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VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
VSC7938
Features
SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control
Applications
* SDH/SONET at 622Mb/s, 1.244Gb/s, 2.488Gb/s, 3.125Gb/s
* Power Supply: 3.3V or 5V 5% * AC-Coupled to Laser Diode * Programmable Modulation Current: 5mA to 60mA * Programmable Bias Current: 1mA to 100mA * Enable/Disable Control * Typical Rise/Fall Times of 60ps * Automatic Optical Average Power Control * Supply Current of 33mA at 3.3V
* Full-Speed Fibre Channel (1.062Gb/s)
General Description
The VSC7938 is a single 3.3V or 5V supply laser diode driver specially designed for SDH/SONET applications up to 3.125Gb/s. External resistors set a wide range of bias and modulation currents for driving the laser. Data and clock inputs accept differential PECL signals. The automatic power control (APC) loop maintains a constant average optical power over temperature and lifetime. The dominant pole of the APC loop can be controlled with an external capacitor. Other features include enable/disable control, short-circuit protection for the modulation and bias inputs, short rise and fall times, programmable slow-start circuit to set laser turn-on delay, and failure-monitor output to indicate when the APC loop is unable to maintain the average optical power. The VSC7938 is available in die form or in a 48-pin TQFP package.
Block Diagram
3.3V Operation
VCC LATCH
IOUT+ IOUTMUX DATA+ DATACLK+ CLKSET
LP
CD CF RF VCC BIAS
D Q
CLR Q
ENABLE APC DISABLE MD FAIL SLWSTRT MODSET BIASMAX CAPC APCSET 1nF
G52349-0, Rev 3.0 01/20/01
(c) VITESSE SEMICONDUCTOR CORPORATION * 741 Calle Plano * Camarillo, CA 93012 Tel: (800) VITESSE * FAX: (805) 987-5896 * Email: prodinfo@vitesse.com Internet: www.vitesse.com
LP
Page 1
VITESSE
SEMICONDUCTOR CORPORATION
SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control
Preliminary Data Sheet
VSC7938
Electrical Characteristics
Table 1: AC Specifications
AC specifications are guaranteed by design and characterization. Typical values are for 3.3V.
Symbol
tSU tH
Parameter
Input Latch Setup Time Input Latch Hold Time Enable/Start-up Delay
Min
100 100
Typ
Max
Units
ps ps
Conditions
LATCH=high LATCH=high
250 60 60 10 80 7 20 80 80 50
ns ps ps ps bits psp-p Jitter BW=12kHz to 20MHz, 0-1 pattern. 20% to 80% 20% to 80% See Notes 1, 2
tR tF PWD CIDMAX tJ
Output Rise Time Output Fall Time Pulse Width Distortion Maximum Consecutive Identical Digits Jitter Generation
NOTES:(1) Measured with 622Mb/s 0-1 pattern, LATCH=high. (2) PWD = (wider pulse - narrower pulse) / 2).
Table 2: DC Specifications Symbol
ICC IBIAS IBIAS-OFF SBIAS
Parameter
Supply Current Bias Current Range Bias Off Current Bias Current Stability Bias Current Absolute Accuracy
Min
Typ
TBD
Max
45 100 100
Units
mA mA A ppm/C % V
Conditions
RMODSET=7.3k RBIASMAX=4.8k IBIAS and IMOD excluded VCC=5V Voltage at BIAS pin=(VCC-1.6) ENABLE=low or DISABLE=high(1) APC Open loop. IBIAS=100mA APC Open loop. IBIAS=1mA Refers to part-to-part variation
1
230 900 15 1.5 18 -480 -15 5 -50 90 15 60 200 15 -480 -50 250 480 1000 480
VRMD IMD
Monitor Diode Reverse Bias Voltage Monitor Diode Reverse Current Range Monitor Diode Bias Setpoint Stability Monitor Diode Bias Absolute Accuracy
A ppm/C % mA A % ppm/C ENABLE=low or DISABLE=high(1) See Note 2 IMOD=60mA IMOD=5mA IMD=1mA(2) IMD=18A(2) Refers to part-to-part variation
IMOD IMOD-OFF
Modulation Current Range Modulation Off Current Modulation Current Absolute Accuracy Modulation Current Stability
NOTES: (1) Both IBIAS and IMOD will turn off if any of the current set pins are grounded. (2) Assumes laser diode to monitor diode transfer function does not change with temperature.
