Micrel, Inc.
MIC3001
August 2004
27
M9999-082404-A
performed using a look-up table (LUT) that stores values
corresponding to each measured temperature over a
128°C span. Four identical tables reside at serial address
A4h as summarized in Table 13. The range of
temperatures spanned by the tables is programmable via
the LUTOFF register. Each table entry is a signed twos
complement number that is used as an offset to the
parameter being compensated. The default value of all
table entries is zero, giving a flat response.
The A/D converter reports a new temperature sample
each tCONV. This occurs at roughly 10Hz. To prevent
temperature oscillation due to thermal or electrical noise,
sixteen successive temperature samples are averaged
together and used to index the LUTs. Temperature
compensation results are therefore updated at 16xtCONV
intervals, or about 1.6 seconds. This can be expressed as
shown in Equation10.
(10)
Each time an updated average value is acquired, a new
offset value for the APC setpoint is read from the
corresponding
look-up
table
(see
Table
14)
and
transferred to the APC circuitry. This is illustrated in
Equation 11. In a same way, new offset values are taken
from similar look-up tables (see Table 15 and Table 16),
added to the nominal values and transferred into the
modulation and fault comparator DACs. The bias current
high alarm threshold, is compensated using a fourth look-
up table (see Table 17). This compensation happens
internally and does not affect any host-accessible
registers.
(11)
If the measured temperature is greater than the maximum
table value, the highest value in each table is used. If the
measured temperature is less than the minimum, the
minimum value is used. Hysteresis is employed to further
enhance noise immunity and prevent oscillation about a
table threshold. Each table entry spans two degrees C. The
table index will not change unless the new temperature
average results in a table index beyond the midpoint of the
next entry in either direction. There is therefore 2 to 3°C of
hysteresis on temperature compensation changes. The
table index will never oscillate due to quantization noise as
the hysteresis is much larger than ±1
2 LSB.
Byte Addresses
Function
00h–3Fh
APC Look-up Table
40h–7Fh
IMOD Look-up Table
80h–BFh
IFLT Look-up Table
C0h–FFh
Bias High Alarm Look-up Table
Table 13. Temperature Compensation Look-up Tables,
Serial Address I2CADR + 4h