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Integrated 3.2Gbps laser driver for vertical cavity surface emitting laser diodes

Milpitas, CA — January 6, 2004 – Linear Technology Corporation introduces a highly integrated 3.2Gbps VCSEL (Vertical Cavity Surface Emitting Laser) diode driver for short reach and medium reach optical communications. The LTC5100 maintains a clean, very low noise transmission eye diagram. It incorporates a feature-rich digital controller with on-chip A/D and D/A converters to provide continuous closed-loop monitoring and control of critical laser parameters. The LTC5100 is targeted for Gigabit Ethernet and Fibre Channel optical links, multi-rate medium (under-20km) and short range fiber optic datalinks, short reach SONET links, and short-haul backplane-interconnect systems.

The LTC5100's digital controller offers programmability and range to accommodate a wide variation of VCSEL laser operating characteristics. The user can set the desired laser bias current, modulation current, laser power, peaking characteristics, and first- and second-order temperature coefficients of a particular laser diode, while storing the setup data in a small and inexpensive external EEPROM. Using a digital feedback loop that monitors the on-chip temperature sensor, the LTC5100's on-board controller drives the laser to the desired optical power levels while compensating for any temperature drift of the laser parameters. The user can select one of two operating modes, Automatic Power Control or Constant Current Control. This closed-loop control removes diode variations, resulting in better manufacturability and consistent quality in the end-product. Moreover, the digital programmability of the laser drive system facilitates automated production setup instead of costly manual adjustments.

The LTC5100 produces consistent and low jitter eye diagrams. Unlike prior laser drivers, the LTC5100 output eye shows minimal variation with temperature, or with laser bias or modulation current level. The precisely controlled true 50 output termination minimizes the problem of reflection-induced distortion of the eye diagram, while allowing placement of the laser diode at an arbitrary distance from the driver output with minimal effect. The LTC5100's grounded cathode output topology significantly reduces EMI (Electro Magnetic Interference) emission. On the input side, the LTC5100's differential port is AC coupled and is 50 terminated, so that no external components are required. The LTC5100 thus enables a high speed, compact optical transmitter to be built in a highly cost-efficient package.

The LTC5100 can be operated in standalone mode, or controlled by an external microprocessor via a built-in I2C Serial Interface port. The LTC5100 has extensive fault detection and eye safety features, including the monitoring of overpower, over-current, under power, and undervoltage.

The LTC5100 operates from a single 3.3V supply. It draws 50 mA of current when transmitting. The LTC5100 is offered in a 4mm x 4mm 16-lead plastic QFN surface mount package, consuming a small PC board space. Pricing starts at $5.15 each in 1,000 piece quantities.

Summary of features:
– 155Mbps to 3.2Gbps laser diode driver for VCSELs
– 60ps rise and fall times, 10ps deterministic jitter
– On-chip ADC and DACs for complete digital closed-loop monitoring and control of laser current and voltage over temperature, to ensure low noise, high quality Eye Diagrams across the modulation range and over temperature
– On-chip digital controller for standalone operation, or microprocessor controlled digital setup via I2C Serial Interface
– Built-in selectable Automatic Power Control or Constant Current Control modes
– Extensive fault detection for laser eye safety
– Fewest external components to fit Small Form Factor applications.

Linear Technology Corporation 1630 McCarthy Boulevard, Milpitas, CA 95035-7417. Tel: 408-432-1900
www.linear.com

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