Page 2
(c) VITESSE SEMICONDUCTOR CORPORATION * 741 Calle Plano * Camarillo, CA 93012 Tel: (800) VITESSE * FAX: (805) 987-5896 * Email: prodinfo@vitesse.com Internet: www.vitesse.com
G52349-0, Rev 3.0 01/20/01
VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
VSC7938
SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control
Table 3: PECL and TTL/CMOS Inputs and Outputs Specifications Symbol
VID VICM IIN VIH VIL
Parameter
Differential Input Voltage Common-Mode Input Voltage Clock and Data Input Current TTL Input High Voltage (ENABLE, LATCH) TTL Input Low Voltage (ENABLE, LATCH) TTL Output High Voltage (FAIL) TTL Output Low Voltage (FAIL)
Min
100 VCC 1.49 -1 2.0
Typ
VCC 1.32
Max
1600 VCC VID/4 10
Units
mVp-p V A V
Conditions
(DATA+)-(DATA-) PECL Compatible
0.8 2.4 0.1 VCC 0.3 VCC 0.44
V V V Sourcing 50A Sinking 100A
Absolute Maximum Ratings(1)
Power Supply Voltage (VCC)............................................................................................................... -0.5V to 6V Current into BIAS.....................................................................................................................-20mA to +150mA Current into OUT+, OUT- ...............................................................................................................................TBD Current into MD .............................................................................................................................-5mA to +5mA Current into FAIL ......................................................................................................................... -10mA to 30mA Voltage at DATA+, DATA-, CLK+, CLK-, ENABLE, LATCH......................................... -0.5V to (VCC + 0.5V) Voltage at APCFILT, MODSET, BIASMAX, APCSET, MD, FAIL, SLWSTRT.......................... -0.5V to +3.0V Voltage at OUT+, OUT- ..................................................................................................... -0.5V to (VCC + 1.5V) Voltage at BIAS .................................................................................................................. -0.5V to (VCC + 0.5V) Continouous Power Dissipation (TA = +85C, TQFP derate 20.8mW/C above +85C) .......................1350mW Operating Junction Temperature Range ...................................................................................... -55C to +150C Storage Temperature Range ........................................................................................................ -65C to +165C
NOTE: (1) CAUTION: Stresses listed under "Absolute Maximum Ratings" may be applied to devices one at a time without causing permanent damage. Functionality at or above the values listed is not implied. Exposure to these values for extended periods may affect device reliability.
Recommended Operating Conditions
Positive Voltage Rail (VCC)..................................................................................................... +3.135V to +5.25V Negative Voltage Rail (GND) ............................................................................................................................0V Modulation Current (IMOD)(1) .......................................................................................................................30mA Ambient Temperature Range (TA)................................................................................................. -40C to +85C
NOTE: (1) VCC = 3.3V, IBIAS = 60mA.
