ONET4201LD
www.ti.com...................................................................................................................................... SLLS677A – NOVEMBER 2005 – REVISED SEPTEMBER 2009
155-Mbps to 4.25-Gbps LASER DRIVER
Check for Samples: ONET4201LD
FEATURES
APPLICATIONS
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Multirate Operation From 155 Mbps up to 4.25
Gbps
Bias Current Programmable From 1 mA
to 100 mA
Modulation Current Programmable From 5 mA
to 85 mA
APC and Fault Detection
Fault Mode Selection
Bias and Photodiode Current Monitors
CML Data Inputs
Temperature Compensation of Modulation
Current
Single 3.3-V Supply
Active Back-Termination at the Output
Surface-Mount, Small-Footprint, 4-mm × 4-mm,
24-Lead QFN Package
SONET/SDH Transmission Systems
Fibre Channel Optical Modules
Fiber Optic Data Links
Digital Cross-Connects
Optical Transmitters
DESCRIPTION
The ONET4201LD is a laser driver for multiple fiber
optic applications up to 4.25 Gbps. The device
accepts CML input data and provides bias and
modulation currents for driving a laser diode. Also
provided are automatic power control (APC),
temperature compensation of modulation current,
fault detection, and current monitor features.
The device is available in a small-footprint, 4-mm ×
4-mm, 24-pin, QFN package. The circuit requires a
single 3.3-V supply.
This power-efficient laser driver is characterized for
operation from –40°C to 85°C.
1
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas
Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2005–2009, Texas Instruments Incorporated
ONET4201LD
SLLS677A – NOVEMBER 2005 – REVISED SEPTEMBER 2009...................................................................................................................................... www.ti.com
This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling and installation procedures can cause damage.
ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more
susceptible to damage because very small parametric changes could cause the device not to meet its published specifications.
DETAILED DESCRIPTION
BLOCK DIAGRAM
A simplified block diagram of the ONET4201LD is shown in Figure 1.
This compact, low-power, 4.25-Gbps laser driver circuit consists of a high-speed data path and a bias and control
block.
The function of the data path is to buffer the input data and then modulate the laser diode current according to
the input data stream.
The bias and control block generates the laser diode bias current, contains automatic power control (APC) to
maintain constant optical output power, generates a modulation current that can be temperature compensated
and controls power-on during start-up and shutdown after failure detection. The circuit design is optimized for
high-speed and low-voltage operation (3.3 V).
The main circuit blocks are described in detail below.
OUTPOL
Current Modulator
MOD+
Input Buffer Stage
DIN+
DIN–
MOD–
Modulation Current Generator
MODSET
MODSET
MODTC
MODTC
MODCTRL
OUT+
IMODEN
IMODMON
OUT–
Active Termination
Bias Current Generator
IBMAX
IBMAX
IBEN
4
VCC
IBMON
IBSET
MONB
MONB
VCC
3
GND
BIAS
BIAS
Reference
Voltage
and Bias
GND Generation
Automatic Power Control
(APC)
IMODEN
IMODMON
Control
IBEN
IBMON
IBSET
CAPC
CAPC
MONP
MONP
APCCTRL
APCCTRL
PD
APCMON
APCSET
DISABLE
DISABLE
APCMON
FLTMODE
FLTMODE
SDOWN
PD
APCSET
SDOWN
B0092-02
Figure 1. Simplified Block Diagram of the ONET4201LD
2
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HIGH-SPEED DATA PATH
The high-speed data path consists of an input buffer stage and a current modulator.
The input buffer stage takes CML compatible differential signals. It provides on-chip 50-Ω termination to VCC.
AC-coupling may be used at the DIN+ and DIN– inputs.
The laser diode current modulator mainly consists of two common-emitter output transistors and the required
driver circuitry. Depending on the input data stream, the modulation current is sunk at the MOD+ or the MOD–
pin.
Modulation current setting is performed by means of the modulation current generator block, which is supervised
by the control circuit block.
The laser diode can be either ac- or dc-coupled. In both cases, the maximum modulation current is 85 mA. The
modulation output is optimized for driving a 20-Ω load.
For optimum performance when driving a laser diode over a 20-Ω transmission line, the ONET4201LD provides
active 20-Ω back-termination, which minimizes jitter caused by reflections.
BIAS AND CONTROL
The bias and control circuitry consists of the bandgap voltage and bias generation block, the bias current
generator, the automatic power control block and the supervising control circuitry.
BANDGAP VOLTAGE AND BIAS GENERATION
The bandgap voltage reference provides process and temperature-independent reference voltages needed to set
bias current, modulation current, and photodiode reference current. Additionally, this block provides the biasing
for all internal circuits.
