X9530 datasheet(21/29 Pages) RENESAS

21 / 29 page

FN8211.1

November 11, 2005

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 (ILD) is a

summation of the Bias Current (IBIAS), Modulation

Current (IMOD) and the Automatic Power Control (APC)

error signal current (IMON). The APC circuit uses the

MPD current (IMON) 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:

– IMODSET : Sets the IMOD level,

– IBIASSET : Sets the IBIAS level,

– IPINSET: 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

IMODSET 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

IBIASSET or IPINSET parameters. The example in Figure

21 uses I2 to control the IPINSET parameter, while

IBIASSET 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 IMON 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. IBIASET 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.

X9530

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