Maxim MAX3948 11.3gbps, low-power, dc-coupled laser driver Datasheet

19-5943; Rev 0; 6/11
EVALUATION KIT AVAILABLE
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
General Description
The MAX3948 is a 3.3V, multirate, low-power laser diode
driver designed for Ethernet, Fibre Channel, and SONET
transmission systems at data rates up to 11.3Gbps. This
device is optimized to drive a differential transmitter
optical subassembly (TOSA) with a 25I flex circuit. The
unique design of the output stage enables DC-coupling
to unmatched TOSAs, thereby lowering transmitter power
consumption by more than 100mW.
The MAX3948 receives differential AC-coupled signals
with on-chip termination. It can deliver laser modulation currents of up to 85mA at an edge speed of 26ps
(20% to 80%) into a 5I external differential load. The
device is designed to have a high-bandwidth differential signal path with on-chip back termination resistors
integrated into its outputs. An input equalization block
can be activated to compensate for SFP+/QSFP+ host
connector losses. The integrated DC circuit provides
programmable laser DC currents up to 61mA. Both the
laser DC current generator and the laser modulator can
be disabled from a single pin.
The device offers one dedicated pin (VSEL) to program up
to four channel addresses for multichannel applications.
The use of a 3-wire digital interface reduces the pin count
while permitting adjustment of input equalization, polarity, output deemphasis, and modulation and DC currents
without the need for external components. The MAX3948
is available in a 3mm x 3mm, 16-pin TQFN package, and
is specified for the -40NC to +95NC extended temperature
range.
Benefits and Features
S Lowest Power Consumption
 168mW Typical IC Power Dissipation at 3.3V
(LDMOD = 40mA, LDDC = 20mA)
 383mW Total Transmitter Power Dissipation at
3.3V Including LDMOD = 40mA, LDDC = 20mA
 Enables < 1W Maximum Total SFP+ Module
Power Dissipation
 Enables < 2.5W Maximum Total QSFP+ Module
Power Dissipation
S Saves Board Space
 Small 3mm x 3mm Package
 DC-Coupling to the Laser Reduces External
Component Count
S Flexibility
 Operate Up to Four MAX3948 ICs Over Single
3-Wire Digital Interface
 Programmable Modulation Current Up to
85mA (5ω Load)
 Programmable DC Current Up to 61mA
(Translates to Up to 100mA Laser Bias Current)
 Programmable Input Equalization and Output
Deemphasis
S Safety
 Supports SFF-8431 SFP+ MSA and SFF-8472
Digital Diagnostic
 Integrated Eye Safety Features with Maskable
Faults
 DC Current Monitor
Applications
40GBASE-LR4 QSFP+ Optical Transceivers
10GBASE-LR SFP+ Optical Transceivers
Ordering Information appears at end of data sheet.
10GBASE-LRM SFP+ Optical Transceivers
OC192-SR SFP+ SDH/SONET Transceivers
�� Maxim Integrated Products 1
For information on other Maxim products, visit Maxim’s website at www.maxim-ic.com.
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
ABSOLUTE MAXIMUM RATINGS
VCC, VCCT.............................................................-0.3V to +4.0V
|VCC - VCCT|...................................................................... < 0.5V
Voltage Range at TIN+, TIN-, DISABLE,
SDA, SCL, CSEL, VSEL, FAULT, and BMON.......-0.3V to VCC
Voltage Range at VOUT and TOUTC........ 0.4V to (VCCT - 0.4V)
Voltage Range at TOUTA............(VCCT - 1.3V) to (VCCT + 1.3V)
Current Range into TIN+ and TIN-................... -20mA to +20mA
Current Range into VOUT.................................. -2mA to +90mA
Current into TOUTC and TOUTA................................... +150mA
Continuous Power Dissipation (TA = +70NC)
TQFN (derate 20.8mW/NC above +70NC)...............1666.7mW
Storage Temperature Range ........................... -55NC to +150NC
Die Attach Temperature . ................................................+400NC
Lead Temperature (soldering, 10s).................................+300NC
Soldering Temperature (reflow).......................................+260NC
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.
PACKAGE THERMAL CHARACTERISTICS (Note 1)
TQFN
Junction-to-Ambient Thermal Resistance (BJA)...........48NC/W
Junction-to-Case Thermal Resistance (BJC)................10NC/W
Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a fourlayer board. For detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial.