AUTOMATIC POWER CONTROL
The ONET4201LD laser driver incorporates an APC loop to compensate for the changes in laser threshold
current over temperature and lifetime. The internal APC is enabled when resistors are connected to the IBMAX
and APCSET pins. A back-facet photodiode mounted in the laser package is used to detect the average laser
output power. The photodiode current IPD, which is proportional to the average laser power, can be calculated by
using the laser-to-monitor transfer ratio, ρMON, and the average power, PAVG:
I PD [A] + PAVG [W] òMON[AńW]
(1)
In closed-loop operation, the APC modifies the laser diode bias current by comparing IPD with a reference current
IAPCSET and generates a bias compensation current. IPD can be programmed by selecting the external resistor
RAPCSET according to:
4.69 V
R
[W] + 4.69 V +
APCSET
I
[A]
P
[W] ò
[AńW]
PD
AVG
MON
(2)
The bias compensation current subtracts from the maximum bias current to maintain the monitor photodiode
current. The maximum bias current is programmed by the resistor connected to IBMAX:
343 V
I
[A] +
BIASMAX
R
[W]
BIASMAX
(3)
This current limit establishes the maximum bias current available in closed loop mode, as well as in transient
fault conditions such as shorts at the PD pin to ground or delayed laser power up.
An external pin MONB is provided as a bias current monitor output. A fraction of the bias current (1/68) is
mirrored and develops a voltage drop across an external resistor to ground, RMONB. The voltage at MONB is
given as:
R
[W] I
[A]
BIAS
V
[V] + MONB
MONB
68
(4)
If the voltage at MONB is greater than the programmed threshold, a fault mode occurs.
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The MONP is also provided as a photocurrent monitor output. The photodiode current, IPD, is mirrored and
develops a voltage across an external resistor to ground, RMONP. The voltage at MONP is given as:
V
[V] + R
[W] I [A]
MONP
MONP
PD
(5)
If the voltage at MONP is greater than the programmed threshold, a fault mode occurs.
As with any negative-feedback system design, care must be taken to assure stability of the loop. The loop
bandwidth must not be too high in order to minimize pattern-dependent jitter. The dominant pole is determined by
the capacitor CAPC. The recommended value for CAPC is 200 nF. The capacitance of the monitor photodiode CPD
adds another pole to the system, and thus it must be small enough to maintain stability. The recommended value
for this capacitance is CPD ≤ 50 pF.
The internal APC loop can be disabled by connecting a 100-kΩ resistor from APCSET to VCC and leaving PD
open. In open-loop operation, the laser diode current is set by IBIASMAX and IMODSET.
MODULATION-CURRENT GENERATOR
The modulation-current generator defines the tail current of the modulator, which is sunk from either MOD+ or
MOD–, depending on the data pattern. The modulation current consists of a modulation current IMOD0 at a
reference temperature T0 = 60°C (set by the resistor RMODSET) and a temperature-dependent modulation current
defined by the resistor RMODTC. The modulation current can be estimated as follows:
I
[A] +
MOD
R
265 V
MODSET
[W]
ǒ ǒ
1)
R
24 W
[W]
MODTC
) 630 ppm
Ǔ
ǒT[ C] * T 0[ C]Ǔ
o
o
Ǔ
(6)
Note that the reference temperature, T0, and the temperature compensation set by RMODTC vary from part to part.
To reduce the variation, IMOD can be calibrated over temperature and set with a microcontroller DAC or digital
potentiometer.
CONTROL
The functions of this block are to control the start-up sequence, detect faults, detect tracking failure of the APC
loop, and provide disable control. The laser driver has a controlled start-up sequence, which helps prevent
transient glitches from being applied to the laser during power on. At start-up, the laser diode is off, SDOWN is
low, and the APC loop is open. Once VCC reaches ~2.8 V, the laser diode bias generator and modulation current
generator circuitry are activated (if DISABLE is low). The slow-start circuitry gradually brings up the current
delivered to the laser diode. From the time that VCC reaches ~2.8 V until the modulation current and bias current
reach 95% of their steady state value, is considered the initialization time. If DISABLE is asserted during power
on, the slow-start circuitry does not activate until DISABLE is negated.
FAULT DETECTION
The fault-detection circuitry monitors the operation of the ONET4201LD. If FLTMODE is set to a low level,
(hard-fault mode) this circuitry disables the bias and modulation circuits and latches the SDOWN output on
detection of a fault. The fault mode is reset by toggling DISABLE (for a minimum time of TRES) or by toggling VCC.
Once DISABLE is toggled, SDOWN is set low and the circuit is re-initialized.
If FLTMODE is set to a high level (soft-fault mode), a fault is indicated at the SDOWN output; however, the bias
and modulation circuits are not disabled. The SDOWN output is reset once the fault causing condition
disappears. Toggling DISABLE or VCC is not required.