G52349-0, Rev 3.0 01/20/01
(c) VITESSE SEMICONDUCTOR CORPORATION * 741 Calle Plano * Camarillo, CA 93012 Tel: (800) VITESSE * FAX: (805) 987-5896 * Email: prodinfo@vitesse.com Internet: www.vitesse.com
Page 3
VITESSE
SEMICONDUCTOR CORPORATION
SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control
Preliminary Data Sheet
VSC7938
Bare Die Descriptions
Figure 1: Pad Assignments
1773m (0.0698")
Pad 11 VCC1 Pad 12 GND1 Pad 13 LATCH Pad 14 ENABLE Pad 15 DISABLE Pad 16 GND1 Pad 17 SLWSTRT Pad 18 VCC1 Pad 19 FAIL Pad 20 GND4 Pad 21 PB_GND Pad 22 APCFILT Pad 23 GND4 Pad 24 VCC4 Pad 25 BIAS Pad 26 PB_GND
Pad 10 CLK-
Pad 9 CLK+
Pad 8 VCC1
Pad 7 GND1
Pad 6 VCC1
Pad 5 DATA-
Pad 4 DATA+
Pad 3 VCC1
Pad 2 GND1 Pad 1 GND2 Pad 48 VCC2 Pad 47 BIASMAX Pad 46 MODSET Pad 45 GND2 Pad 44 APCSET Pad 43 AUTSEL Pad 42 GND2 Pad 41 PB_GND Pad 40 GND3 Pad 39 PB_GND Pad 38 CAPC Pad 37 VCC3 Pad 36 GND3
20m (0.0008")
2233m (0.0879")
VSC7938
Pad 27 VCC4
Pad 28 DB_OUT+
Pad 29 OUT+
Pad 30 OUT-
Pad 31 DB_OUT-
Pad 32 VCC4
Pad 33 GND4
Pad 34 GND3
Pad 35 MD
Die Size: Die Thickness: Pad Pitch: Pad Size:
1773m x 2233m (0.0698" x 0.0879") 625m (0.0246") 115m (0.0045")
75m (0.0030")
Pad to Pad Clearance:
95m x 95m (0.0037" x 0.0037") 20m (0.0008") Pad Passivation Opening: 95m x 95m (0.0037" x 0.0037") Scribe Size: 75m (0.0030")
Page 4
(c) VITESSE SEMICONDUCTOR CORPORATION * 741 Calle Plano * Camarillo, CA 93012 Tel: (800) VITESSE * FAX: (805) 987-5896 * Email: prodinfo@vitesse.com Internet: www.vitesse.com
G52349-0, Rev 3.0 01/20/01
VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
VSC7938
Table 4: Pad Coordinates Signal Name
GND2 GND1 VCC1 DATA+ DATAVCC1 GND1 VCC1 CLK+ CLKVCC1 GND1 LATCH ENABLE DISABLE GND1 SLWSTRT VCC1 FAIL GND4 PB_GND APCFILT GND4 VCC4
SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control
Pad Number
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Coordinates (m) X
1613.55 1414.525 1289.525 1174.525 1059.525 944.525 829.525 714.525 599.525 484.525 369.525 159.45 159.45 159.45 159.45 159.45 159.45 159.45 159.45 159.45 159.45 159.45 159.45 159.45
Y
1863.475 2073.55 2073.55 2073.55 2073.55 2073.55 2073.55 2073.55 2073.55 2073.55 2073.55 1863.475 1748.475 1633.475 1518.475 1403.475 1288.475 1058.475 1058.475 828.475 828.475 598.475 598.475 483.475
Signal Name
BIAS PB_GND VCC4 DB_OUT+ OUT+ OUTDB_OUTVCC4 GND4 GND3 MD GND3 VCC3 CAPC PB_GND GND3 PB_GND GND2 AUTSEL APCSET GND2 MODSET BIASMAX VCC2
Pad Number
25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
Coordinates (m) X
159.45 369.525 484.525 599.525 714.525 829.525 944.525 1059.525 1174.525 1289.525 1404.525 1613.55 1613.55 1613.55 1613.55 1613.55 1613.55 1613.55 1613.55 1613.55 1613.55 1613.55 1613.55 1613.55
Y
368.475 159.45 159.45 159.45 159.45 159.45 159.45 159.45 159.45 159.45 159.45 368.475 483.475 598.475 713.475 828.475 943.475 1058.475 1173.475 1288.475 1403.475 1518.475 1633.475 1748.475