ELECTRICAL CHARACTERISTICS
(VCC = VCCT = 2.95V to 3.63V, TA = -40NC to +95NC; typical values are at VCC = VCCT = 3.3V, TA = +25NC, LDDC = 20mA, LDMOD
= 40mA, and 14I single-ended electrical output load, unless otherwise noted. See Figure 1 for electrical setup.) (Note 2)
PARAMETER
SYMBOL
CONDITIONS
Power-Supply Current
ICC
Excludes output current through the external pullup inductors (Note 3)
Power-Supply Voltage
VCCT, VCC
MIN
TYP
MAX
UNITS
51
62
mA
3.63
V
2.75
V
POWER SUPPLY
2.95
POWER-ON RESET
VCC for Enable High
2.55
VCC for Enable Low
2.3
2.45
1
10.3
V
DATA INPUT SPECIFICATION
Input Data Rate
Launch amplitude into FR4 transmission
line P 12in,
SET_TXEQ[1:0] = 01b,
SET_TXEQ[1:0] = 11b
Differential Input Voltage
VIN
Common-Mode Input Voltage
VCM
Differential Input Resistance
RIN
0.2
11.3
Gbps
0.8
VP-P
SET_TXEQ[1:0] = 01b,
SET_TXEQ[1:0] = 11b,
outside of optimized range
0.15
SET_TXEQ[1:0] = 00b
0.15
1.0
1.0
2.15
75
100
V
125
I
�� Maxim Integrated Products 2
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
ELECTRICAL CHARACTERISTICS (continued)
(VCC = VCCT = 2.95V to 3.63V, TA = -40NC to +95NC; typical values are at VCC = VCCT = 3.3V, TA = +25NC, LDDC = 20mA, LDMOD
= 40mA, and 14I single-ended electrical output load, unless otherwise noted. See Figure 1 for electrical setup.) (Note 2)
PARAMETER
SYMBOL
SCD11
Differential Input S-Parameters
(Note 4)
SDD11
CONDITIONS
MIN
-40
f P 4.1GHz
-19
4.1GHz P f P 11.3GHz
-16
SCC11
IDCMAX
Current into VOUT pin
Minimum DC DAC Current
IDCMIN
Current into VOUT pin
DC-Off Current
IDC-OFF
50
DC DAC LSB Size
DC DAC Integral Nonlinearity
INL
DC DAC Differential Nonlinearity
DNL
2.5mA P IDC P 50mA
Guaranteed monotonic at 8-bit resolution,
SET_IDC[8:1]
2.5mA P IDC P 50mA, VVOUT = VCCT - 1.5V
(Notes 6, 7)
DC Current DAC Stability
DC Compliance Voltage at VOUT
BMON Current Gain
GBMON
GBMON = IBMON/IDC, external resistor to
GND defines voltage
Compliance Voltage at BMON
UNITS
dB
61
mA
2.5
mA
0.1
mA
116
FA
±0.5
%FS
±0.5
LSB
1
4
%
VCCT
-2
VCCT
- 1.5
VCCT
-1
V
15
16.7
20
mA/A
1.5
5
%
1.8
V
2.5mA P IDC P 50mA, VVOUT = VCCT - 1.5V
(Notes 6, 7)
BMON Current Gain Stability
MAX
-15
1GHz P f P 11.3GHz, ZCM_SOURCE = 25W
DC CURRENT GENERATOR (Note 5, Figure 3)
Maximum DC DAC Current
TYP
0.1GHz P f P 11.3GHz
0
LASER MODULATOR (Note 8)
Maximum Laser Modulation
Current
LDMODMAX
Current into TOUTC pin, 5I laser load,
6.25% deemphasis
Minimum Laser Modulation
Current
LDMODMIN
Current into TOUTC pin, 5I laser load,
6.25% deemphasis
10
mAP-P
Current into TOUTC pin
0.1
mA
Modulation-Off Laser Current
LDMOD-
Modulation DAC Full-Scale
Current
IMOD-FS
OFF
85
99.7
Modulation DAC LSB Size
Modulation DAC Integral
Nonlinearity
INL
Modulation DAC Differential
Nonlinearity
DNL
TOUTA and TOUTC
Instantaneous Output
Compliance Voltage
Guaranteed monotonic at 8-bit resolution,
SET_IMOD[8:1]
mAP-P
130
mA
247
FA
±1
%FS
±0.5
LSB
VTOUTA
With external inductive pullup to VCCT
VCCT - 1
VCCT + 1
VTOUTC
With external inductive pullup to VOUT
0.6
VCCT - 1
V
�� Maxim Integrated Products 3
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
ELECTRICAL CHARACTERISTICS (continued)