A functional representation of the fault detection circuitry is shown in Figure 2.
4
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TRES Counter
DISABLE
IMODEN
RES
START
IBEN
Inverter
Comparator
Inverter
Flipflop
+
VCC
Q
R
+
-
IPD
S
2.8 V
Q
MUX
I0 Q
I1
CMOS
Buffer
SDOWN
MUX
IBIAS/68
I1 Q
Comparator
I0
+
MONB
Q
-
Comparator
+
MONP
Q
+
-
MODTC
SHORT
MODTC
MODSET
MODSET
APCSET
APCSET
IBMAX
1.25 V
Short Circuit
to VCC or
GND Detect
IBMAX
FLTMODE
B0093-01
Figure 2. Functional Representation of the Fault Detection Circuitry
A fault mode is produced if the laser cathode is grounded and the photocurrent causes MONP to exceed its
programmed threshold. Another fault mode can be produced if the laser diode end-of-life condition causes
excessive bias current and photodiode current that results in monitor voltages (MONP, MONB) being greater
than their programmed threshold. Other fault modes can occur if there are any I/O pin single-point failures (short
to VCC or GND) and the monitor voltages exceed their programmed threshold (see Table 1).
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Table 1. Response to I/O-Pin Shorts to VCC or GND
FLTMODE = LOW
PIN
Response to Short to GND
Response to Short to VCC
FLTMODE = HIGH
Response to Short to GND
Response to Short to VCC
APCSET
SDOWN latched high, IBIAS and
IMOD disabled
No fault, IMOD unaffected
SDOWN high, IBIAS and IMOD No fault
unaffected
BIAS
SDOWN latched high, IMOD
disabled
No fault, IBIAS goes to zero
SDOWN high, IMOD
unaffected
No fault, IMOD unaffected
CAPC
No fault
No fault, IBIAS goes to zero
No fault, IMOD unaffected
No fault, IBIAS goes to zero
DIN+
No fault, IMOD disabled
No fault
No fault, IMOD disabled
No fault
DIN–
No fault, IMOD disabled
No fault
No fault, IMOD disabled
No fault
DISABLE
Normal circuit operation
Normal circuit operation
Normal circuit operation
Normal circuit operation
IBMAX
SDOWN latched high, IBIAS and
IMOD disabled
SDOWN latched high, IBIAS
and IMOD disabled
SDOWN high, IMOD
unaffected
SDOWN high, IMOD unaffected
MOD+
SDOWN latched high, IBIAS and
IMOD disabled
No fault
SDOWN high, IBIAS
unaffected
No fault
MOD–
SDOWN latched high, IBIAS and
IMOD disabled
No fault
SDOWN high, IBIAS
unaffected
No fault
MODSET
SDOWN latched high, IBIAS and
IMOD disabled
No fault, disables IMOD
SDOWN high, IBIAS
unaffected
No fault, disables IMOD
MODTC
SDOWN latched high, IBIAS and
IMODdisabled
No fault
SDOWN high, IBIAS and IMOD No fault
unaffected
MONB
No fault
SDOWN latched high, IBIAS
and IMOD disabled
No fault
SDOWN high, IBIAS and IMOD
unaffected
MONP
No fault
SDOWN latched high, IBIAS
and IMOD disabled
No fault
SDOWN high, IBIAS and IMOD
unaffected
OUTPOL
No fault, polarity reverses
No fault
No fault, polarity reverses
No fault
PD
No fault, IMOD unaffected
No fault, IBIAS goes to zero
No fault, IMOD unaffected
No fault, IBIAS goes to zero
SDOWN
No fault
No fault
No fault
No fault
PACKAGE
For the ONET4201LD, a small-footprint, 4-mm × 4-mm, 24-lead QFN package is used, with a lead pitch of 0,5
mm. The pinout is shown in Figure 3.
In order to achieve the required low thermal resistance of about 38 K/W, which keeps the maximum junction
temperature below 115°C, a good thermal connection of the exposed die pad is mandatory.
6
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PD
CAPC
IBMAX
OUTPOL
APCSET
DISABLE
RGE PACKAGE
(TOP VIEW)
17
VCC
3
16
MOD–
DIN–
4
15
MOD+
VCC
5
14
VCC
GND
6
13
12
BIAS
8
9
10 11
MODSET
7
MODTC
VCC
DIN+
SDOWN
GND
2
FLTMODE
24 23 22 21 20 19
18
MONB
1
MONP
GND
P0024-03
Figure 3. Pinout of the ONET4201LD in a 4-mm × 4-mm, 24-Lead QFN Package (Top View)
TERMINAL FUNCTIONS
TERMINAL
NAME
NO.
I/O
DESCRIPTION
APCSET
23
Analog-in
BIAS
13
Analog-out
CAPC
20
Analog
APC loop capacitor
DIN+
3
CML-in
Noninverted data input. On-chip, 50-Ω terminated to VCC.