G52349-0, Rev 3.0 01/20/01
(c) VITESSE SEMICONDUCTOR CORPORATION * 741 Calle Plano * Camarillo, CA 93012 Tel: (800) VITESSE * FAX: (805) 987-5896 * Email: prodinfo@vitesse.com Internet: www.vitesse.com
Page 5
VITESSE
SEMICONDUCTOR CORPORATION
SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control
Preliminary Data Sheet
VSC7938
Package Pin Descriptions
Figure 2: Pin Diagram
RESERVED
BIASMAX
MODSET
APCSET
CAPC 38
GND
GND
GND
VCC
48
47
46
45
44
43
42
41
40
39
GND GND VCC DATA+ DATAVCC GND VCC CLK+ CLKVCC GND
1 2 3 4 5 6 7 8 9 10 11 12
37
VCC
NC
NC
36 35 34 33 32 31
GND MD GND GND VCC NC OUTOUT+ NC VCC NC BIAS
VSC7938
30 29 28 27 26 25
13
14
15
16
17
18
19
20
21
22
23 GND
LATCH
ENABLE
DISABLE
SLWSTRT
VCC
FAIL
APCFILT
GND
GND
Page 6
(c) VITESSE SEMICONDUCTOR CORPORATION * 741 Calle Plano * Camarillo, CA 93012 Tel: (800) VITESSE * FAX: (805) 987-5896 * Email: prodinfo@vitesse.com Internet: www.vitesse.com
VCC
NC
24
G52349-0, Rev 3.0 01/20/01
VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
VSC7938
Table 5: Pin Identifications Pin Name
GND VCC DATA+ DATACLK+ CLKLATCH ENABLE DISABLE SLWSTRT FAIL NC APCFILT BIAS OUT+ OUTMD CAPC RESERVED APCSET MODSET BIASMAX
SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control
Pin Number
1, 2, 7, 12, 16, 20, 23, 33, 34 36, 40, 42, 45 3, 6, 8, 11, 18, 24, 27, 32, 37, 48 4 5 9 10 13 14 15 17 19 21, 26, 28, 31, 39, 41 22 25 29 30 35 38 43 44 46 47 Ground Power Supply Positive Data Input (PECL) Negative Data Input (PECL)
Description
Positive Clock Input (PECL). Connect to VCC if data retiming is not used. Negative Clock Input (PECL). Leave unconnected if data retiming is not used. Latch Input (TTL/CMOS). Connect to VCC for data retiming and GND for direct data. Enable Input (TTL/CMOS). If used, connect DISABLE to GND. Connect to VCC for normal operation and GND to disable laser bias and modulation currents. Disable Input (TTL/CMOS). If used, leave ENABLE pin floating. Connect to GND for normal operation and VCC to disable laser bias and modulation currents. Connect capacitor to GND to delay turn on time of bias and modulation currents. Output (TTL/CMOS). When low, it indicates APC failure. No Connection. Leave these pins unconnected. No effect on device operation. Laser Bias Current Output Positive Modulation-Current Output. IMOD flows when input data is high. Negative Modulation-Current Output. IMOD flows when input data is low. Monitor Diode Input. Connect to monitor photodiode anode. Connect capacitor to GND to filter high-speed AC monitor photocurrent. Capacitor to GND sets dominant pole of the APC feedback loop. Do not connect. Resistor to GND sets desired average optical power. If APC is not used connect 100k resistor to GND. Connect resistor to GND to set desired modulation current. Connect resistor to GND to set maximum bias current. The APC function can subtract from this value, but it cannot add to it.
