MAXIM MAX3795

19-3387; Rev 0; 8/04
1Gbps to 4.25Gbps Multirate VCSEL
Driver with Diagnostic Monitors
♦ 3.3V ±10% Single Supply
♦ 2mA to 15mA Modulation Current
♦ 1mA to 15mA Bias Current
♦ 52ps Transition Time
♦ 8.4ps Deterministic Jitter
♦ Optional Peaking Current to Improve VCSEL Edge
Speed
♦ Supports Common-Cathode and Differential
Configuration
♦ Safety Circuits Compliant with SFF and SFP
MSAs
♦ Pin Compatible to MAX3740A
Ordering Information
PART
TEMP RANGE
-40°C to +85°C 24 Thin QFN (4mm x 4mm)
MAX3795ETG+
-40°C to +85°C 24 Thin QFN (4mm x 4mm)
+Denotes lead-free package.
Pin Configuration
REF
MD
COMP
VCC
BIASMON
23
22
21
20
19
Fibre-Channel Optical Transmitters
PWRMON
TOP VIEW
Gigabit Ethernet Optical Transmitters
24
Applications
GND
1
18
TX_DISABLE
2
17
BIASSET
IN+
3
16
VCC
IN-
4
15
OUT+
FAULT
5
14
OUT-
SQUELCH
6
13
GND
9
10
TC2
GND
12
8
TC1
11
7
MAX3795
VCC
Multirate (1Gbps to 4.25Gbps) SFP/SFF Modules
PIN-PACKAGE
MAX3795ETG
MODSET
The MAX3795 safety circuit detects faults that could
cause hazardous light levels and disables the VCSEL
output. The safety circuits are compliant with SFF and
SFP multisource agreements (MSAs).
The MAX3795 is available in a compact 4mm ✕ 4mm,
24-pin thin QFN package and operates over the -40°C
to +85°C temperature range. The MAX3795 is pin-forpin compatible with the MAX3740A and is available in
lead-free packages.
♦ Supports All SFF-8472 Digital Diagnostics
PEAKSET
The MAX3795 is a high-speed VCSEL driver for smallform-factor (SFF) and small-form-factor pluggable (SFP)
fiber optic transmitters. It contains a bias generator, a
laser modulator, and comprehensive safety features.
The automatic power control (APC) adjusts the laser
bias current to maintain average optical power over
changes in temperature and laser properties. The driver accommodates common-cathode and differential
configurations.
The MAX3795 operates up to 4.25Gbps. It can switch
up to 15mA of laser modulation current and source up
to 15mA of bias current. Adjustable temperature compensation is provided to keep the optical extinction
ratio within specifications over the operating temperature range. The MAX3795 interfaces with the Dallas
DS1856/DS1859 to meet SFF-8472 timing and diagnostic requirements. The MAX3795 accommodates various
VCSEL packages, including low-cost TO-46 headers.
Features
BIAS
THIN QFN (4mm x 4mm)
EXPOSED PAD IS CONNECTED TO GND
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1
MAX3795
General Description
MAX3795
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (VCC) ............................................-0.5V to +4.0V
Voltage at TX_DISABLE, IN+, IN-, FAULT,
SQUELCH, TC1, TC2, MODSET, PEAKSET, BIASSET,
BIAS, BIASMON, COMP, MD, REF,
PWRMON ...............................................-0.5V to (VCC + 0.5V)
Voltage at OUT+, OUT- .........................(VCC - 2V) to (VCC + 1V)
Current into FAULT ............................................ -1mA to +25mA
Current into OUT+, OUT- ....................................................60mA
Continuous Power Dissipation (TA = +85°C)
24-Pin Thin QFN
(derate 20.8mW/°C above +85°C).................................1354mW
Storage Temperature Range .............................-55°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
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 in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VCC = +2.97V to +3.63V, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA =
+25°C, unless otherwise noted.)