DIN–
4
CML-in
Inverted data input. On-chip, 50-Ω terminated to VCC.
DISABLE
24
LVTTL-in
Disable modulation and bias current outputs.
FLTMODE
10
CMOS-in
Fault mode selection input. If a low level is applied to this pin, any fault event is latched and the
bias and modulation currents are disabled in a fault condition. Toggling of DISABLE or VCC
resets the fault condition. If pin is set to a high level, fault events are flagged at the SDOWN
output but not latched. The bias and modulation currents are not disabled. SDOWN is reset
once the fault condition disappears.
1, 6, 18, EP
Supply
GND
Set photodiode reference current with resistor to GND.
Laser diode bias current sink. Connect to laser cathode.
Circuit ground. The exposed die pad (EP) must be grounded.
IBMAX
21
Analog-in
MOD+
15
Analog-out
Laser modulation current output. Connect to laser cathode. Avoid usage of vias on board.
MOD–
16
Analog-out
Complementary laser modulation current output. Connect to VCC adjacent to anode of laser
diode. Avoid usage of vias on board.
MODSET
11
Analog-in
Set temperature-independent modulation current with resistor to GND.
MODTC
12
Analog-in
Set modulation-current temperature compensation with resistor to GND.
MONB
8
Analog-out
Bias current monitor sources 1/68 of the bias current
MONP
7
Analog-out
Photodiode current monitor sources a current identical to the photodiode current
OUTPOL
22
LVTTL-in
Alters modulation current output polarity. Open or high: normal polarity, low: inverted polarity.
OUTPOL is pulled up internally. Normal polarity: when DIN+ is high, current is sunk into MOD+.
PD
19
Analog-in
Monitor photodiode input. Connect to photodiode anode for APC. Sinks the photodiode current
to GND.
9
LVTTL-out
Fault detection flag
2, 5, 14, 17
Supply
SDOWN
VCC
Set maximum laser diode current with resistor to GND.
3.3 V ±10% supply voltage
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ABSOLUTE MAXIMUM RATINGS
over operating free-air temperature range (unless otherwise noted) (1)
VALUE
UNIT
VCC
Supply voltage (2)
–0.3 to 4
V
IIBIAS
Current into BIAS
–20 to 120
mA
IIMOD+, IIMOD–
Current into MOD+, MOD–
–20 to 120
mA
IPD
Current into PD
–5 to 5
mA
VDIN+, VDIN–, VDISABLE,
VMONB, VMONP, VFLTMODE,
VSDOWN
Voltage at DIN+, DIN–, DISABLE, MONB, MONP, FLTMODE, SDOWN (2)
–0.3 to 4
V
VCAPC, VIBMAX, VMODSET,
VAPCSET, VMODTC
Voltage at CAPC, IBMAX, MODSET, APCSET, MODTC (2)
–0.3 to 3
V
VMOD+, VMOD-
Voltage at MOD+, MOD– (2)
0.6 to VCC+1.5
V
VBIAS
Voltage at BIAS (2)
1 to 3.5
V
ESD
ESD rating at all pins except MOD+, MOD–
2
ESD rating at MOD+, MOD-
1
kV (HBM)
TJ,max
Maximum junction temperature
150
°C
Tstg
Storage temperature range
–65 to 150
°C
TA
Characterized free-air operating temperature range
–40 to 85
°C
TLEAD
Lead temperature 1,6 mm (1/16 inch) from case for 10 seconds
260
°C
(1)
(2)
Stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. These are stress ratings
only, and functional operation of the device at these or any other conditions beyond those indicated under recommended operating
conditions is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
All voltage values are with respect to network ground terminal.
RECOMMENDED OPERATING CONDITIONS
over operating free-air temperature range (unless otherwise noted)
VCC
Supply voltage
TA
Operating free-air temperature
8
MIN
NOM
MAX
3
3.3
3.6
V
85
°C
–40
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UNIT
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DC ELECTRICAL CHARACTERISTICS
over recommended operating conditions (unless otherwise noted)
PARAMETER
TEST CONDITIONS
VCC
Supply voltage
IVCC
Supply current
IBIAS
Bias current range
IBIAS-OFF
Bias off-current
DISABLE = high or hard-fault mode; VBIAS ≤ 3.5 V
Bias overshoot
During module hot plugging. VCC turn on time must be
≤ 0.8 s
Bias current temperature
stability
APC open loop
Bias current absolute
accuracy (1)
IBIAS ≥ 1 mA
Bias current monitor gain
IBIAS/IMONB
MONB and MONP threshold
range
A fault is never detected for VMONB/P ≤ 1 V and a fault
always occurs for VMONB/P ≥ 1.35 V
PD current monitor gain
IPD/IMONP
VID
MAX
3.3
3.6
–480
IOH = 100 μA sourcing
SDOWN output low voltage
IOL = 1 mA sinking
mA
55
mA
100
mA
25
μA
480
–15%
ppm/°C
15%
±15%
68
1
1.25
mA/ mA
1.35
1
200
SDOWN output high voltage
V
10%
IBIAS = 1 mA, TA = 25°C
Differential input signal
UNIT
32
IMOD = 60 mA, IBIAS = 100 mA (excluding IMOD, IBIAS)
V
mA/mA
1600
mVp-p
2.4
4.7
DISABLE input high voltage
V
7.4
0.4
V
10
kΩ
2
V
DISABLE input low voltage
0.8
V
Monitor diode voltage
1.6
V
1500
μA
Monitor diode dc current range
(1)
TYP
3
IMOD = 30 mA, IBIAS = 20 mA (excluding IMOD, IBIAS)
DISABLE input impedance
VPD
MIN
18
Absolute accuracy refers to part-to-part variation.
