78M6612 Data Sheet
DS_6612_001
3.2 Temperature Measurement
Measurement of absolute temperature uses the on-chip temperature sensor while applying the following
formula:
(N(T) − Nn )
T =
+Tn
Sn
In the above formula, T is the temperature in °C, N(T) is the ADC count at temperature T, Nn is the ADC
count at 25°C, Sn is the sensitivity in LSB/°C as stated in the Electrical Specifications, and Tn is +25°C.
Example: At 25°C a temperature sensor value of 518,203,584 (Nn) is read by the ADC by a 78M6612 in
the 64-pin LQFP package. At an unknown temperature T the value 449.648.000 is read at (N(T)). The
absolute temperature is then determined by dividing both Nn and N(T) by 512 to account for the 9-bit
shift of the ADC value and then inserting the results into the above formula, using –2220 for LSB/°C:
449.648.000 -518,203,584
T =
+ 25C = 85.3°C
512⋅(−2220)
It is recommended to base temperature measurements on TEMP_RAW_X which is the sum of two
consecutive temperature readings thus being higher by a factor of two than the raw sensor readings.
3.3 Temperature Compensation
Temperature Coefficients: The internal voltage reference is calibrated during device manufacture.
The temperature coefficients TC1 and TC2 are given as constants that represent typical component
behavior (in µV/°C and µV/°C2, respectively).
Since TC1 and TC2 are given in µV/°C and µV/°C2, respectively, the value of the VREF voltage
(1.195V) has to be taken into account when transitioning to PPM/°C and PPM/°C2. This means
that PPMC = 26.84*TC1/1.195, and PPMC2 = 1374*TC2/1.195).
Temperature Compensation: The CE provides the bandgap temperature to the MPU, which then may
digitally compensate the power outputs for the temperature dependence of VREF, using the CE register
GAIN_ADJ. Since the band gap amplifier is chopper-stabilized via the CHOP_EN bits, the most significant
long-term drift mechanism in the voltage reference is removed.
The MPU, not the CE, is entirely in charge of providing temperature compensation. The MPU applies the
following formula to determine GAIN_ADJ (address 0x12). In this formula TEMP_X is the deviation from
nominal or calibration temperature expressed in multiples of 0.1°C:
TEMP _ X ⋅ PPMC TEMP _ X 2 ⋅ PPMC2
GAIN _ ADJ =16385+
+
214
223
In a power and energy measurement unit, the 78M6612 is not the only component contributing to
temperature dependency. A whole range of components (e.g. current transformers, resistor dividers,
power sources, filter capacitors) will contribute temperature effects.
Since the output of the on-chip temperature sensor is accessible to the MPU, temperature-
compensation mechanisms with great flexibility are possible. MPU access to GAIN_ADJ permits a
system-wide temperature correction over the entire unit rather than local to the chip.
64
Rev. 1.2
TERIDIAN [ TERIDIAN SEMICONDUCTOR CORPORATION ]