PARAMETER
Supply Current
SYMBOL
ICC
CONDITIONS
MIN
SQUELCH set low,
IMOD = 2mAP-P
TX_DISABLE set low,
peaking is not used
IMOD = 15mAP-P
(Note 1)
MAX
UNITS
35
71
81
mA
Additional current when peaking is used,
RPEAK = 1.18kΩ
ICC-SHDN
TYP
15
Additional current when SQUELCH is high
5
Total current when TX_DISABLE is high
7
10
FAULT OUTPUT
Output High Voltage
VOH
RLOAD = 10kΩ to 2.97V
Output Low Voltage
VOL
RLOAD = 4.7kΩ to 3.63V
2.4
V
0.4
V
10.0
kΩ
TX_DISABLE INPUT
Input Impedance
RPULL
4.7
Input High Voltage
VIH
2.0
Input Low Voltage
VIL
V
0.8
The time for ICC to reach ICC-SHDN when
TX_DISABLE transitions high
Power-Down Time
8
50
V
µs
SQUELCH
Squelch Threshold
25
Squelch Hysteresis
85
6
mVP-P
mVP-P
Time to Squelch Data
(Note 3)
0.02
5.00
µs
Time to Resume from Squelch
(Note 3)
0.02
5.00
µs
BIAS GENERATOR
Maximum Bias Pin Voltage
Bias Current
Accuracy of Programmed Bias
Current
2
VBIAS-MAX
IBIAS
∆BIAS
Referenced to VCC
-0.65
Minimum
Maximum
V
1
15
5mA ≤ IBIAS ≤ 15mA
-8
+8
1mA ≤ IBIAS ≤ 5mA
-12
+12
_______________________________________________________________________________________
mA
%
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
(VCC = +2.97V to +3.63V, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA =
+25°C, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
Bias Current During Fault
IBIAS_OFF
BIASMON Gain
GBIASMON IBIASMON / IBIAS
BIASMON Stability
MIN
Current out of the BIAS pin
TYP
MAX
UNITS
1.5
10
µA
1mA < IBIAS < 3mA
0.0875
0.105
0.1375
3mA ≤ IBIAS < 15mA
0.085
0.105
0.125
(Notes 2, 4)
-10
mA/mA
+10
%
V
AUTOMATIC POWER CONTROL (APC)
MD Nominal Voltage
VMD
Voltage at REF
VREF
APC loop is closed
1
VREF 0.2
2
1.2
1.8
2.2
MD Voltage During Fault
0
MD Input Current
Normal operation (FAULT = low)
APC Time Constant
CCOMP = 0.047µF, ∆IPD / ∆ILASER = 0.02
PWRMON Nominal Gain
VPWRMON / (VREF - VMD)
-2
0.7
1.85
2.15
V
V
+2
µA
2.45
V/V
90
µs
LASER MODULATOR (Load is 50Ω AC-Coupled to OUT+)
Differential Input Voltage
VID
Input Common-Mode Voltage
VCM
Minimum
0.25
Maximum
2.4
1.75
S11
f < 4GHz
12.7
dB
SDD11
f < 4GHz
11
dB
IMOD
Laser Modulation During Fault or
Squelch Active
IMOD_OFF
Current into OUT+
RLOAD ≤ 50Ω
Minimum
Maximum
115
Ω
RIN
Single-Ended Input Return Loss
Modulation Current
100
V
Differential Input Resistance
Differential Input Return Loss
85
VP-P
2
15
DC tested
15
mA
50
µAP-P
+10
%
Tolerance of Programmed
Modulation Current
TC1 is shorted to TC2
Minimum Peaking Current
RPEAKSET = 10kΩ
0.2
mA
Maximum Peaking Current
RPEAKSET = 1kΩ
2
mA
-10
Peaking Current Duration
75
ROUT
Output Resistance
Single-ended resistance
42
Minimum Programmable
Temperature Coefficient
Maximum Programmable
Temperature Coefficient
Modulation Transition Time
(Note 2)
Temperature range 0°C to +70°C
tR
50Ω load, no peaking,
5mA ≤ IMOD ≤ 15mA
tF
50Ω load, no peaking,
5mA ≤ IMOD ≤ 15mA
50
ps
58
Ω
0
ppm/°C
+5000
ppm/°C
-40°C to +85°C
49
+100°C
58
-40°C to +85°C
56
+100°C
64
72
79
ps
_______________________________________________________________________________________
3
MAX3795
ELECTRICAL CHARACTERISTICS (continued)
MAX3795
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +2.97V to +3.63V, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA =
+25°C, unless otherwise noted.)
