MIC3003 datasheet(26/75 Pages) MICREL

26 / 75 page

Micrel, Inc.

MIC3003

November 2009

M9999-111209-C

hbwhelp@micrel.com or (408) 955-1690

Furthermore, the reference level for the APC circuit is

selectable 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 6.

The APCCAL bit in OEMCAL0 is used to turn the APC

function on and off. It will be turned on in the MIC3003’s

default state as shipped from the factory. When the APC

is on, the value in the selected APCSETx register is

added to the signed compensation value taken from the

APC look-up table and loaded into the VBIAS DAC. When

the 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.

Figure 5. MIC3003 APC and Modulation Control

Block Diagram

Figure 6. Programmable Feedback Resistor

Laser Modulation Control

As shown in Figure 5, a temperature-compensated DAC

is provided to set and control the laser modulation

current via an external laser driver circuit. The MODREF

bit 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 MIC3003’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 8.

When the 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 the selected MODSETx register, and loaded

into the VMOD DAC. When the APC is off, the

compensation 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.

Figure 7. Transmitter Configurations

Supported by MIC3003

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