MIC3003GML datasheet(30/75 Pages) MICREL

30 / 75 page

Micrel, Inc.

MIC3003

November 2009

M9999-111209-C

hbwhelp@micrel.com or (408) 955-1690

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 tCONV. This occurs at roughly 10 Hz when 8-bit

temperature

resolution

selected.

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 16xtCONV

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

Byte

Addresses

Function

00h–3Fh

APC Look-up Table

40h–7Fh

IMOD Look-up Table

80h–BFh

IFLT Look-up Table

Base address

+4h

C0h–FFh

Bias High Alarm Look-up Table

58h–63h

APC Look-up Table (cont.)

64h–6Fh

IMOD Look-up Table (cont.)

70h–7Bh

IFLT Look-up Table (cont.)

Base address

+6h

7Ch–87h

Bias High Alarm Look-up Table

(cont.)

Table 16. Temperature Compensation Look-up Tables

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