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AC ELECTRICAL CHARACTERISTICS
Typical operating condition is at VCC = 3.3 V, IMOD = 30 mA, IBIAS = 20 mA and TA = 25°C,
over recommended operating conditions (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
Data Rate
IMOD
Modulation current range
Current into MOD+/MOD– pin;
VMOD+, VMOD– ≥ 0.6 V
IMOD-OFF
Modulation off-current
DISABLE = high or hard-fault occurred
Modulation current stability
Modulation current absolute
accuracy (1)
TYP MAX
4.25
UNIT
Gbps
5
85
25
μA
–600
600
ppm/°C
IMOD = 10 mA
±40%
IMOD = 50 mA
±25%
IMOD = 80 mA
±20%
8300
mA
Modulation current
temperature compensation (2)
RMODTC = 3.125 kΩ
tr
Output rise time (20% to 80%)
VMOD+ ≥ 1 V, VMOD– ≥ 1 V, IMOD = 30 mA
55
75
ps
tf
Output fall time (20% to 80%)
VMOD+ ≥ 1 V, VMOD– ≥ 1 V, IMOD = 30 mA
55
75
ps
tOFF
Disable assert time (see Figure 4)
Time from rising edge of DISABLE to when output
currents fall below the maximum limits of IMOD-OFF
and IBIAS-OFF
0.06
5
μs
tON
Disable negate time (see Figure 5)
Time from falling edge of DISABLE to when output is
90% of nominal
80
μs
tINIT
Time to initialize
From power on or negation of SDOWN using
DISABLE
80
μs
tFAULT
Fault assert time
Time from fault to SDOWN rising edge
3.3
tRESET
DISABLE reset (see Figure 6)
Maximum spike pulse length at DISABLE being
ignored
RMODTC = Open
Time DISABLE must be high to reset SDOWN
Output overshoot/undershoot
Random jitter
DJ
Deterministic jitter (3)
–13.5%
10
μs
0.8
μs
6
μs
13.5
%
0.6
0.9
psRMS
10 mA ≤ IMOD ≤ 60 mA, with K28.5 pattern
at 4.25 Gbps
15
30
psp-p
10 mA ≤ IMOD ≤ 60 mA, with 223 – 1 PRBS or
equivalent pattern at 2.67 Gbps
13
32
psp-p
223 – 1 PRBS or equivalent pattern at 155 Mbps
(3)
50
IMOD = 60 mA
K28.5 pattern at 1.06 Gbps
(1)
(2)
ppm/°C
630
5
psp-p
10
psp-p
Absolute accuracy refers to part-to-part variation.
For a given external resistor connected to the MODTC pin, the modulation current temperature compensation will vary due to
part-to-part variations.
Jitter measured at positive edge and negative edge crossing of eye diagram.
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VHIGH
SDOWN
VLOW
t
VHIGH
DISABLE
VLOW
t
IMOD
IMOD
IMOD-OFF
t
IBIAS
IBIAS
IBIAS-OFF
t
tOFF
T0102-01
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VHIGH
SDOWN
VLOW
t
VHIGH
DISABLE
VLOW
t
IMOD
IMOD
IMOD-OFF
t
IBIAS
IBIAS
IBIAS-OFF
t
tON
T0103-01
12
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VHIGH
SDOWN
VLOW
t
VHIGH
DISABLE
VLOW
t
IMOD
IMOD
IMOD-OFF
t
IBIAS
IBIAS
IBIAS-OFF
t
tRESET
tRESET
tON
T0104-01
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TYPICAL CHARACTERISTICS
Typical operating condition is at VCC = 3.3 V, IMOD = 30 mA, IBIAS = 20 mA and TA = 25°C (unless otherwise noted)
ELECTRICAL EYE-DIAGRAM AT 2.125 Gbps
WITH K28.5 PATTERN, IMOD = 30 mA
Single-Ended Output Voltage [50 mV/Div]
Single-Ended Output Voltage [50 mV/Div]
ELECTRICAL EYE-DIAGRAM AT 4.25 Gbps
WITH K28.5 PATTERN, IMOD = 30 mA
Time [50 ps/Div]
Time [100 ps/Div]
G001
G002
Figure 8.
