AK2572 Datasheet, PDF (23/50 Pages) Asahi Kasei Microsystems – APC for Burst Mode Applicable Direct Modulation Laser Diode

ASAHI KASEI
[AK2572]
4.4 On-chip Temperature Sensor (TEMPSENS) Characteristics
Characteristic of the On-chip temperature sensor (TEMPSENS) in Figure 4-14, and the relation of AD code
(R_TEMP [7:1] : EEPROM address equivalent) versus Temperature relation in Table 4-10 are shown.
The On-chip temperature sensor characteristic is expressed as :
V [V]＝－0.01156 * t + 1.62 [Typ.]
where temperature t ［℃］、and output voltage V [V]. The output voltage of the detected temperature by the
temperature sensor has a negative slope characteristic with - 11.56 mV/℃±7% (Reference value by design).
Output voltage from the temperature sensor is A-to-D converted by 8 bits ADC (Max. input voltage＝2.2 V
[Typ.]) into the digital code R_TEMP [7:0] and inverted.
7 bits data of the digital code R_TEMP [7:1] is used as EEPROM address, and temperature compensation
data for Bias current (E_BIAS_TC) and Modulation current (E_MOD_TC) are read out.
The relation of Temperature and A-to-D converted code is expressed as :
8 bits AD code : R_TEMP［7:0］＝255－int(- 1.334*t +188.3) [Typ.]
7 bits AD code : R_TEMP［7:1］＝127－int(- 0.667*t +94.0) [Typ.]
Be noted that as output voltage from On-chip temperature sensor is A-to-D converted into digital code and
inverted, AD code against the detected temperature of On-chip temperature sensor exhibits a positive slope
characteristic. Temperature change per each 1 LSB in 8 bits AD code “R_TEMP [7:0]” is +0.75 ℃/LSB [Typ.]
and temperature change per each 1 LSB in 7 bits AD code “R_TEMP [7:1]” is +1.5 ℃/LSB [Typ.].
Therefore temperature compensation data for Bias current (E_BIAS_TC) and Modulation current
(E_MOD_TC) are written in every 1.5 ℃ increment. However, as the temperature compensation data is
derived from a linear interpolation of the detected temperature by the On-chip temperature sensor (refer to
Section 4.1), resolution of R_BAIS_FF and R_MOD_FF data to be loaded to DAC is 0.75 ℃. As On-chip
temperature sensor detects the chip surface temperature, temperature difference exists between the LD
temperature and the detected temperature by the On-chip temperature sensor.
Temperature detect error of the On-chip temperature sensor can be tuned as follows :
(1) Slope calculation at 2 different temperature points :
Read the TEMPSENS output (TEMPMON) or the A-to-D converted code (R_TEMP) at 2 different
temperature points and calculate the slope characteristic. It can adjust and correct the characteristics
including the error on the slope of the On-chip temperature sensor.
(2) Tuning at a single temperature point :
Read the A-to-D converted code (R_TEMP) while LD adjustment is made, and calculate the AD code at a
given temperature, based on the +0.75 ℃ / LSB slope characteristic. It cannot compensate the error of
On-chip temperature sensor in this method.
Figure 4-14 On-chip TEMPSENS Characteristics [Typ.]
2.5
2.0
1.5
1.0
0.5
< MS0290-E-01>
0.0
-40 -20
0
20 40 60 80 100 120
Temperature t [℃]
-23-
2004/8