MIC3001 Datasheet, PDF (25/74 Pages) Micrel Semiconductor – SFP Management IC with Internal Calibration

MIC3001
Temperature Compensation
Since the performance characteristics of laser diodes and
photodiodes change with operating temperature, the MIC3001
provides a facility for temperature compensation of the A.P.C.
loop setpoint, laser modulation current, bias current fault
comparator threshold, and bias current high alarm flag thresh-
old. Temperature compensation is performed using a look-up
table (LUT) that stores values corresponding to each mea-
sured temperature over a 128°C span. Four identical tables
reside at serial address A4_h as summarized in Table 13. The
range of temperatures spanned by the tables is program-
mable 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
t_CONV. This occurs at roughly 10Hz. To prevent temperature
oscillation due to thermal or electrical noise, sixteen succes-
sive temperature samples are averaged together and used to
index the LUTs. Temperature compensation results are
therefore updated at 16×t_CONV intervals, or about 1.6 sec-
onds. This can be expressed as shown in Equation10.
T_COMPm
=
T_n
+
T_n+1 +
T_{n + 2}
16
+ • • •T_n+15
(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 trans-
ferred 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.
Micrel
APCSET_m ^{= APCSETx + APCLUT(TCOMPm )}
Table _ min≤ T_COMPm ≤ Table _ max
= APCSETx + APCLUT(max)
APCSET_m
T_COMP > Table _ max
= APCSETx + APCLUT(min)
APCSET_m
T_COMP < Table _ min
(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 mini-
mum value is used. Hysteresis is employed to further en-
hance 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
80h–BFh
I_MOD Look-up Table
IFLT Look-up Table
C0h–FFh
Bias High Alarm Look-up Table
Table 13. Temperature Compensation Look-up Tables,
Serial Address I2CADR + 4_h
August 2004
25
M9999-082404