PARAMETER
SYMBOL
Deterministic Jitter
DJ
Random Jitter
RJ
CONDITIONS
MIN
5mA ≤ IMOD ≤ 15mA,
4.25Gbps, K28.5 (Notes 2, 5) +100°C
-40°C to +85°C
TYP
MAX
8.4
15.6
12.7
APC closed loop
0.5
APC open loop (Note 2)
0.5
0.9
UNITS
psP-P
psRMS
SAFETY FEATURES (see the Typical Operating Characteristics)
High-Current Fault Threshold
VBIAS Fault Threshold
VBMTH
VBTH
VBIASMON > VBMTH causes a fault
VBIAS referenced to VCC
0.7
0.8
0.9
V
-0.250
-0.2
-0.150
V
0.7
0.8
0.9
V
Power-Monitor Fault Threshold
VPMTH
VPWRMON > VPMTH causes a fault
TX Disable Time
t_OFF
Time from rising edge of TX_DISABLE to
IBIAS = IBIAS_OFF and IMOD = IMOD_OFF
(Note 2)
1.8
5
µs
TX Disable Negate Time
t_ON
Time from rising edge of TX_DISABLE to
IBIAS and IMOD at 99% of steady state
(Note 2)
55
500
µs
Fault Reset Time
t_INIT1
Time to set VFAULT = low after power-on or
after rising edge of TX_DISABLE (Note 2)
60
200
ms
Power-On Time
t_INIT2
Time after power-on to transmitter-on with
TX_DISABLE low (Note 2)
60
200
ms
Fault Assert Time
t_FAULT
Time from fault occurrence to VFAULT =
high; CFAULT < 20pF, RFAULT = 4.7kΩ
(Note 2)
1.4
50
µs
Fault Delay Time
t_FLTDLY
Time from fault to IBIAS = IBIAS_OFF and
IMOD = IMOD_OFF; measured with a
continuously occurring fault (Note 2)
1
5
µs
TX_DISABLE Reset
t_RESET
Time TX_DISABLE must be held high to
reset FAULT (Note 2)
1
µs
Note 1: Supply current measurements exclude IBIAS from the total current.
Note 2: AC characteristics guaranteed by design and characterization.
Note 3: Measured by applying a pattern that contains 20µs of K28.5, followed by 5µs of zeros, then 20µs of K28.5, followed by 5µs
of ones. Data rate is equal to 2.5Gbps, with inputs filtered using 1.8GHz Bessel filters.
Note 4: Variation of bias monitor gain for any single part over the range of VCC, temperature, 3mA < IBIAS < 15mA.
Note 5: Deterministic jitter measured at 4.25Gbps with a K28.5 pattern (00111110101100000101).
4
_______________________________________________________________________________________
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
ELECTRICAL EYE DIAGRAM
OPTICAL EYE DIAGRAM
ELECTRICAL EYE DIAGRAM
MAX3795 toc01
MAX3795 toc03
MAX3795 toc02
4.25Gbps, K28.5, 10mA MODULATION,
PEAKING OFF
1Gbps, K28.5, -3dBm, 850nm VCSEL
ADVANCED OPTICAL COMPONENTS,
1
HFE4191-541
1Gbps, K28.5, 10mA MODULATION,
RPEAKSET = 1.4kΩ
75mV/div
75mV/div
2
3
40ps/div
135ps/div
152ps/div
IBIASMON vs. BIAS CURRENT
OPTICAL EYE DIAGRAM
MAX3795 toc04
MAX3795 toc05
1.8
3.125Gbps, K28.5, -7dBm, 850nm VCSEL,
ADVANCED OPTICAL COMPONENTS
HFE4191-541
1.6
1.4
IBIASMON (mA)
4.25Gbps, K28.5, -7dBm, 850nm VCSEL,
ADVANCED OPTICAL COMPONENTS
HFE4191-541
MAX3795 toc06
OPTICAL EYE DIAGRAM
1.2
1.0
0.8
0.6
0.4
0.2
0
34ps/div
0
50ps/div
4
8
16
12
BIAS CURRENT (mA)
20
15
10
2.5
2.0
1.5
1.0
0
0
0
2
4
6
8
10 12 14 16 18 20
MODULATION CURRENT (mAP-P)
FALL TIME
50
40
RISE TIME
30
20
0.5
5
MEASURED FROM 20%
60
TRANSITION TIME (ps)
25
IBIAS = 5mA
3.0
RANDOM JITTER (psRMS)
30
70
MAX3795 toc08
35
DETERMINISTIC JITTER (psP-P)
3.5
MAX3795 toc07
40
TRANSITION TIME
vs. MODULATION CURRENT
RANDOM JITTER
vs. MODULATION CURRENT
MAX3795 toc09
DETERMINISTIC JITTER
vs. MODULATION CURRENT
10
0
2
4
6
8
10 12 14 16 18 20
MODULATION CURRENT (mAP-P)
0
2
4
6
8
10
12
14
MODULATION CURRENT (mA)
_______________________________________________________________________________________
5
MAX3795
Typical Operating Characteristics
(VCC = +3.3V, RTC = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, TA = +25°C, unless otherwise
noted.)
