MIC3000 datasheet(17/68 Pages) MICREL

17 / 68 page

October 2004

M9999-101204

MIC3000

Micrel

Laser Diode Bias Control

The MIC3000 can be configured to generate a constant bias

current using electrical feedback, or regulate average trans-

mitted optical power using a feedback signal from a monitor

photodiode, see Figure 3. An operational amplifier is used to

control laser bias current via the VBIAS output. The VBIAS pin

can drive a maximum of ±10mA. An external bipolar transistor

provides current gain. The polarity of the op amp’s output is

programmable BIASREF in OEMCFG1 in order to accommo-

date either NPN or PNP transistors that drive common anode

and common cathode laser, respectively. Additionally, the

polarity of the feedback signal is programmable for use with

either common-emitter or emitter-follower transistor circuits.

Furthermore, the reference level for the APC circuit is select-

able to accommodate electrical, i.e., current feedback, or

optical feedback via a monitor photodiode. Finally, any one of

seven different internal feedback resistors can be selected.

This internal resistor can be used alone or in parallel with an

external resistor. This wide range of adjustability (50:1)

accommodates a wide range of photodiode current, i.e, wide

range of transmitter output power. The APC operating point

can be kept near the mid-scale value of the APC DAC,

insuring maximum SNR, maximum effective resolution for

digital diagnostics, and the widest possible DAC adjustment

range for temperature compensation, etc. See Figure 4.

The APCCAL bit in OEMCAL0 is used to turn the APC

function on and off. It will be turned off in the MIC3000’s

default state as shipped from the factory. When APC is on,

the value in the selected APCSETx register is added to the

signed value taken from the APC look-up table and loaded

into the VBIAS DAC. When APC is off, the VBIAS DAC may be

written directly via the VBIAS register, bypassing the look-up

table entirely. This provides direct control of the laser diode

bias during setup and calibration. In either case, the VBIAS

DAC setting is reported in the APCDAC register. The APCCFG

bits determine the DACs response to higher or lower numeric

values.

DAC

INV

GAIN

APC

Look-Up

Table

Temp

Sensor

VMOD

Look-Up

Table

APCSET

DAC

MODSET

MODREF

VMOD–

VMOD

COMP

VBIAS

RFB[2:0]

VDD

VOUT

VDD-OUT

VOUT

VDD-OUT

BIASREF

Figure 3. MIC3000 APC and Modulation Control

Block Diagram

APC Op-Amp

RFB[2:0]

BIASREF

51.2k

25.6k

12.8k

6.4k

3.2k

1.6k

0.8k

VDD

Figure 4. Programmable Feedback Resistor

Laser Modulation Control

As shown in Figure 3, a temperature-compensated DAC is

provided to set and control the laser modulation current via an

external laser driver circuit. MODREF in OEMCFG0 selects

whether the VMOD DAC output swings up from ground or

down from VDD. If the laser driver requires a voltage input to

set the modulation current, the MIC3000’s VMOD output can

drive it directly. If a current input is required, a fixed resistor

can be used between the driver and the VMOD output. Several

different configurations are possible, as shown in Figure 6.

When APC is on, i.e., the APCCAL bit in OEMCAL0 is set to

0, the value corresponding to the current temperature is taken

from the MODLUT look-up table, added to MODSET, and

loaded into the VMOD DAC. When APC is off, the value in

VMOD is loaded directly into the VMOD DAC, bypassing the

look-up table entirely. This provides for direct modulation

control for setup and calibration. The MODREF bit deter-

mines the DACs response to higher or lower numeric values.

VDD

Common-cathode with

monitor photodiode

Common-anode with

monitor photodiode

Common-anode

Common-cathode

VDD

Figure 5. Transmitter Configurations

Supported by MIC3000

MIC3000 Datasheet(PDF) 17 Page - Micrel Semiconductor

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