(VCC = VCCT = 2.95V to 3.63V, TA = -40NC to +95NC; typical values are at VCC = VCCT = 3.3V, TA = +25NC, LDDC = 20mA, LDMOD
= 40mA, and 14I single-ended electrical output load, unless otherwise noted. See Figure 1 for electrical setup.) (Note 2)
PARAMETER
Modulation Output Termination
SYMBOL
Modulation Current DAC Stability
Modulation Current Rise/Fall
Time
Deterministic Jitter (Note 6)
Random Jitter
tR, tF
DJ
RJ
Differential S-Parameters
(Note 4)
CONDITIONS
MIN
TYP
MAX
UNITS
19
25
31
I
10mA P LDMOD P 85mA,
VVOUT = VCCT - 1.5V (Notes 6, 7)
1.5
4
%
20% to 80%, 10mA P LDMOD P 85mA
(Note 6)
26
36
ps
10mA P LDMOD P 85mA, 8.5Gbps with
K28.5 pattern
4
10mA P LDMOD P 85mA, 10.3125Gbps
(Note 9)
6
12
psP-P
10mA P LDMOD P 85mA, 11.3Gbps
(Note 9)
8
13
10mA P LDMOD P 85mA (Note 6)
0.19
0.55
0.1GHz P f P 4.1GHz, ZCM_SOURCE =
12.5W
-10
4.1GHz < f P 11.3GHz, ZCM_SOURCE =
12.5W
-6
0.1GHz< f P 11.3GHz, ZDIFF_SOURCE =
50W
-13
ROUT
SCC22
SDD22
psRMS
dB
SAFETY FEATURES
Fault never occurs for VVOUT R VCCT - 2V,
fault always occurs for VVOUT < VCCT 2.8V, referenced to VCCT
VCCT 2.8
VCCT
-2
Fault never occurs for VVOUT R 1.7V, fault
always occurs for VVOUT < 1.35V,
referenced to GND,
SET_IMOD[8:6] = 111b
1.35
1.7
Fault never occurs for VVOUT R 0.57V, fault
always occurs for VVOUT < 0.43V,
referenced to GND,
SET_IMOD[8:6] = 000b
0.43
0.57
Threshold Voltage at TOUTC
Fault never occurs for VTOUTC R 0.48V,
fault always occurs for VTOUTC < 0.35V
0.35
0.48
V
Threshold Voltage at TOUTA
Fault never occurs for VTOUTA R VCCT 1.45V, fault always occurs for VTOUTA <
VCCT - 1.88V
VCCT 1.88
VCCT 1.45
V
Threshold Voltage at VCCT
Fault never occurs for VCCT R VCC - 0.15V,
fault always occurs for VCCT < VCC - 0.4V
VCC 0.4
VCC 0.15
V
Threshold Voltage at VOUT
V
�� Maxim Integrated Products 4
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
ELECTRICAL CHARACTERISTICS (continued)
(VCC = VCCT = 2.95V to 3.63V, TA = -40NC to +95NC; typical values are at VCC = VCCT = 3.3V, TA = +25NC, LDDC = 20mA, LDMOD
= 40mA, and 14I single-ended electrical output load, unless otherwise noted. See Figure 1 for electrical setup.) (Note 2)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
TIMING REQUIREMENTS (Notes 5, 6, 8)
Initialization Time
tINIT
LDDC = 25mA, LDMOD = 65mA, DC and
modulation DAC are both H0x00, time from
TX_EN = high to LDDC and LDMOD at 90%
of steady state
DISABLE Assert Time
tOFF
Time from rising edge of DISABLE input
signal to LDDC and LDMOD at 10% of
steady state (Note 6)
25
75
ns
DISABLE Negate Time
tON
Time from falling edge of DISABLE to LDDC
and LDMOD at 90% of steady state (Note 6)
250
600
ns
Time from negation of latched fault using
DISABLE to LDDC and LDMOD at 90% of
steady state
250
600
ns
Time from fault to FAULT = high, CFAULT ≤
20pF, RFAULT = 4.7kW
0.7
3
Fs
FAULT Reset Time
tRECOVER
FAULT Assert Time
tFAULT
Time DISABLE must be held high to reset
fault
DISABLE to Reset Time
250
ns
4
Fs
DIGITAL I/O SPECIFICATIONS (SDA, SCL, CSEL, FAULT, DISABLE)
Input High Voltage
Input Low Voltage
Input Hysteresis
VIH
1.8
VIL
0
VHYST
Input Capacitance
VCC
0.8
80
CIN
DISABLE Input Resistance
Input Leakage Current
(DISABLE)
