.S. Pat. No. 5,099,381. For example U.S. Pat. No. '381 describes a circuit where a bandgap voltage from a Brokaw cell is compared to a V
be
multiplier voltage. To avoid electrically and/or thermally induced instability about the threshold temperature some local positive feedback may also be applied to provide the switching point with some hysteresis. A temperature detection circuit employing a bandgap voltage source and feedback to provide hysteresis is described in U.S. Pat. No. 5,149,199. General background prior art in the field of temperature detection can be found in U.S. Pat. No. 6,181,121, US 2002/0093325, U.S. Pat. No. 6,188,270, U.S. Pat. No. 6,366,071, U.S. Pat. No. 5,327,028, U.S. Pat. No. 4,789,819 and U.S. Pat. No. 5,095,227.
The IEEE Journal of Solid-State Circuits, vol. 31, no. 7, July 1996, pages 933 to 937, “Micropower CMOS Temperature Sensor with Digital Output”, A Bakker and J H Huijsing, describes a CMOS temperature sensor in which a current proportional to a V
be
voltage is compared to a reference current which is substantially independent of temperature formed by the addition of the PTAT (proportional to absolute temperature) current to a base-emitter voltage referenced current. The sum of these two currents is approximately temperature independent because they have opposite temperature coefficients, positive for the PTAT current and negative for the V
be
current. However the circuit of Bakker and Huijsing is relatively complicated (see, for example,
FIG. 4
) and its sensitivity could be improved.
Another temperature detection circuit is described in U.S. Pat. No. 5,980,106, which again uses a bandgap reference.
FIGS. 1A and 1B
, which are taken from U.S. Pat. No. '106 illustrate the principle of this circuit. Broadly speaking two current sources
10
,
20
with respective positive and negative temperature coefficient characteristics
12
,
22
are applied to a detection node A coupled to an output circuit, in
FIG. 1A
inverter
30
. As can be seen from inspection of
FIGS. 1A and 1B
the inverter output will switch where the voltage of point A crosses the switching threshold for the inverter, in
FIG. 1B
at threshold temperature TD. U.S. Pat. No. '106 also teaches the application of feedback to detection node A as shown, for example, in
FIG. 3A
of '106. A detailed temperature detection circuit (
FIG. 4
) is also described in which a thermal voltage (VT)-based current Ith is combined (compared) with a current derived from a bandgap reference Ibg at node A (negative temperature coefficients introduced by resistors in the circuit cancelling). Again, however, the circuit of U.S. Pat. No. '106 is relatively complex and includes floating bipolar transistors as well as MOSFETs.
It is desirable to be able to provide a simpler, cheaper and easier to fabricate temperature sensor. A bandgap voltage is often present in circuits such as voltage regulators but is unnecessary in applications such as speaker amplifiers, so that an arrangement not reliant on an explicit bandgap voltage generator would be preferable. Furthermore, it has been recognized that fundamentally it should be possible to construct a temperature detector merely by comparing two quantities with different temperature coefficients and predictable absolute values, or at least with predictable relative values at some reference temperature from which temperature coefficients may be referred. Also, increasingly circuits are being manufactured using CMOS rather than bipolar technology, even in traditionally bipolar areas such as loudspeaker power amplifiers (see, for example, the Fairchild FAN 7021). The use of CMOS precludes the application of many prior art techniques.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is therefore provided a temperature sensor comprising: a current mirror with an input and at least two outputs; a first reference current generator having a first current input and a first current output and configured to generate a first reference current with a positive temperature coefficient at said first current output in response to said first current input; a second reference current generator having a second current input and a second current output and configured to generate a second reference current with a negative temperature coefficient at said second current output in response to said second current input; and wherein one of said first and second reference generators has a respective current output coupled to said input of said current mirror; said first current input of said first reference generator and said second current input of said second reference generator share an input node coupled to a first of said current mirror outputs; and the other of said first and second reference generators has a respective current output coupled to a second of said current mirror outputs to thereby provide a current sense node; and wherein said first reference current generator comprises a thermal voltage referenced current source, and said second reference current generator comprises a temperature dependent semiconductor characteristic referenced current source.
In this specification the term current source includes negative current sources, that is sources in which a current flows into the source (sometimes alternatively referred to as “sinks”), and current may therefore flow into a current source output. Broadly speaking, two reference current sources are provided, both interacting with the same current mirror, one of the current sources being referred or substantially proportional to a bipolar transistor base-emitter voltage (negative temperature coefficient), the other of the current sources being referred or substantially proportional to a bipolar transistor thermal voltage (in mathematical terms kT/q where k is Boltzman's constant, T is the absolute temperature in Kelvin and q is the ch

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