G52349-0, Rev 3.0 01/20/01
(c) VITESSE SEMICONDUCTOR CORPORATION * 741 Calle Plano * Camarillo, CA 93012 Tel: (800) VITESSE * FAX: (805) 987-5896 * Email: prodinfo@vitesse.com Internet: www.vitesse.com
Page 7
VITESSE
SEMICONDUCTOR CORPORATION
SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control
Preliminary Data Sheet
VSC7938
Detailed Description
The VSC7938 is a high-speed laser driver with Automatic Power Control. The device is designed to operate up to 3.125Gb/s with a 3.3V or 5V supply. The data and clock inputs support PECL inputs as well as other inputs that meet the common-mode voltage and differential voltage swing specifications. The differential pair output stage is capable of sinking up to 60mA from the laser with typical rise and fall times of 60ps. This output may be DC-coupled for 5V operation. To allow for larger output swings during 3.3V operation, the VSC7938 was designed to be AC-coupled to the laser cathode with a pull-up inductor for DC-biasing. This configuration will isolate laser forward voltage from the output circuitry and will allow the output at OUT+ to swing above and below the supply voltage VCC. The key features of the VSC7938 are Automatic Power Control, low power supply current, and fast rise and fall times. The VSC7939 and VSC7940 are other Vitesse laser drivers with similar features in a 32-pin TQFP package. These devices also have pins for monitoring modulation and bias currents. The VSC7940 is a special version of the VSC7939 designed to drive 100mA into a DC-coupled load with a 5V supply.
Automatic Power Control To ensure constant average optical power, the device utilizes an Automatic Power Control loop. A photodiode mounted in the laser package provides optical feedback to compensate for changes in average laser output power due to changes that affect laser performance such as temperature and laser lifetime. The laser bias current is adjusted by the APC loop according to the reference current set at APCSET by an external resistor. An external capacitor at CAPC controls the time constant for the APC feedback loop. The recommended value for CAPC is 0.1F. This value reduces pattern-dependent jitter associated with the APC feedback loop and guarantees stability. Because the APC loop noise is internally filtered, APCFILT is not internally connected and does not need to be connected to any external components. The device's performance will not be affected if a capacitor is connected to APCFILT. If the APC loop cannot adjust the bias current to track the desired monitor current, FAIL is set low. The device may be operated with or without APC. To utilize APC, a capacitor must be connected at CAPC (0.1F) and a resistor must be connected at APCSET to set the average optical power. For open-loop operation (no APC), a 100k resistor should be connected between APCSET and GND. CAPC has no effect on openloop operation. In both modes of operation, resistors to ground should be placed at BIASMAX and MODSET to set the bias and modulation currents. Data Retiming The VSC7938 provides inputs for differential PECL clock signals for data retiming to minimize jitter at high speeds. To incorporate this function, LATCH should be connected to VCC. If this function is unused, CLK+ should be connected to VCC, CLK- should be left unconnected, and LATCH should be connected to GND. Short-Circuit Protection If BIASMAX or MODSET are shorted to ground, the output modulation and bias currents will be turned off.
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(c) VITESSE SEMICONDUCTOR CORPORATION * 741 Calle Plano * Camarillo, CA 93012 Tel: (800) VITESSE * FAX: (805) 987-5896 * Email: prodinfo@vitesse.com Internet: www.vitesse.com
G52349-0, Rev 3.0 01/20/01
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 DISABLE 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 approximately 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: ENABLE(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-topeak optical power, PAVE is the average power, re is the extinction ratio, and is the laser slope efficiency:
IMOD = Pp-p / = 2 * PAVE * (re-1) / (re+1) / A resistor at MODSET controls the output bias current. Graphs of IMODSET vs. RMODSET in Typical Operating Characteristics for both 3.3V and 5V operation describe the relationship between the resistor at MODSET and the output modulation current at 25C. 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 25C. 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 maximum end-of-life bias current at 85C 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 calculate from the desired optical average power, PAVE,, and the laser-to-monitor transfer, MON, for a specific laser using the following equation: IMD = PAVE * MON
G52349-0, Rev 3.0 01/20/01
(c) VITESSE SEMICONDUCTOR CORPORATION * 741 Calle Plano * Camarillo, CA 93012 Tel: (800) VITESSE * FAX: (805) 987-5896 * Email: prodinfo@vitesse.com Internet: www.vitesse.com
Page 9
VITESSE
SEMICONDUCTOR CORPORATION
SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control
Preliminary Data Sheet
VSC7938
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. For example, if the laser diode has a resistance of 5, a 20 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 and 2pF, respectively. However, these values should be adjusted until an optical output waveform is obtained. Reducing Pattern-Dependent Jitter Three design values significantly affect pattern-dependent jitter; the capacitor at CAPC, the pull-up inductor at the output (LP), and the AC-coupling capacitor at the output (CD). As previously stated, the recommended value for the capacitor at CAPC is 0.1F. This results in a 10kHz loop bandwidth which makes the patterndependent jitter from the APC loop negligible. For 2.5Gb/s data rates, the recommended value for CD is 0.056F. The time constant at the output is dominated by LP. The variation in the peak voltage should be less that 12% of the average voltage over the maximum consecutive identical digit (CID) period. The following equation approximates this time constant for a CID period, t, of 100UI = 40ns:
LP = -t / ln(1-12%) = 7.8t = LP / 25
Therefore, the inductor LP should be a 7.8H SMD ferrite bead inductor for this case.