Typical Operating Characteristics (continued)
(VCC = +3.3V, RTC = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, TA = +25°C, unless otherwise
noted.)
BIAS CURRENT vs. RBIASSET
100µ
10
100
0.1
1
RBIASSET (Ω)
10
90
IMOD = 15mA
OUTPUT RETURN LOSS
DIFFERENTIAL
MEASUREMENT
AT IN±
-5
0
MAX3795 toc14
MAX3795 toc13
0
-5
-15
S22 (dB)
S11 (dB)
60
-15
-20
IMOD = 2mA
-20
-25
-30
-25
40
SINGLE-ENDED
MEASUREMENT
-10
-10
70
10
RPWRSET (Ω)
INPUT RETURN LOSS
SUPPLY CURRENT vs. TEMPERATURE
50
1
0
100
RMODSET (kΩ)
100
80
10µ
1µ
1
1
MAX3795 toc12
1m
MAX3795 toc15
1
10
10m
MONITOR DIODE CURRENT (A)
10
MAX3740A toc11
MAX3795 toc10
MODULATION CURRENT (mAP-P)
-35
30
-30
20
-35
-40
-15
10
35
60
-40
-45
100
85
10G
1G
TEMPERATURE (°C)
1G
FREQUENCY (Hz)
MODULATION CURRENT
vs. TEMPERATURE
MODULATION CURRENT TEMPCO
vs. RTC
10
RTC = 1kΩ
9
RTC = 5kΩ
8
RTC = 10kΩ
7
RTC = 60kΩ
RTC = 100kΩ
6
RTC = 500kΩ
5
REFERENCED TO +25°C
4500
TEMPCO (ppm/°C)
RMODSET = 1.8kΩ
RTC = 100Ω
5500
MAX3795 toc16
11
MODULATION CURRENT (mAP-P)
100
FREQUENCY (Hz)
MAX3795 toc17
BIAS CURRENT (mA)
100
0.1
3500
2500
1500
500
4
-500
0
10
20
30
40
50
60
TEMPERATURE (°C)
6
MONITOR DIODE CURRENT
vs. RPWRSET
MODULATION CURRENT vs. RMODSET
100
SUPPLY CURRENT (mA)
MAX3795
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
70
80
90
100
1k
10k
100k
1M
RTC (Ω)
_______________________________________________________________________________________
10G
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
MONITOR DIODE CURRENT
vs. TEMPERATURE
MAX3795 toc20
MAX3795 toc19
MAX3795 toc18
275
MONITOR DIODE CURRENT (µA)
STARTUP WITH SLOW RAMPING SUPPLY
HOT PLUG WITH TX_DISABLE LOW
300
3.3V
3.3V
VCC
250
VCC
0V
0V
FAULT
FAULT
225
LOW
LOW
200
TX_DISABLE
175
LOW
150
LOW
LASER
OUTPUT
LASER
OUTPUT
125
t_INIT = 62ms
TX_DISABLE
t_INIT = 60ms
100
-40
-15
10
35
60
85
20ms/div
20ms/div
TEMPERATURE (°C)
TX_DISABLE NEGATE TIME
TRANSMITTER DISABLE
MAX3795 toc21
3.3V
3.3V
VCC
t_OFF = 2.2µs
FAULT
HIGH
TX_DISABLE
t_ON = 131µs
HIGH
LOW
LOW
HIGH
TX_DISABLE
LOW
LOW
LASER
OUTPUT
LASER
OUTPUT
LASER
OUTPUT
40µs/div
1µs/div
FAULT RECOVERY TIME
4µs/div
FREQUENT ASSERTION OF TX_DISABLE
MAX3795 toc24
EXTERNAL
FAULT
REMOVED
VPWRMON
FAULT
t_FAULT = 2.16µs
FAULT
LOW
LOW
TX_DISABLE
MAX3795 toc23
EXTERNALLY
FORCED
VPWRMON
FAULT
VCC
FAULT
RESPONSE TO FAULT
MAX3795 toc22
MAX3795 toc25
VPWRMON
EXTERNALLY
FORCED FAULT
FAULT
HIGH
LOW
HIGH
TX_DISABLE
LOW
LOW
t_INIT = 54µs
LASER
OUTPUT
40µs/div
TX_DISABLE
LASER
OUTPUT
200µs/div
_______________________________________________________________________________________
7
MAX3795
Typical Operating Characteristics (continued)
(VCC = +3.3V, RTC = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, TA = +25°C, unless otherwise
noted.)
