X9530 Datasheet, PDF (20/30 Pages) Xicor Inc. – Temperature Compensated Laser Diode Controller

X9530
APPLICATIONS INFORMATION
Temperature Sensing
The X9530’s on-chip temperature sensor functions
similarly to other semiconductor temperature sensors.
The surface mount package (TSSOP) and the Chip
Scale Package both allow good thermal conduction
from the PC board to the die, so the X9530 will provide
an accurate measure of the temperature of the board.
If there is no ambient air movement over the device
package or the board, then the measured temperature
will be very close to that of the board. If there is air
movement over the package and the air temperature
is substantially different from that of the PC board,
then the measured temperature will be at a value
between that of the board and the air. If the X9530 is
intended to sense the temperature of a particular
component on the board, the X9530 should be located
as close as possible to that component to minimize
contributions from other devices or the differential
temperatures across the board.
X9530 LASER DIODE BIAS APPLICATION
EXAMPLE
The X9530 is ideally suited to the control of temperature
sensitive parameters in fiber optic applications. Figure
20 shows the typical topology of a laser driver circuit
used in many fiber optic transceiver modules.
This example uses a common anode connected Laser
Diode (LD), in conjunction with a PIN Monitor Photo-
Diode (MPD). The laser diode current (I_LD) is a
summation of the Bias Current (I_BIAS), Modulation
Current (I_MOD) and the Automatic Power Control (APC)
error signal current (I_MON). The APC circuit uses the
MPD current (I_MON) as an input, and ensures that a
constant average optical power output of the LD is
maintaned. The modulation circuitry is driven by an
external high speed data source.
Typical control parameters of a LD driver circuit such
as the one shown in Figure 20 may be:
– I_MODSET : Sets the I_MOD level,
– I_BIASSET : Sets the I_BIAS level,
– I_PINSET: Sets the average optical power output.
Figure 21 shows how the X9530 may be used to
control these parameters while providing accurate
temperature compensation.
In this example the I1 output of the X9530 drives the
I_MODSET input of the laser diode circuit. By loading the
appropriate values into the look-up table (LUT1) of the
device, it can dynamically change the modulation
current of the driver circuit. This may be used to
compensate for the effect of reduced laser light output
at elevated temperatures.
Depending upon the type of driver circuit used, the I2
output of the X9530 may be used to control either
I_BIASSET or I_PINSET parameters. The example in Figure
21 uses I2 to control the I_PINSET parameter, while
I_BIASSET is set at a fixed value using a Intersil Digital
potentiometer.
Similar to the control of the modulation current, I2 may
be used to compensate for changes in I_MON over
temperature. By loading the appropriate values into
the look-up table (LUT2) of the device, this would have
the effect of dynamically controlling the average
optical power output of the LD (via the APC circuit)
over temperature.
The lookup table values for this fiber optic application
could be determined in two ways. One way is to use
well-defined data for LD and monitor photo diode drift
over temperature, and calculate the appropriate I1 and
I2 values needed at each temperature setting. Another
way is to test the assembled module over temperature
and load values into the tables at each setting. This
will require APC on/off control to determine each
MODSET value. See Intersil application note AN156
for a full design analysis with LD driver application.
If design requirements are such that no temperature
compensation is necessary for the average optical
power output of the LD, then the I2 output pin could be
used to set the bias current. I_BIASET of the driver circuit
may be controlled by I2 of the X9530, and the same
current level could be set with control 4 register. This
would provide a constant (temperature independant)
setting for the bias current.
As previously described, the X9530 also contains
general purpose EEPROM memory which may be
accessed by the 2 wire serial bus. In the case of
pluggable fiber optic applications such as GBIC, SFP
or SFF this memory may be used for the storage of
transceiver module parameters.
20
FN8211.0
March 10, 2005