VSC7940 datasheet(9/16 Pages) VITESSE

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VITESSE

SEMICONDUCTOR CORPORATION

Preliminary Data Sheet

VSC7940

SONET/SDH 3.125Gb/s

Laser Diode Driver with Automatic Power Control

G52357-0, Rev 3.2

Page 9

05/11/01

Tel: (800) VITESSE • FAX: (805) 987-5896 • Email: prodinfo@vitesse.com

Internet: www.vitesse.com

Enable/Disable

Two pins are provided to allow either ENABLE or DISABLE control. If ENABLE is used, connect DIS-

ABLE to ground. Is DISABLE is used, leave ENABLE floating. Both modulation and bias currents are turned

off when ENABLE is low or DISABLE is high. Typically, ENABLE or DISABLE responds to within approxi-

mately 250ns.

Controlling the Modulation Current

The output modulation current may be determined from the following equation where Pp-p is the peak-to-

peak optical power, PAVE is the average power, re is the extinction ratio, and η is the laser slope efficiency:

IMOD = Pp-p / η= 2 * PAVE * (re-1) / (re+1) / η

A resistor at MODSET controls the output bias current. Graphs of IMODSET vs. RMODSET in Typical Operat-

ing Characteristics describe the relationship between the resistor at MODSET and the output modulation cur-

rent at 25

°C. After determining the desired output modulation current, use the graph to determine the

appropriate resistor value at MODSET.

Controlling the Bias Current

A resistor at BIASMAX should be used to control the output bias current. Graphs of IBIASMAX vs. RBIASMAX

in Typical Operating Characteristics describe the relationship between the resistor at BIASMAX and the output

bias current at 25

°C. If the APC is not used, the appropriate resistor value at BIASMAX is determined by first

selecting the desired output bias current, and then using the graph to determine the appropriate resistor value at

BIASMAX. When using APC, BIASMAX sets the maximum allowed bias current. After determining the max-

imum end-of-life bias current at 85

°C for the laser, refer to the graph of I

BIASMAX vs. RBIASMAX in Typical Oper-

ating Characteristics to select the appropriate resistor value.

Controlling the APC Loop

To select the resistor at APCSET, use the graph of IMD vs. RAPCSET in Typical Operating Characteristics.

The graph relates the desired monitor current to the appropriate resistance value at APCSET. IMD may be calcu-

late from the desired optical average power, PAVE,, and the laser-to-monitor transfer, ρMON, for a specific laser

using the following equation:

IMD = PAVE * ρMON

Laser Diode Interface

An RC shunt network should be placed at the laser output interface. The sum of the resistor placed at the

output and the laser diode resistance should be 25

Ω. For example, if the laser diode has a resistance of 5Ω, a

Ω resistor should be placed in series with the laser. For optimal performance, a bypass capacitor should be

placed close to the laser anode.

A “snubber network” consisting of a capacitor CF and resistor RF should be placed at the laser output to

minimize reflections from the laser (see Block Diagram). Suggested values for these components are 80

Ω and

2pF, respectively, however, these values should be adjusted until an optical output waveform is obtained.

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