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
MAX3795
Pin Description
8
PIN
NAME
FUNCTION
1, 10, 13
GND
2
TX_DISABLE
3
IN+
Noninverted Data Input
4
IN-
Inverted Data Input
5
FAULT
6
SQUELCH
7, 16, 20
VCC
+3.3V Supply Voltage
8
TC1
Temperature Compensation Set Pin 1. A resistor placed between TC1 and TC2 (RTC) programs the
temperature coefficient of the laser modulation current.
9
TC2
Temperature Compensation Set Pin 2. A resistor placed between TC1 and TC2 (RTC) programs the
temperature coefficient of the laser modulation current.
11
MODSET
Modulation Set. A resistor connected from MODSET to ground (RMODSET) programs the desired
modulation current amplitude.
12
PEAKSET
Peaking Current Set. A resistor connected between PEAKSET and ground (RPEAKSET) programs the
peaking current amplitude. To disable peaking, leave PEAKSET open.
Ground
Transmit Disable. Driver output is disabled when TX_DISABLE is high or left unconnected. The
driver output is enabled when the pin is asserted low.
Fault Indicator. Open-drain output with ESD protection. FAULT is asserted high during a
fault condition.
Squelch Enable. Squelch is enabled when the pin is set high. Squelch is disabled when the pin is
set low or left open.
14
OUT-
Inverted Modulation Current Output
15
OUT+
Noninverted Modulation Current Output
17
BIASSET
18
BIAS
19
BIASMON
Bias-Current Monitor. The output of BIASMON is a sourced current proportional to the bias current.
A resistor connected between BIASMON and ground (RBIASMON) can be used to form a groundreferenced bias monitor.
21
COMP
Compensation Pin. A capacitor between COMP and MD compensates the APC. A typical value of
0.047µF is recommended. For open-loop configuration, short the COMP pin to GND to deactivate
the APC circuit.
22
MD
Monitor Diode Connection
23
REF
Reference Pin. Reference monitor used for APC. A resistor between REF and MD (RPWRSET) programs
the photomonitor current when the APC loop is closed.
24
PWRMON
EP
Exposed Pad
Bias-Current Set. When a closed-loop configuration is used, connect a 1.7kΩ resistor between
ground and BIASSET to program the maximum bias current. When an open configuration is used,
connect a resistor between BIASSET and ground (RBIASSET) to program the VCSEL bias current.
Bias-Current Output
Average Power Monitor. The pin is used to monitor the transmit optical power. For open-loop
configuration, connect PWRMON to GND.
Ground. Must be soldered to the circuit board ground for proper thermal and electrical
performance. See the Layout Considerations section.
_______________________________________________________________________________________
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
MAX3795
PWRMON
REF
1.8V
(2VBE + 0.2)
RPWRSET
CURRENT
AMPLIFIER
2X
MAX3795
POWERCONTROL
AMPLIFIER
MD
ENABLE
IBIAS
34
BIAS
BIAS GENERATOR
FERRITE
BEAD
SMOOTHSTART
IPD
BIASMON
1.6V
(2VBE)
IBIAS
9
1V
RBIASMON
200Ω
COMP
BIASSET
RBIASSET
CCOMP
Figure 1. Bias Generator
Detailed Description
The MAX3795 contains a bias generator with APC,
safety circuit, and a laser modulator with optional peaking compensation (see the Functional Diagram).
Bias Generator
Figure 1 shows the bias-generator circuitry that contains
a power-control amplifier and smooth-start circuitry. An
internal pnp transistor provides DC laser current to bias
the laser in a light-emitting state. The APC circuitry
adjusts the laser-bias current to maintain average power
over temperature and changing laser properties. The
smooth-start circuitry prevents current spikes to the laser
during power-up or enable, ensuring compliance with
safety requirements and extending the life of the laser.
