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VSC7938

SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control

Vitesse Semiconductor 

VITESSE

SEMICONDUCTOR CORPORATION

Preliminary Data Sheet

VSC7938

SONET/SDH 3.125Gb/s

Laser Diode Driver with Automatic Power Control

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 within approxi-

mately 250ns.

Slow-Start

For laser safety, the VSC7938 offers a slow-start mechanism via the SLWSTRT pin which provides delay

for enabling the laser diode. To disable Slow-Start, leave SLWSTRT open. An external capacitor to ground sets

the delay by the following equation:

τENABLE(ns) = CSLWSTRT(pF) * 20 + 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 for both 3.3V and 5V operation describe the relationship between the resistor at MODSET

and the output modulation current 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 for both 3.3V and 5V operation 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 deter-

mine the appropriate resistor value at BIASMAX. When using APC, BIASMAX sets the maximum allowed

bias current. After determining the maximum end-of-life bias current at 85°C for the laser, refer to the graph of

IBIASMAX vs. RBIASMAX in Typical Operating 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

G52349-0, Rev 3.0

01/20/01

© VITESSE SEMICONDUCTOR CORPORATION • 741 Calle Plano • Camarillo, CA 93012

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

Internet: www.vitesse.com

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