Electronic Design

Automatic Monitoring And Control Define 3.2-Gbit/s VCSEL Drives

The LTC5100 vertical-cavity surface-emitting laser (VCSEL) driver has set its sights on short- to medium-range (< 20 km) applications, including Gigabit Ethernet, Fibre Channel, short-reach Sonet links, and backplane interconnections.

It boasts unprecedented programmability, continuous closed loop monitoring, and control of all critical laser parameters. These features support fully automated production. No external potentiometers or manual adjustments are required for initial setup. External components like coupling capacitors, ferrite beads, or transistors are also eliminated, simplifying board layout.

Data input is differential and capacitive-coupled with on-chip capacitors. The output is a single-ended MOSFET driver designed to operate common cathode VCSEL diodes. The laser, which features user-settable bias, may be modulated with a current over the 1- to 12-mA range. Laser output is monitored with an external photodiode.

Key characteristics of the LTC5100 include 155-Mbit/s to 3.2-Gbit/s operation, 60-ps rise and fall times, and 10-ps max deterministic jitter. The on-chip digital controller contains a 10-bit analog-to-digital converter (ADC) that monitors six different circuit variables, including all critical laser currents and voltages as well as temperature.

The digital controller uses these digitized inputs to set the outputs of four digital-to-analog converters (DACs), which control some aspect of the laser. The automatic power control and constant-current-control feedback modes are user-selectable. Such control functions optimize the eye diagram for best transmission (see the figure).

The LTC5100 is fully programmable via an I2C serial interface that an external controller can use to monitor selected chip parameters for feedback or statistical process control. Operating from a single 3.3-V supply, it draws 50 mA of current when transmitting. It's housed in a 4- by 4-mm, 16-lead plastic QFN surface-mount package. Along with its lack of external components, its small size suits it for SFF and SFP optical transceiver modules.

Available now, the LTC5100 costs $5.15 in 1000-unit quantities.

Linear Technology Corp.
(408) 432-1900, ext. 2233

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