The MD input is connected to the cathode of a monitor
diode, which is used to sense laser power. The BIAS
output is connected to the anode of the laser through an
inductor or ferrite bead. The power-control amplifier drives a current amplifier to control the laser’s bias current.
During a fault condition, the bias current is disabled.
The PWRMON output provides a voltage proportional to
average laser power given by:
VPWRMON = 2 x IPD x RPWRSET
where VPWRMON = 0.4V (typ)
The BIASMON output provides a current proportional to
the laser bias current given by:
IBIASMON = IBIAS x GBIASMON
When APC is not used (no monitor diode), connect the
COMP and PWRMON pins to GND. In this mode, bias
current is set by the resistor (RBIASSET) between the
BIASSET pin and GND. When a closed-loop configuration is used, connect a 1.7kΩ resistor between ground
and BIASSET to set the maximum bias current.
Safety Circuit
The safety circuit contains an input disable
(TX_DISABLE), a latched fault output (FAULT), and fault
detectors (Figure 2). This circuit monitors the operation
of the laser driver and forces a shutdown (disables
laser) if a fault is detected (Table 1). Table 2 contains
the circuit’s response to various single-point failures.
The transmit fault condition is latched until reset by a
toggle of TX_DISABLE or VCC. The FAULT pin should
be pulled high with a 4.7kΩ to 10kΩ resistor.
Table 1. Fault Conditions
PIN
FAULT CONDITION
BIAS
VBIAS > VCC - 0.2V
BIASMON
VBIASMON > 0.8V
PWRMON
VPWRMON > 0.8V
_______________________________________________________________________________________
9
MAX3795
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
Table 2. Circuit Response to Various Single-Point Faults (Closed-Loop APC Configuration)
PIN NAME
FAULT
CIRCUIT RESPONSE TO
VCC SHORT
CIRCUIT RESPONSE TO
GND SHORT
CIRCUIT RESPONSE TO
OPEN
Does not affect laser power.
Does not affect laser power.
Does not affect laser power.
Modulation and bias current are
disabled.
Normal condition for circuit
operation.
Modulation and bias current are
disabled.
IN+
Does not affect laser power.
Does not affect laser power.
Does not affect laser power.
IN-
Does not affect laser power.
Does not affect laser power.
Does not affect laser power.
SQUELCH
Does not affect laser power.
Does not affect laser power.
Does not affect laser power.
TC1
Does not affect laser power.
Does not affect laser power.
The laser modulation is decreased,
but average power is not affected.
TC2
The laser modulation is increased,
but average power is not affected.
Modulation current is disabled.
The laser modulation is decreased,
but average power is not affected.
MODSET
Modulation current is disabled.
The laser modulation is increased,
but average power is not affected.
The laser modulation is decreased,
but average power is not affected.
PEAKSET
Does not affect laser power.
Does not affect laser power.
Does not affect laser power.
OUT+
Modulation current is disabled.
Modulation current is disabled.
Modulation current is disabled.
OUT-
Does not affect laser power.
Does not affect laser power.
Does not affect laser power.
Laser bias is disabled.
Fault state* occurs.
Laser bias is disabled.
Fault state* occurs. Note that VCSEL
emissions may continue. Care must
be taken to prevent this condition.
This disables the VCSEL.
This disables the VCSEL.
Fault state* occurs.
Does not affect laser power.
Fault state* occurs.
COMP
The bias current is reduced, and the
average power of the laser output is
reduced.
IBIAS increases to the value
determined by RBIASSET. If the biasmonitor fault threshold is exceeded,
a fault is signaled.
APC loop will be unstable.
If the bias-monitor fault threshold is
exceeded, a fault is signaled.
MD
IBIAS increases to the value
determined by RBIASSET. If the biasmonitor fault threshold is exceeded,
a fault is signaled.
The bias current is reduced, and the
average power of the laser output is
reduced.
IBIAS increases to the value
determined by RBIASSET. If the bias
monitor fault threshold is exceeded,
a fault is signaled.
REF
IBIAS increases to the value
determined by RBIASSET. If the biasmonitor fault threshold is exceeded,
a fault is signaled.
The bias current is reduced, and the
average power of the laser output is
reduced.
The bias current is reduced, and the
average power of the laser output is
reduced.
Fault state* occurs.
Does not affect laser power.
Does not affect laser power.
TX_DISABLE
BIASSET
BIAS
BIASMON
PWRMON
*A fault state asserts the FAULT pin, disables the modulator output, and disables the bias output.