Input/Output Considerations Although the VSC7938 is PECL compatible, this is not required to drive the device. The inputs must only meet the common-mode voltage and differential voltage swing specifications. Power Consumption The following equation provides the device supply current (IS) in terms of quiescent current (IQ), modulation current (IMOD), and bias current (IBIAS):
IS = IQ + 0.47 * IMOD + 0.15 * IBIAS For 3.3V operation, IQ is 15mA. For 5V operation, IQ is 20mA. This equation may be used to determine the estimated power dissipation: PDIS = VCC * IS For example, if the device were operated at 3.3V with a 30mA modulation current and a 10mA bias current, the supply current would be: IS = 15mA + 0.47 * 30mA + 0.15 * 10mA = 31 This corresponds to a power dissipation of 3.3V * 31mA = 102mW.
Page 10
(c) VITESSE SEMICONDUCTOR CORPORATION * 741 Calle Plano * Camarillo, CA 93012 Tel: (800) VITESSE * FAX: (805) 987-5896 * Email: prodinfo@vitesse.com Internet: www.vitesse.com
G52349-0, Rev 3.0 01/20/01
VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
VSC7938
SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control
Typical Operating Characteristics
IMODSET vs. RMODSET
T=25C VCC = 3.3V
IMODSET vs. RMODSET
T=25C VCC = 5V
IBIASMAX vs. RBIASMAX
T=25C, VCC = 3.3V
IBIASMAX vs. RBIASMAX
T=25C, VCC = 5V
G52349-0, Rev 3.0 01/20/01
(c) 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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VITESSE
SEMICONDUCTOR CORPORATION
SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control
IMD vs. RAPCSET, T=25C, VCC = 3.3V
Preliminary Data Sheet
VSC7938
IMD vs. RAPCSET, T=25C, VCC = 5V
Rise and Fall Times
T=85C, V=3.3V
Monte Carlo Simulation of ICC
Page 12
(c) VITESSE SEMICONDUCTOR CORPORATION * 741 Calle Plano * Camarillo, CA 93012 Tel: (800) VITESSE * FAX: (805) 987-5896 * Email: prodinfo@vitesse.com Internet: www.vitesse.com
G52349-0, Rev 3.0 01/20/01
VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
VSC7938
Applications Information
SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control
The following is a typical design example for the VSC7938 assuming 3.3V operation with APC.
Select a Laser The Table 5 provides specifications for a typical communication-grade laser capable of operating at 2.5Gb/s. Table 6: Typical Laser Characteristics Symbol
PAVE Ith MON TC
Parameter
Wavelength Average Optical Output Power Threshold Current Laser to Monitor Transfer Laser Slope Efficiency Operating Temperature Range
Value
1310 6 6 0.04 0.4 -40 to +85
Units
nm mW mA mA/mW mW/mA C
Select Resistor for APCSET The monitor diode current is estimated by IMD = PAVE * MON = 6mW * 0.04mA/mW = 0.24mA. The IMD vs. RAPCSET in Typical Operating Characteristics shows the resistor at APCSET should be 5k. Select Resistor for MODSET To ensure some minimum extinction ratio over temperature and lifetime, assume an optimal extinction ratio of 20 (13dB) at 25C. The modulation current may be calculated from the following equation:
IMOD = Pp-p / = 2 * PAVE * (re-1) / (re+1) / = 2 * 6mA * (20-1) / (20 + 1) / 0.4 = 27.1mA The graph of IMODSET vs. RMODSET in Typical Operating Characteristics shows the resistor for MODSET should be 8.5k.
