AK2572 Datasheet, PDF (15/50 Pages) Asahi Kasei Microsystems – APC for Burst Mode Applicable Direct Modulation Laser Diode

ASAHI KASEI
[AK2572]
< Linear interpolation of E_EXTRA_TC >
Given that the detected temperature data are R_TEMP［7:0］＝z＝8y, 8y+1, ･･･, 8y+7, R_TEMP［7:3］＝y,
and the temperature compensated data retained in EEPROM is E_EXTRA_TC(y), and data calculated by
linear interpolation is R_EXTRA(z) (DAC loading E_EXTRA_TC data is selected by RE_MODV_SEL ),
R_EXTRA(z)＝E_EXTRA_TC(y－1)＋｛E_EXTRA_TC(y)－E_EXTRA_TC(y－1)｝×R_TEMP［2:0］/ 8
But at y＝0 (R_TEMP［7:0］＝z＝0 ~ 7), E_EXTRA_TC(y)＝E_EXTRA_TC(y－1)＝E_EXTRA_TC(0)
In order to keep the optical power of LD constant by APC_FF method, regardless of environmental temperature
changes, it is necessary to write and store the data of Bias current and Modulation current at each in the
temperature-corresponding EEPROM address when to adjust each LD module. In normal operation, On-chip
oscillator for temperature compensation of the current to drive LD modules automatically executes the
temperature detection and the current setting.
Those temperature compensated data for Bias current, Modulation current and EXTRA_DAC which are all
derived from the linear interpolation, have approximately 0.75 ℃ resolution and can automatically adjust LD
current and reference voltage for external circuit in approximately 0.75 ℃ step. The On-chip temperature sensor
is designed to cover the temperature range from - 40 ℃ ~ +115 ℃ under the ADC operating voltage range (0 ~
2.2 V [Typ.]). As to the relation between temperature sensor and ADC code, please refer to Section “4.4 On-chip
Temperature Sensor Characteristics“.
Figure 4-2 APC_FF Functional Block Diagram
Output voltage from
TEMPSENS is
A-to-D converted
Read out the data in
EEPROM with referring
Digital code as EEPROM
address
Set the read out data into
I-DAC1(V-DAC3) and
I-DAC2, then control LD
current
On-chip Temperature
Sensor (TEMPSENS)
ADC
(8bit)
ADC output
R_TEMP[7:1]
EEPROM
Digital code changes in
response to temperature
change
Temperature t [℃]
Address Data
R_MOD_FF _I-DAC1
(V-DAC3)
R_BIAS_FF
I-DAC2
LDD
Imod
LD
Ibias
TEMPSENS output
Output voltage proportionally
changes in response to
temperature change
Temperature t [℃]
Memory for Imod
(I-DAC1 or V-DAC3)
128 address
Memory for Ibias
(I-DAC2)
128 address
A proper current value data
should be written in advance
at each address location
which corresponds to each
temperature value
R_MOD_FF, R_BIAS_FF (Linear interpolation of APC_FF data):
R(z) = E(x-1) + {E(x)-E(x-1)} *
R_TEMP[0]
2
E(x): E_BIAS_TC or E_MOD_TC at x=R_TEMP[7:1]
R(z): R_BIAS_FF or R_MOD_FF at z=R_TEMP[7:0]=2x, 2x+1
4.2 APC_FB Function
APC_FB functional block diagram is shown in Figure 4-3.
In APC_FB block, an amplified PDIN voltage by gain value (vpd) and DAC_APC output voltage (vapc_ref) are
compared at APC_COMP, and the feedback current (R_APC_FB) is calculated at digital filter so that vpd and
vapc_ref are equal. The cut-off frequency (fpd), which is fixed by Rpd and Cpd, should be set as follows:
5 kHz＜fpd＜10 kHz
-15-
<MS0290-E-01>
2004/8