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Mixed signal verification of temp sensor

Posted: 22 Oct 2013     Print Version  Bookmark and Share

Keywords:temperature sensor  thermometers  thermistors  BJT  Analog TSense 

A temperature sensor can be implemented with an analogue electronic circuit that can sense and show the ambient temperature. These sensors use a solid-state technique to determine the temperature. That is to say, they don't use mercury (like old thermometers), bimetallic strips (like in some home thermometers or stoves), nor do they use thermistors (temperature sensitive resistors). Instead, they use the fact that, as temperature increases, the voltage across a diode (p-n junction) increases at a known rate.

This concept can be realistically implemented in a BJT where a diode (p-n junction) is present across the base and emitter terminals and the voltage drop between these two terminals – that is VBE—of a transistor changes in accordance with the ambient temperature. By precisely amplifying this voltage change, it is easy to generate an analogue signal that is directly proportional to the temperature. Now let us look at some of the basic BJT based formulas. We know that for a BJT:

VBE = VG0(1 – T/T0 ) + VBE0(T/T0) + nKT/q ln(T0/T) + kT/q ln(Ic/IC0)

Where,
T = temperature in Kelvin
T0 = reference temperature
VG0 = bandgap voltage at absolute zero
VBE0 = bandgap voltage at temperature T0 and current IC0
K = Boltzmann's constant
q = charge on an electron
n = a device-dependent constant

By comparing the bandgap voltages at two different currents, IC1 and IC2, many of the variables in the above equation can be eliminated, resulting in the relationship:

VBE1 – VBE2 = kT/q ln(IC1/IC2)

If we can maintain a constant ratio of N:1 in IC1 and IC2, then by measuring the VBE1 – VBE2 one can measure the temperature and this is how a generic temperature sensor works. There are 2 basic ways of tracking this change. If the voltage increases as temperature increases it is called Proportional To Absolute Temperature implementation or PTAT. If the voltage decreases with increase in temperature, it is called Complimentary To Absolute Temperature or CTAT. So a temperature sensor circuit can be PTAT based or CTAT based.

From a microcontroller perspective, the knowledge of temperature is very crucial. Based on temperature the parameters that are affected are:

Performance of the chip: P-N junctions and devices inside the chip change their state very rapidly as temperature changes and hence the performance of the chip is impacted. At high temperature there are chances that the frequency of operation of the chip increases automatically.

Leakage of the Chip: Leakage increases at high temperature and hence the chip might start burning more and more current as temperature is increased.

Other thermal issues might also arise, like Channel Hot Carrier Effect and all that affect the performance of the chip.

If the microcontroller is aware of the temperature of the surroundings it can make multiple intelligent decisions to fix these issues. For example, it can trim its clocks in such a way that frequency is always in a desired range. It can enable a well biasing protocol to reduce leakage. It can shutdown certain peripherals that are sensitive to temperature, e.g. a micro controlling a motor might shutdown the motor if the temperature goes too high as it may damage the insulation in the windings, etc. So we understand that from an SoC perspective temperature sensors are crucial blocks and they must be verified thoroughly.

The methodology described in this paper can identify design contentions which can potentially change the output of the temperature sensor (e.g. Charge sharing at the output due to potential pull-ups or pull-downs). In fact, any variation in the temperature sensor output due to design inefficiency can be identified.

This paper contains a Mixed Signal Verification Method for temperature sensors. It basically talks about verifying this block in RTL+SPICE or RTL+VAMS based abstraction of the SoC, where the analogue part of the design has an electrical based modelling (VAMS/SPICE) and rest of the design has a behavioural model (RTL). For IPs like a temperature sensor that is integrated in an SoC, a faithful verification can be done with an electrical modelling of these blocks. The accuracy of the data that comes out of such IPs is very critical for the customer.

After being integrated in the SoC, the variations in supply of this block and its integration with other IPs like the ADC, need to be thoroughly reviewed. The accuracy of this verification improves many times if done in a Mixed Signal Environment. As a generic methodology we propose a simple linear formula that can be applied on any sub-system having a temperature sensor (TSense) block. This formula gives a value for every temperature which can be compared against an Analog TSense output. As discussed earlier, this methodology becomes very powerful if mixed signal simulations are being done.

Methodology
Initially the Analog TSense block has to be simulated in a stand-alone testbench across all the temperatures. The voltage output of the temperature sensor (Vout) is tabulated for all the temperatures in the interested range of operation. As expected from the stand-alone IP, this output voltage would vary linearly with temperature. We therefore draw this linear variation behaviour. For all these voltages to lie down exactly on the best-fit line, we may need to make minor adjustments in the voltage output. We took a pilot IP of a temperature sensor that had to work in the range of -40C to 150C. We followed all the steps, made the necessary changes and came up with the table.

Table: TSense output across temperature.


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