Select Resistor for BIASMAX The maximum threshold current at +85C and end of life must be determined. A graph of a typical laser's Ith versus TC reveals a maximum threshold current of 30mA at 85C. Therefore, the maximum bias can be approximated by: IBIASMAX = ITH-MAX + IMOD / 2 = 30mA + 27.1mA / 2 = 43.6mA
The graph of IBIASMAX vs. RBIASMAX in Typical Operating Characteristics shows the resistor for BIASMAX should be 5k.
Wire Bonding For best performance, gold ball-bonding techniques are recommended. Wedge bonding is not recommended. For best performance and to minimize inductance keep wire bond lengths short.
PCB Layout Guidelines
Use high frequency PCB layout techniques with solid ground planes to minimize crosstalk and EMI. Keep high speed traces as short as possible for signal integrity. The output traces to the laser diode must be short to minimize inductance. Short output traces will provide best performance.
G52349-0, Rev 3.0 01/20/01 (c) VITESSE SEMICONDUCTOR CORPORATION * 741 Calle Plano * Camarillo, CA 93012 Tel: (800) VITESSE * FAX: (805) 987-5896 * Email: prodinfo@vitesse.com Internet: www.vitesse.com
Page 13
VITESSE
SEMICONDUCTOR CORPORATION
SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control
Preliminary Data Sheet
VSC7938
Package Information - 48 Pin TQFP
1. All dimensioning and tolerancing conform to ANSI Y14.5-1982. 2. Controlling dimension: millimeter. 3. This outline conforms to JEDEC Publication 95 Registration MS-026.
Page 14
(c) VITESSE SEMICONDUCTOR CORPORATION * 741 Calle Plano * Camarillo, CA 93012 Tel: (800) VITESSE * FAX: (805) 987-5896 * Email: prodinfo@vitesse.com Internet: www.vitesse.com
G52349-0, Rev 3.0 01/20/01
VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
VSC7938
Ordering Information
SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control
The order number for this product is formed by a combination of the device type and package type.
VSC7938 xx
Device Type SONET/SDH 3.125Gb/s Laser Diode Driverwith Automatic Power Control
Package RO: 48-Pin TQFP, 7x7x1.4mm W: Dice Waffle Pack
Notice
Vitesse Semiconductor Corporation ("Vitesse") provides this document for informational purposes only. This document contains pre-production information about Vitesse products in their concept, development and/or testing phase. All information in this document, including descriptions of features, functions, performance, technical specifications and availability, is subject to change without notice at any time. Nothing contained in this document shall be construed as extending any warranty or promise, express or implied, that any Vitesse product will be available as described or will be suitable for or will accomplish any particular task. Vitesse products are not intended for use in life support appliances, devices or systems. Use of a Vitesse product in such applications without written consent is prohibited.
G52349-0, Rev 3.0 01/20/01
(c) VITESSE SEMICONDUCTOR CORPORATION * 741 Calle Plano * Camarillo, CA 93012 Tel: (800) VITESSE * FAX: (805) 987-5896 * Email: prodinfo@vitesse.com Internet: www.vitesse.com
Page 15
VITESSE
SEMICONDUCTOR CORPORATION
SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control
Preliminary Data Sheet
VSC7938
Page 16
(c) VITESSE SEMICONDUCTOR CORPORATION * 741 Calle Plano * Camarillo, CA 93012 Tel: (800) VITESSE * FAX: (805) 987-5896 * Email: prodinfo@vitesse.com Internet: www.vitesse.com
G52349-0, Rev 3.0 01/20/01

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