Many applications with a single power regulator can benefit from the monitoring and control features of a power supply manager, but most power supply manager ICs have more than one channel. In an application that only has one power supply, there will be an unused set of DAC and ADC pins. Instead of letting the unused channel go to waste, we can use these pins, and a bit of microcontroller code, to sense remote temperature. The LTC®2970 is a 2-channel power supply monitor and controller. Each channel has two 14-bit ADCs to measure voltage and current, and one 8-bit DAC to servo the power supply voltage. It can drive an attached bipolar junction transistor to make a delta-VBE measurement, and the microcontroller can use the measured voltages to calculate temperature. The cost of the added components is as little as 3 or 4 pennies.
To make a remote temperature sensor with the LTC2970, run two different currents (ILOW and IHIGH) through the transistor and measure VBE at both currents (VBE_LOW and VBE_HIGH). The difference between the two VBE measurements (ΔVBE) is a direct function of temperature. Use Equation 1 to calculate temperature in the BJT.
Shown in the figure, the LTC2970 has everything that we need in one of its two channels. The IDAC output can drive a programmable current between 0µA and 255µA. One of the ADC inputs can directly measure VBE across the BJT.
The other ADC input can measure a useful proxy for current by sampling the voltage across a resistor. This arrangement avoids several traditional problems with semiconductor temperature sensors. The first is knowing how much of the measured VBE voltage is due to voltage drop in the wiring between the current source and the transistor. Here we measure VBE directly at the transistor terminal, not at the DAC output pin, making resistance in the line less important (though transistor internal resistance will affect the measurement slightly). The second difficulty is knowing the precise ratio of ILOW and IHIGH currents. Because we measure voltage across a resistor, instead of relying on the IDAC to be accurate, we have very good knowledge of the ratio of currents. We don’t need to know if the DAC is supplying precise ratios of currents, and we also don’t need to know what the resistor value actually is. The voltage across it will be a pure function of current, and the ratio of two voltages will be equal to the ratio of the two currents.