Feature: Medical
Figure 9: Connection between CBT patch and interface board
Figure 10: Functional diagram of MAX30208
Table 1: MAX30208 I/O power specifications
Thermal error mitigation of the electrical system Tere are two main areas of concern in the electrical system: (1) the heat generated by the MAX30208 (for example, self-heating), and (2) the heat generated by the PCB trace(s) (for example, heat radiation). Both sources can adversely affect the performance of the thermal system by sourcing or sinking heat energy into or out of the CBT patch. Figure 10 shows the MAX30208 circuit diagram. Te CBT patch electrical system was powered by a regulated
C pull-up resistors, being a significant source of heat energy, are located on the MCU board. Note that there are no I2
1.8 VDC power supply located on the MCU interface board. Te I2
C pull-up
resistors implemented on the CBT patch rigid-flex PCB. Table 1 shows the current and voltage levels on each I/O pin
for operation at 37°C. Tese values were extrapolated from the MAX30208 data sheet electrical table and associated TOC figures. Most of the power dissipation comes from the I2
Figure 11: A CBT patch schematic diagram Although package thermal resistances are reported on data
sheets, the designer must be cautious with their use in heat-flow estimates. Both θjA
θjC C signal lines,
and the power supply line yielding about 810µW if operated continuously. As the temperature signal doesn’t change very quickly, we can use periodic sampling – this help with data management and it reduces the overall power requirements, which in turn mitigates the heat dissipation of the MAX30208 and its signal and power traces. MAX30208 implements a typical 15ms integration period.
With a sampling rate of 1Hz, the average power consumption is thus:
(thermal resistance, junction-to-ambient) and (thermal resistance, junction-to-case) are evaluated per JEDEC
specs, which can be quite different from the actual application. Tey are commonly used as figure-of-merit values for comparing competing devices. Terefore, it is not recommended to use the ambient temperature to predict the junction temperature, especially since the temperature sensors in this application are sandwiched between insulating and non-insulating materials. Since the MAX30208 temperature measuring circuit is on the
integrated circuit, the associated self-heating of the die is what we are most interested in. Tis device was designed to measure the external temperature on the top and bottom of the package. Assuming the case and die temperature are the same, we can estimate the temperature error due to self-heating as follows:
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