71M6521DE Datasheet, PDF (60/101 Pages) Teridian Semiconductor Corporation – Energy Meter IC

71M6521DE/71M6521FE
Energy Meter IC
DATASHEET
JANUARY 2008
Temperature Measurement
Measurement of absolute temperature uses the on-chip temperature sensor while applying the following formula:
T
=
(N (T ) −
S_n
N_n)
+ T_n
In the above formula T is the temperature in °C, N(T) is the ADC count at temperature T, N_n is the ADC count at 25°C, S_n is
the sensitivity in LSB/°C as stated in the Electrical Specifications, and T_n is +25°C.
Example: At 25°C a temperature sensor value of 518,203,584 (N_n) is read by the ADC by a 71M6521FE 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 N_n 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:
T = ^{449.648.000 - 518,203,584} + 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.
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:
GAIN
_
ADJ
= 16385 +
TEMP _ X ⋅ PPMC
2¹⁴
+
TEMP _
X ² ⋅ PPMC2
2²³
In a production electricity meter, the 71M6521DE/FE 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 meter rather than local to the chip.
Page: 60 of 101
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