RPULL
IIH
IIL
IIH
Input Leakage Current (SDA)
Input Leakage Current (SCL,
CSEL)
Internal pullup resistor
4.7
Input connected to VCC
Input connected to GND
7.5
5
pF
10
kI
775
IIL
Input connected to VCC
Input connected to GND; internal pullup is
75kW typical
IIH
Input connected to VCC; internal pulldown
is 75kW typical
35
75
IIL
Input connected to GND
-2
+2
Output High Voltage (SDA,
FAULT)
VOH
External pullup is (4.7kI to 10kI) to VCC
Output Low Voltage (SDA,
FAULT)
VOL
External pullup is (4.7kI to 10kI) to VCC
V
mV
10
440
V
-2
+2
35
75
VCC - 0.1
FA
FA
FA
V
0.4
V
�� Maxim Integrated Products 5
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
ELECTRICAL CHARACTERISTICS (continued)
(VCC = VCCT = 2.95V to 3.63V, TA = -40NC to +95NC; typical values are at VCC = VCCT = 3.3V, TA = +25NC, LDDC = 20mA, LDMOD
= 40mA, and 14I single-ended electrical output load, unless otherwise noted. See Figure 1 for electrical setup.) (Note 2)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
400
1000
kHz
3-WIRE DIGITAL INTERFACE TIMING CHARACTERISTICS (Figure 5)
SCL Clock Frequency
fSCL
SCL Pulse-Width High
tCH
500
ns
SCL Pulse-Width Low
tCL
500
ns
SDA Setup Time
tDS
100
ns
SDA Hold Time
tDH
100
ns
tD
5
ns
SCL Rise to SDA Propagation
Time
CSEL Pulse-Width Low
tCSW
500
ns
CSEL Leading Time Before the
First SCL Edge
tL
500
ns
CSEL Trailing Time After the Last
SCL Edge
tT
500
ns
SDA, SCL Load
CB
Total bus capacitance on one line with
4.7kI pullup to VCC
20
pF
VCC
V
VSEL FOUR-LEVEL DIGITAL INPUT (Note 10, Table 2)
Input Voltage High
3-wire address, ADDR[6:5] = 11b
5/6VCC
+ 0.2
Input Voltage Mid-High
3-wire address, ADDR[6:5] = 10b
3/6VCC
+ 0.2
2/3 x
VCC
5/6VCC
- 0.2
V
Input Voltage Mid-Low
3-wire address, ADDR[6:5] = 01b
1/6VCC
+ 0.2
1/3 x
VCC
3/6VCC
- 0.2
V
Input Voltage Low
3-wire address, ADDR[6:5] = 00b
0
1/6VCC
- 0.2
V
Note 2: Specifications at TA = -40NC and +95NC are guaranteed by design and characterization.
Note 3: VOUT is connected to 1.9V. TOUTA is connected to VCCT through pullup inductors, and TOUTC is connected to VOUT
through pullup inductors.
Note 4: Measured with Agilent 8720ES + ATN-U112A and series RC (39I and 0.3pF) between TOUTC and TOUTA (Figure 1).
Note 5: LDDC = IDC + IMOD x (DE + R x (1 - DE)/(50 + R)/2), where LDDC is the effective laser DC current, IDC is the DC DAC
current, IMOD is the modulation DAC current, DE is the deemphasis percentage, and R is the differential laser load resistance. Example: For R = 5I and DE = 6.25%, LDDC = IDC + 0.105 x IMOD.
Note 6: Guaranteed by design and characterization.
Note 7: Stability is defined as [(IMEASURED) - (IREFERENCE)]/(IREFERENCE) over the listed current/temperature range and VCCT =
VCC = VCCREF Q5%, VCCREF = 3.3V. Reference current measured at VCCREF and TREF = +25NC.
Note 8: LDMOD = IMOD x (1 - DE) x 50/(50 + R), where LDMOD is the effective laser modulation current, IMOD is the modulation
DAC current, DE is the deemphasis percentage, and R is the differential laser load resistance. Example: For RI = 5 and
DE = 6.25%, LDMOD = 0.852 x IMOD.
Note 9: Equivalent 223 - 1 PRBS pattern = 27 - 1 PRBS + 72 zeros + 27 - 1 PRBS + 72 ones.