Modulation Circuit
The modulation circuitry consists of an input buffer, a
current mirror, and a high-speed current switch (Figure
3). The modulator drives up to 15mA of modulation into
a 50Ω VCSEL load.
10
The amplitude of the modulation current is set with
resistors at MODSET and temperature coefficient (TC1,
TC2) pins. The resistor at MODSET (R MODSET) programs the temperature-stable portion of the modulation
current, and the resistor between TC1 and TC2 (RTC)
programs the temperature coefficient of the modulation
______________________________________________________________________________________
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
MAX3795
TX_DISABLE
BIAS
VBIAS FAULT
FAULT
OUTPUT
VCC - 0.2V
FAULT
BIASMON
HIGH-CURRENT FAULT
R
0.8V
Q
ENABLE
S
R-S LATCH
PWRMON
HIGH-POWER FAULT
0.8V
POR
TX_DISABLE
SAFETY CIRCUIT
MAX3795
Figure 2. Safety Circuit
VCC
MAX3795
ROUT
ROUT
INPUT BUFFER
OUT+
CURRENT SWITCH
IN+
OUTSIGNAL
DETECT
100Ω
IN-
PEAKING
CONTROL
PEAKSET
SQUELCH
ENABLE
CURRENT AMPLIFIER 40x
MODULATION
CURRENT GENERATOR
RPEAKSET
TEMPERATURE
COMPENSATION
1V
200Ω
TC2
TC1
MODSET
RMODSET
RTC
Figure 3. Modulation Circuit
______________________________________________________________________________________
11
MAX3795
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
current. For appropriate RTC and RMODSET values, see
the Typical Operating Characteristics.
Design Procedure
Select Laser
Select a communications-grade laser with a rise time of
90ps or better for 4.25Gbps applications. Use a highefficiency laser that requires low modulation current and
generates a low-voltage swing. Trim the leads to reduce
laser package inductance. The typical package leads
have inductance of 25nH per inch (1nH/mm). This
inductance causes a large voltage swing across the
laser. A compensation filter network can also be used to
reduce ringing, edge speed, and voltage swing.
Programming Modulation Current
A resistor (RMODSET) placed between the MODSET pin
and ground controls the modulation current out of the
MAX3795 to the VCSEL. The modulation current is
given by the following:

 


ROUT
1
IMOD = 
× 40 × 


 200 + RMODSET 
  ROUT + RLOAD 
It is important to note that the load impedance of the
VCSEL affects the modulation current being sourced by
the MAX3795. The Modulation Current vs. RMODSET
graph in the Typical Operating Characteristics shows the
current into a 50Ω load. Capacitance at the MODSET pin
should be ≤20pF.
Programming Modulation-Current Tempco
Compute the required modulation tempco from the
slope efficiency of the laser at TA = +25°C and at a
higher temperature. Then select the value of RTC from
the Typical Operating Characteristics. For example,
suppose a laser has a slope efficiency (SE) of
0.021mW/mA at +25°C, which reduces to 0.018mW/mA
at +85°C. The temperature coefficient is given by the
following:
Laser tempco =
(SE85 − SE25 )
SE25 × (85 − 25)
= −2380ppm / °C
× 106
From the Typical Operating Characteristics, the value
of RTC, which offsets the tempco of the laser, is 9kΩ. If
modulation temperature compensation is not desired,
short TC1 and TC2.
Programming the APC Loop
Program the average optical power by adjusting
R PWRSET . To select the resistance, determine the
desired monitor current to be maintained over temperature and lifetime. See the Monitor Diode Current vs.
RPWRSET graph in the Typical Operating Characteristics,
and select the value of RPWRSET that corresponds to the
required current.
VCC
Programming Bias Current
The bias current output of the MAX3795 is controlled by
a resistor (RBIASSET) placed between the BIASSET pin
and ground. In open-loop operation, BIASSET controls
the bias current level of the VCSEL. In closed-loop
operation (APC); the RBIASSET controls the maximum
allowed bias current. The open-loop bias current is
given by the following:


1.2
IBIAS = 
 × 34
 200 + RBIASSET 
The Bias Current vs. R BIASSET graph in the Typical
Operating Characteristics shows the current into a 50Ω
load. Capacitance at the BIASSET pin should be
≤20pF.
