MLX90615

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<strong>MLX90615</strong>Infra Red Thermometer8.6.2 Customizing the temperature range for PWM outputThe calculated ambient and object temperatures are stored in RAM with a resolution of 0.02 °C (15 bit). ThePWM operates with a 10-bit number so the transmitted temperature is rescaled in order to fit in the desiredrange.For this goal 2 cells in EEPROM are foreseen to store the desired temperature range, PWM T min and PWMT range.Thus the output range can be programmed with an accuracy of 0.02 °C.The data for PWM is rescaled according to the following equation:TRAM−TMINTEEPROMRANGEEEPROMTPWM=, KPWM=KPWM1023The T RAM is the linearized temperature, 15-bit (2D8A…4BD0h, 2D8A for -40°C and 4BD0h for +115°C) andthe result is a 10-bit word, in which 000h corresponds to PWM T MIN [°C], 3FFh corresponds to PWM T MAX [°C]TMAX − Toand 1LSB corresponds toMIN[°C] , (T MAX = T MIN + T RANGE )1023T T ∗ 50 LSBMIN=EEPORM MINMAX= TEEPORM MAXT∗ 50 LSB3901090615 Page 18 of 30 Data SheetRev 00128/Aug/2008
<strong>MLX90615</strong>Infra Red Thermometer8.7 Principle of operationThe IR sensor consists of series connected thermo-couples with cold junctions placed at thick chip substrateand hot junctions, placed over thin membrane. The IR radiation absorbed from the membrane heats (orcools) it. The thermopile output signal isV ir4 4( Ta, To) A.( To − Ta )= ,Where To is the object temperature absolute (Kelvin) temperature, Ta is the sensor die absolute (Kelvin)temperature, and A is the overall sensitivity.An additional sensor is needed for the chip temperature. After measurement of the output of both sensors,the corresponding ambient and object temperatures can be calculated. These calculations are done by theinternal DSP, which produces digital outputs, linearly proportional to measured temperatures.8.7.1 Ambient temperature TaThe Sensor die temperature is measured with a PTAT element. All the sensors’ conditioning and dataprocessing is handled on-chip and the linearized sensor die temperature Ta is available in RAM.The resolution of the calculated Ta is 0.02 ˚C. The sensor is factory calibrated for the range -40 to +85 ˚C. InRAM cell , 6h, 2D89h corresponds to -40 ˚C and 45F3h corresponds to +85 ˚C. Conversion RAM content toreal Ta is easy:Ta [ ° K]= Tareg × 0.028.7.2 Object temperature ToThe result has a resolution of 0.02 ˚C and is available in RAM (address 7h). To is derived from RAM as:To [ ° K]= Toreg × 0.023901090615 Page 19 of 30 Data SheetRev 00128/Aug/2008
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MLX90615

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