ELECTRICAL EYE-DIAGRAM AT 1.0625 Gbps
WITH K28.5 PATTERN, IMOD = 30 mA
DETERMINISTIC JITTER
vs
MODULATION CURRENT
Single-Ended Output Voltage [50 mV/Div]
Deterministic Jitter Including PWD − psp−p
Figure 7.
60
50
40
30
20
10
0
10
15
Time [200 ps/Div]
G003
Figure 9.
14
20
25
30
35
40
45
Modulation Current − mA
50
55
60
G004
Figure 10.
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TYPICAL CHARACTERISTICS (continued)
Typical operating condition is at VCC = 3.3 V, IMOD = 30 mA, IBIAS = 20 mA and TA = 25°C (unless otherwise noted)
RANDOM JITTER
vs
TEMPERATURE
3.0
3.0
2.5
2.5
Random Jitter − psRMS
Random Jitter − psRMS
RANDOM JITTER
vs
MODULATION CURRENT
2.0
1.5
1.0
0.5
1.5
1.0
0.5
0.0
10
15
20
25
30
35
40
45
50
55
Modulation Current − mA
0.0
−40 −30 −20 −10 0
60
10 20 30 40 50 60 70 80 90
TA − Free-Air Temperature − °C
G005
G006
Figure 11.
Figure 12.
RISE TIME AND FALL TIME
vs
MODULATION CURRENT
BIAS-MONITOR CURRENT GAIN IMONB/IBIAS
vs
BIAS CURRENT IBIAS
20
Bias Monitor Current Gain − mA/A
80
Rise Time and Fall Time − ps
2.0
Rise Time
70
60
50
Fall Time
40
30
20
19
18
17
16
15
14
13
12
11
10
10
15
20
25
30
35
40
45
Modulation Current − mA
50
55
60
10
15
G007
Figure 13.
20
25
30
35
40
45
50
55
Bias Current − mA
60
G008
Figure 14.
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TYPICAL CHARACTERISTICS (continued)
Typical operating condition is at VCC = 3.3 V, IMOD = 30 mA, IBIAS = 20 mA and TA = 25°C (unless otherwise noted)
BIAS CURRENT IBIAS IN OPEN LOOP MODE
vs
EXTERNAL RESISTOR RBIASMAX
MODULATION CURRENT IMOD
vs
EXTERNAL RESISTOR RMODSET
100
IMOD − Modulation Current − mA
IBIAS − Bias Current − mA
120
100
80
60
40
20
0
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
RBIASMAX − External Resistor − kΩ
100
0
10
30
40
50
60
70
80
G009
Figure 15.
Figure 16.
MONITOR DIODE CURRENT IPD
vs
EXTERNAL RESISTOR RAPCSET
PHOTODIODE MONITOR GAIN IMONP/IPD
vs
TEMPERATURE
90
100
G010
3.0
Photodiode Monitor Gain − mA/mA
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
10
20
30
40
50
60
70
80
RAPCSET − External Resistor − kΩ
90
100
2.5
2.0
1.5
1.0
0.5
0.0
−40 −30 −20 −10 0
G011
Figure 17.
16
20
RMODSET − External Resistor − kΩ
1.8
IPD − Monitor Diode Current − mA
90
10 20 30 40 50 60 70 80 90
TA − Free-Air Temperature − °C
G012
Figure 18.
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ONET4201LD
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TYPICAL CHARACTERISTICS (continued)
Typical operating condition is at VCC = 3.3 V, IMOD = 30 mA, IBIAS = 20 mA and TA = 25°C (unless otherwise noted)
BIAS CURRENT MONITOR GAIN IMONB/IBIAS
vs
TEMPERATURE
SUPPLY CURRENT (excl. IMOD and IBIAS)
vs
TEMPERATURE
80
70
18
Supply Current − mA
Bias Current Monitor Gain − mA/A
20
16
14
60
50
40
30
12
20
10
−40 −30 −20 −10 0
10
−40 −30 −20 −10 0
10 20 30 40 50 60 70 80 90
TA − Free-Air Temperature − °C
10 20 30 40 50 60 70 80 90
TA − Free-Air Temperature − °C
G013
Figure 19.
Figure 20.