Note 10:These limits are based on simulated values.
�� Maxim Integrated Products 6
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
VCC
4.7kI
VOUT
SDA
CSEL
SCL
VOUT = VCCT - 2V TO VCCT - 1V
0.01µF
VCCT
VCCT
VCC
0.01µF
VCC
2.2µH
100I
0.01µF
0.1µF
40I
25I
50I
MAX3948
50I
100nH
0.01µF
Z0 = 50I
TIN+
50I
50I
50I
39I
TOUTC
VOUT
0.1µF
0.01µF
Z0 = 50I
TOUTA
TIN-
0.3pF
50I
100nH
VCC
50I
50I
SAMPLING
OSCILLOSCOPE
VCCT
0.1µF
VCC
50I
25I
0.01µF
40I
EP
BMON
FAULT
VSEL
DISABLE
2.2µH
VCCT
VCC
50I
100I
VCCT
0.01µF
2kI
4.7kI
Figure 1. AC Test Setup
�� Maxim Integrated Products 7
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Typical Operating Characteristics
(Typical values are at VCC = VCCT = 3.3V, TA = +25°C, data pattern = 27 - 1 PRBS + 72 zeros + 27 - 1 PRBS (inverted) + 72 ones,
unless otherwise noted.)
INPUT DIFFERENTIAL RETURN LOSS
vs. FREQUENCY
10.3Gbps ELECTRICAL EYE DIAGRAM
MAX3948 toc02
MAX3948 toc01
0
223-1, PRBS
MAX3948 toc03
10.3Gbps OPTICAL EYE DIAGRAM
-10
SDD11 (dB)
-20
-30
-40
-50
-60
20ps/div
100
1000
10,000
100,000
FREQUENCY (MHz)
-5
-10
-10
-30
-20
-40
-25
-50
-30
-60
1000
10,000
-15
SDD22 (dB)
-15
100
100,000
-20
-25
-30
-35
-40
-45
1000
100
10,000
100,000
100
1000
10,000
100,000
FREQUENCY (MHz)
FREQUENCY (MHz)
OUTPUT COMMMON-MODE RETURN LOSS
vs. FREQUENCY
RANDOM JITTER
vs. MODULATION CURRENT (AT LOAD)
SUPPLY CURRENT vs. TEMPERATURE
(LDMOD = 40mAP-P, LDDC = 20mA)
0.8
-10
RJ (psRMS)
0.7
-15
-20
0.6
0.5
0.4
0.3
-25
0.2
-30
CURRENT INTO VCC AND VCCT PINS
DIFFERENTIAL LASER LOAD = 5Ω
70
SUPPLY CURRENT (mA)
-5
11.3Gbps, 1111 0000 PATTERN
0.9
80
MAX3948 toc08
MAX3948 toc07
1.0
MAX3948 toc09
FREQUENCY (MHz)
0
SCC22 (dB)
0
-5
-20
SCD11 (dB)
SCC11 (dB)
-10
OUTPUT DIFFERENTIAL RETURN LOSS
vs. FREQUENCY
MAX3948 toc05
0
MAX3948 toc04
0
INPUT DIFFERENTIAL TO COMMON-MODE
RETURN LOSS vs. FREQUENCY
MAX3948 toc06
INPUT COMMON-MODE RETURN LOSS
vs. FREQUENCY
60
50
40
0.1
0
-35
100
1000
10,000
FREQUENCY (MHz)
100,000
0
10
20
30
40
50
60
MODULATION CURRENT (mAP-P)
70
80
30
-40 -25 -10
5
20
35
50
65
80
95
TEMPERATURE (°C)
�� Maxim Integrated Products 8
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Typical Operating Characteristics (continued)
(Typical values are at VCC = VCCT = 3.3V, TA = +25°C, data pattern = 27 - 1 PRBS + 72 zeros + 27 - 1 PRBS (inverted) + 72 ones,
unless otherwise noted.)