MAX3795
PACKAGE
IN+
1kΩ
VCC
1nH
0.5pF
50Ω
VCC
50Ω
IN-
1nH
0.5pF
Figure 4. Simplified Input Structure
12
______________________________________________________________________________________
15pF
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
Interface Models
50W
PACKAGE
50W
1nH
OUT-
0.5pF
1nH
Figures 4 and 5 show simplified input and output circuits
for the MAX3795 laser driver. Figure 6 shows the fault circuit interface.
Layout Considerations
OUT+
0.5pF
To minimize inductance, keep the connections between
the MAX3795 output pins and laser diode as short as
possible. Use multilayer boards with uninterrupted
ground planes to minimize EMI and crosstalk.
Exposed-Pad (EP) Package
MAX3795
Figure 5. Simplified Output Structure
VCC
Laser Safety and IEC 825
MAX3795
FAULT
Figure 6. Fault Circuit Interface
IPD =
VREF − VMD
0.2V
≈
RPWRSET
RPWRSET
The low frequency cutoff of a transmitter using APC is
given by:
f3dB ≈
The exposed pad on the 24-pin thin QFN provides a very
low thermal resistance path for heat removal from the IC.
The pad is also electrical ground on the MAX3795 and
must be soldered to the circuit board ground for proper
thermal and electrical performance. Refer to Maxim
Application Note HFAN-08.1: Thermal Considerations for
QFN and Other Exposed-Pad Packages for additional
information.
∆IPD
1
×
∆ ILASER 2 × π × C APC × 50
Input Termination Requirements
The International Electrotechnical Commission (IEC)
determines standards for hazardous light emissions from
fiber-optic transmitters. IEC 825 defines the maximum
light output for various hazard levels. The MAX3795 provides features that facilitate compliance with IEC 825. A
common safety precaution is single-point fault tolerance,
whereby one unplanned short, open, or resistive connection does not cause excess light output. Using this laser
driver alone does not ensure that a transmitter design is
compliant with IEC 825. The entire transmitter circuit and
component selections must be considered. Customers
must determine the level of fault tolerance required by
their applications, recognizing that Maxim products are
not designed or authorized for use as components in
systems intended for surgical implant into the body, for
applications intended to support or sustain life, or for any
other application where the failure of a Maxim product
could create a situation where personal injury or death
may occur.
The MAX3795 data inputs are SFP MSA compatible. Onchip, 100Ω differential input impedance is provided for
optimal termination (Figure 4). Because of the on-chip
biasing network, the MAX3795 inputs self-bias to the
proper operating point to accommodate AC-coupling.
______________________________________________________________________________________
13
MAX3795
Applications Information
VCC
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
MAX3795
Functional Diagram
COMP
FAULT
MD
REF
BIAS
BIAS
GENERATOR
WITH APC
SAFETY
CIRCUITRY
TX_DISABLE
BIASMON
PWRMON
BIASSET
ENABLE
VCC
LASER
MODULATOR
MAX3795
ROUT
ROUT
SQUELCH
OUTOUT+
IN+
SIGNAL
DETECT
100Ω
PEAKING
CONTROL
INMODULATION CURRENT
GENERATOR
ENABLE
TC1
TC2
MODSET
Package Information
Chip Information
TRANSISTOR COUNT: 3806
PROCESS: SiGe BIPOLAR
For the latest package outline information, go to
www.maxim-ic.com/packages.
PART
14
PEAKSET
PACKAGE TYPE
PACKAGE CODE
MAX3795ETG
24 Thin QFN
(4mm x 4mm x 0.8mm)
T2444-1
MAX3795ETG+
24 Thin QFN
(4mm x 4mm x 0.8mm)
T2444-1
______________________________________________________________________________________
1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
+3.3V
4.7kن
VCC
FAULT
PWRMON
MODSET
TX_DISABLE
RMODSET
SQUELCH
MAX3795
0.1µF
REF
IN+
RPWRSET
COMP
0.047µF
IN-
MD
TC1
BIAS
0.1µF
RTC†
L1*
0.01µF
TC2
L1*
0.01µF
OUT+
CF†
BIASSET
RBIASSET
CF†
OUTGND PEAKSET
BIASMON
RPEAKSET†
0.01µF
50Ω
RF†
0.01µF
56Ω
RF†
RBIASMON
L2*
†OPTIONAL COMPONENT
SINGLE-ENDED DRIVE
DIFFERENTIAL DRIVE
*FERRITE BEAD
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 15
© 2004 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
MAX3795
Typical Application Circuit