DISABLE ASSERT TIME tOFF
DISABLE NEGATE TIME tON
∆t = 2.21 µs
G014
∆t = 240 µs
VSDOWN
VSDOWN
VDISABLE
VDISABLE
IMOD+
IMOD+
IBIAS
IBIAS
Time [100 µs/Div]
Time [500 ns/Div]
G016
G017
Figure 21.
Figure 22.
SHUTDOWN RESET TIME tRESET
∆t = 12.8 µs
VSDOWN
VDISABLE
IMOD+
IBIAS
Time [5 µs/Div]
G018
Figure 23.
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APPLICATION INFORMATION
CAPC
RAPCSET
RBIASMAX
Figure 24 shows the ONET4201LD connected with a dc-coupled interface to the laser diode, alternatively the
ONET4201LD laser driver can be ac-coupled.
OUTPOL
DISABLE
PD
CAPC
IBMAX
OUTPOL
DISABLE
GND
APCSET
VCC
GND
VCC
VCC
DIN+
DIN+
MOD–
DIN–
DIN–
MOD+
VCC
VCC
Monitor
Photodiode
20 W
RD
MODTC
MODSET
FLTMODE
SDOWN
MONB
MONP
GND
ONET4201LD
24-Lead QFN
LaserDiode
BIAS
RMODTC
SDOWN
RMODSET
MONB
RMONB
FLTMODE
RMONP
MONP
S0154-02
Figure 24. Basic Application Circuit With DC-Coupled Interface Between
the ONET4201LD and the Laser Diode
APC loop instability may occur with large inductive loading on the BIAS pin. To ensure loop stability in this case,
it is recommended to connect a 1-nF capacitor to ground at the BIAS pin.
18
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SELECT A LASER
In the design example according to Figure 24, the ONET4201LD is dc coupled to a typical communication-grade
laser diode capable of operating at 4.25 Gbps with the specifications shown in Table 2.
Table 2. Laser Diode Specifications
PARAMETER
VALUE
UNITS
λ
Wavelength
PAVG
Average optical output power
ITH
Threshold current
10
mA
ρMON
Laser-to-monitor transfer
0.05
mA/mW
η
Laser slope efficiency
0.2
mW/mA
1310
nm
5
mW
SELECT APCSET RESISTOR
When the APC loop is activated, the desired average optical output power PAVG is defined by characteristics of
the monitor diode and by the APCSET resistor RAPCSET. The relation between the monitor photodiode current IPD
and the average optical output power PAVG is given by Equation 7:
I
[A] + P
[W] ò
[AńW]
PD
AVG
MON
(7)
The RAPCSET resistor is calculated by Equation 8:
4.69 V
R
[W] + 4.69 V +
APCSET
I
[A]
P
[W] ò
[AńW]
PD
AVG
MON
(8)
For the laser diode specified in Table 2 and the desired average optical output power of 5 mW, RAPCSET is
calculated as seen in Equation 9:
4.69 V
4.69 V
R
[W] +
+
+ 18.75 kW
APCSET
P
[W] ò
[AńW]
5 mW 0.05 mAńmW
AVG
MON
(9)
Note that the monitor photodiode current IPD must not exceed 1.5 mA corresponding to a minimum APCSET
resistor RAPCSET,MIN = 3.1 kΩ.
SELECT MODSET RESISTOR
Modulation current IMOD is dependent on the required optical output peak-to-peak power Pp-p or the average
optical power PAVG. IMOD can be calculated using the laser slope efficiency η and the desired extinction ratio re:
I
2
Pp*p[W]
[A] +
+
MOD
h[WńA]
P
re*1
re)1
[W]
AVG
h[WńA]
(10)
Using the laser diode parameters from Table 2 and assuming an extinction ratio re = 8 dB ( 6.3) for an average
optical power PAVG = 5 mW the required modulation current results as:
2
I
MOD
+
5 mW
6.3*1
6.3)1
0.2 mWńmA
+ 36.3 mA
(11)
The modulation current is adjustable with a selectable temperature coefficient TC according to the relation:
I
[A] + I
[A]
MOD
MOD0
ǒ1 ) TC ǒT[ C] * T0[ C]ǓǓ
o
o
(12)
where T is the ambient temperature in °C and T0 is the reference temperature (T0 = 60°C).
The temperature coefficient of the modulation current TC is typically adjustable between 630 ppm/°C and 8300
ppm/°C.
For calculation of the required external resistor RMODSET for a given modulation current and a given temperature,
the formula can be modified as follows:
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R
MODSET
ǒ1 ) TC ǒT[ C] * T0[ C]ǓǓ
[W] + 265 V
I
[A]
MOD
o
o
(13)
If 4000 ppm/°C is the desired temperature coefficient and the modulation current from the example above,
36.3 mA, is required at a temperature of 25°C, the MODSET resistor RMODSET is given by Equation 14.
