MIC3003 Datasheet, PDF (30/75 Pages) Micrel Semiconductor – FOM Management IC with Internal Calibration

Micrel, Inc.
MIC3003
Temperature Measurement
The temperature-to-digital converter is built around a
switched current source and an eight-bit/nine-bit analog-
to-digital converter. The temperature is calculated by
measuring the forward voltage of a diode junction at two
different bias current levels. An internal multiplexer directs
the current source’s output to a diode junction. This data
is also used as the input to the temperature
compensation look-up tables. Each time temperature is
sampled and an updated value acquired, new corrective
values for modulation current and the APC setpoint are
read from the corresponding tables, added to the set
values, and transferred to the DACs.
Diode Faults
The MIC3003 is designed to respond in a failsafe manner
to hardware faults in the temperature sensing circuitry. If
there is a fault with the on-chip sensing diode, the
temperature data reported by the A/D converter will be
forced to its full-scale value (+127 °C). The diode fault
flag, DFLT, will be set in OEMCFG0, TXFAULT will be
asserted, and the high temperature alarm and warning
flags will be set. The reported temperature will remain at
+127 °C until the fault condition is cleared. Diode faults
may be reset by toggling TXDISABLE, as with any other
fault. Diode faults will not be detected at power up until
the first A/D conversion cycle is completed.
Temperature Compensation
Since the performance characteristics of laser diodes and
photodiodes change with operating temperature, the
MIC3003 provides a facility for temperature compensation
of the APC. loop set-point, laser modulation current, bias
current fault comparator threshold, and bias current high
alarm flag threshold. Temperature compensation is
performed using a look-up table (LUT) that stores values
corresponding to each measured temperature over a
150°C span. Four identical tables reside at serial address
A4h and A6h as summarized in Table 16. Each table
entry is a signed twos complement integer 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 10 Hz when 8-bit
temperature resolution is selected. To prevent
temperature oscillation due to thermal or electrical noise,
sixteen successive temperature samples are averaged
together and used to index the LUT.s. Temperature
compensation results are therefore updated at 16xt_CONV
intervals, or about 1.6 seconds. This can be expressed as
shown in Equation 8:
(8)
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 17) 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 18 and Table 19), 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 20).
This compensation happens internally and does not affect
any host-accessible registers.
(9)
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. 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.
Serial
Address
Base address
+4h
Base address
+6h
Byte
Addresses
00h–3Fh
40h–7Fh
80h–BFh
C0h–FFh
58h–63h
64h–6Fh
70h–7Bh
7Ch–87h
Function
APC Look-up Table
IMOD Look-up Table
IFLT Look-up Table
Bias High Alarm Look-up Table
APC Look-up Table (cont.)
IMOD Look-up Table (cont.)
IFLT Look-up Table (cont.)
Bias High Alarm Look-up Table
(cont.)
Table 16. Temperature Compensation Look-up Tables
November 2009
30
M9999-111209-C
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