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VSC7960RO

3.125Gb/s CML Limiting Amplifier with LOS Detect and Laser Driver with Automatic Power Control

Vitesse Semiconductor 

VITESSE

SEMICONDUCTOR CORPORATION

Advance Product Information

VSC7960

3.125Gb/s CML Limiting Amplifier with LOS Detect

and Laser Driver with Automatic Power Control

ation (no APC), a 100kΩ resistor should be connected between APCSET_MD and GND. CAPC has no effect

on open-loop operation. In both modes of operation, resistors to ground should be placed at BIASMAX and

MODSET to set the bias and modulation currents.

Laser Driver Short-Circuit Protection

If BIASMAX or MODSET are shorted to ground, the output modulation and bias currents will be turned

off.

Laser Driver Enable/Disable

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

to ground. If 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 approximately

250ns.

Controlling the Laser Driver Modulation Current

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

optical power, PAVE is average power, re is extinction ratio, and η is 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 Laser Driver 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 IBIASMAX Vs. RBIASMAX in Typical Oper-

ating Characteristics to select the appropriate resistor value.

Controlling the Laser Driver APC Loop

To select the resistor at APCSET_MD, use the graph of IMD vs. RAPCSET in Typical Operating Characteris-

tics. The graph relates the desired monitor current to the appropriate resistance value at APCSET_MD. IMD may

be calculate 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

G52359-0, Rev 2.1

5/01/01

© 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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