4000 ppm
(25 oC * 60 oC) + 6.3 kW
R
[W] + 265 V
1)
oC
MODSET
36.3 mA
(14)
ǒ
Ǔ
Note that the modulation current IMOD must not exceed 85 mA over the complete temperature range,
corresponding to a minimum MODSET resistor RMODSET,MIN = 3.1 kΩ.
SELECT MODTC RESISTOR
The RMODTC resistor is used to program a modulation temperature coefficient that can be used to compensate for
the decreased slope efficiency of the laser at a higher temperature. The temperature coefficient TCLD of the laser
can be calculated using the slope efficiency η1 at temperature T1 and η2 at temperature T2 as shown in
Equation 15:
h [WńA] * h [WńA]
1 +
2
1
TC
106
LD oC
o
o
h 1[WńA]
T 2[ C] * T 1[ C]
(15)
ƪ ƫ
ǒ
Ǔ
As an example, for the laser in Table 2, the slope efficiency at temperature T1 = 25°C is η1 = 0.2 mW/mA. At
temperature T2 = 85°C the slope efficiency is η2 = 0.15 mW/mA. The corresponding temperature coefficient TCLD
laser can be calculated:
0.15 mWńmA * 0.2 mWńmA
TC
+
10 6 + * 4167 o1
LD
C
0.2 mWńmA (85 oC * 25 oC)
(16)
The MODTC resistor RMODTC can be used to compensate the laser temperature coefficient TCLD in order to
maintain the same optical output swing within a range of 630 ppm up to 8300 ppm. For this, RMODTC may be
programmed as follows:
24 W
R
+
MODTC
oC
(TC * 630 ppm) o1
C
(17)
ƪ ƫ
To compensate for the decreased slope efficiency of the laser in Table 2, TC must be 4167 ppm/°C.
This leads to the following MODTC resistor RMODTC:
24 W
R
+
+ 6.8 kW
MODTC
4167 ppm * 630 ppm o
C
oC
(18)
SELECT BIASMAX RESISTOR
The BIASMAX resistor RBIASMAX is used to limit the bias current applied to the laser diode.
To calculate RBIASMAX, the maximum threshold current at 85°C and end of life must be determined. The
maximum bias current for the dc-coupled interface can be approximated by Equation 19.
I
[A] + I
[A]
BIASMAX
THMAX
(19)
RBIASMAX can be set by Equation 20.
20
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R
BIASMAX
[W] +
343 V
I
BIASMAX
[A]
+
343 V
I
THMAX
[A]
(20)
For the example laser diode, the maximum threshold current is 40 mA at 85°C. Therefore, RBIASMAX can be
approximated by Equation 21.
R
+ 343 V + 8.6 kW
BIASMAX
40 mA
(21)
SELECT VMONB AND VMONP RANGE
Monitoring the bias current is achieved by taking the fractional (1/68) bias current and developing a voltage
across an external resistor to ground. Equation 22 provides the value for VMONB for a resistor value equal to
768 Ω.
R
[W] I
[A]
768 W I
[A]
BIAS
BIAS
V
[V] + MONB
+
+ 11.29 W I
[A]
MONB
BIAS
68
68
(22)
Monitoring of the photodiode current is achieved by taking a mirror of IPD and developing a voltage across an
external resistor to ground. Equation 23 provides the value for VMONP for a resistor equal to 200 Ω.
V
[V] + R
[W] I [A] + 200 W I [A]
MONP
MONP
PD
PD
(23)
LASER DIODE INTERFACE
The output stage of the ONET4201LD is optimized for driving a 20-Ω load. The combination of a damping
resistor, RD, along with the resistance of the laser diode must be 20 Ω for impedance matching. The suggested
typical value for RD is 6 Ω to 15 Ω. A bypass capacitor of 10 nF placed close to the laser anode also helps to
optimize performance.
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REVISION HISTORY
Changes from Original (November 2005) to Revision A ................................................................................................ Page
•
Changed 200us to 80us for typ value of tON ....................................................................................................................... 10
•
Changed 200us to 80us for typ value of tINIT ...................................................................................................................... 10
•
Changed 10us to 0.8us for max value of tRESET max spike ................................................................................................. 10
•
Changed 20us min to 6us max value of tRESET time DISABLE ........................................................................................... 10
22
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PACKAGE OPTION ADDENDUM
www.ti.com
14-Oct-2022
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
Samples
(4/5)
(6)
ONET4201LDRGER
ACTIVE
VQFN
RGE
24
3000
RoHS & Green
NIPDAU
Level-2-260C-1 YEAR
-40 to 85
ONET
4201L
Samples
ONET4201LDRGET
ACTIVE
VQFN
RGE
24
250
RoHS & Green
NIPDAU
Level-2-260C-1 YEAR
-40 to 85
ONET
4201L
Samples
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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