MAX3948 toc11
70
60
50
IDC (mA)
125
120
115
40
30
20
110
105
10
100
0
5
20
35
50
65
80
95
100
0
200
300
400
500
5
4
3
25Ω DIFFERENTIAL LOAD
100
0
600
200
0
100
500
600
IDC = 50mA
600
IDC = 25mA
500
400
IDC = 10mA
300
100
80
400
MAX3948 toc14
700
1
60
300
800
200
0
120
-40 -25 -10
SET_TXDE[6:0]
5
20
35
50
65
80
95
TEMPERATURE (°C)
EDGE SPEED vs. MODULATION CURRENT
EDGE SPEED vs. DEEMPHASIS SETTING
50
MAX3948 toc15
10.3Gbps, 1111 0000 PATTERN
20% TO 80%
SET_IMOD[8:0] = 230d
20% TO 80%
10.3Gbps, 1111 0000 PATTERN
45
40
EDGE SPEED (ps)
40
FALL TIME
30
25
RISE TIME
20
20
900
2
35
10Ω DIFFERENTIAL LOAD
30
1000
MAX3948 toc13
6
45
40
DC MONITOR CURRENT
vs. TEMPERATURE
7
50
50
SET_IMOD[8:0]
BMON CURRENT (µA)
DEEMPHASIS (%)
8
40
60
SET_IDC[8:0]
SET_IMOD[8:0] = 230d
TXDE_MD[1:0] = 2d
20
70
0
MODULATION CURRENT DEEMPHASIS
vs. MANUAL DEEMPHASIS SETTING
9
5Ω DIFFERENTIAL LOAD
80
10
TEMPERATURE (°C)
10
90
35
FALL TIME
30
25
RISE TIME
20
15
MAX3948 toc16
-40 -25 -10
100
MODULATION CURRENT (mAP-P)
130
EDGE SPEED (ps)
SUPPLY CURRENT (mA)
135
CURRENT INTO VCC AND VCCT PINS PLUS
MODULATION, DEEMPHASIS, AND DC DAC
CURRENT, DIFFERENTIAL LASER LOAD = 5Ω
MAX3948 toc10
140
MODULATION CURRENT (AT LOAD)
vs. DAC SETTING
DC CURRENT vs. DAC SETTING
MAX3948 toc12
TOTAL CURRENT vs. TEMPERATURE
(LDMOD AT LOAD = 40mAP-P, LDDC = 20mA)
15
10
0
20
40
60
IMOD (mA)
80
100
10
20
45
70
95
120
SET_TXDE[6:0]
�� Maxim Integrated Products 9
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Typical Operating Characteristics (continued)
(Typical values are at VCC = VCCT = 3.3V, TA = +25°C, data pattern = 27 - 1 PRBS + 72 zeros + 27 - 1 PRBS (inverted) + 72 ones,
unless otherwise noted.)
TRANSMITTER DISABLE
TRANSMITTER ENABLE
MAX3948 toc17
VCC
3.3V
VCC
RESPONSE TO FAULT
MAX3948 toc18
3.3V
MAX3948 toc19
EXTERNAL FAULT
VOUT
tON = 400ns
FAULT
HIGH
LOW
HIGH
FAULT
HIGH
DISABLE
DISABLE
OPTICAL
OUTPUT
OPTICAL
OUTPUT
OUTPUT
200ns/div
FAULT RECOVERY
DISTRIBUTION OF RISE TIME
(WORST CASE CONDITIONS)
FREQUENT ASSERTION OF DISABLE
MAX3948 toc20
MAX3948 toc21
45
40
EXTERNAL FAULT
VOUT
EXTERNAL FAULT
REMOVED
1µs/div
LOW
FAULT
LOW
HIGH
DISABLE
FAULT
HIGH
DISABLE
LOW
LOW
HIGH
VCC = 2.95V
TA = 95°C
20% to 80%
30
25
20
15
10
5
OUTPUT
OUTPUT
35
MAX3948 toc22
80ns/div
4µs/div
0
4µs/div
29.5 30.0 30.5 31.0 31.5 32.0 32.5 33.0
RISE TIME (ps)
MAX3948 3-WIRE ADDRESS
vs. VSEL VOLTAGE (DATA FROM SIMULATION)
PERCENT OF UNITS (%)
40
35
VCC = 2.95V
TA = 95°C
20% to 80%
30
25
20
15
10
VCC
VSEL VOLTAGE (FRACTION OF VCC)
45
MAX3948 toc23
DISTRIBUTION OF FALL TIME
(WORST CASE CONDITIONS)
2VCC/3
VCC/3
INDETERMINATE
ADDR[6:5] = 01
1/6xVCC±200mV
GND
INDETERMINATE
ADDR[6:5] = 10
3/6xVCC±200mV
5
0
ADDR[6:5] = 11
5/6xVCC±200mV
MAX3948 toc24
VOUT
LOW
LOW
PERCENT OF UNITS (%)
DISABLE
LOW
FAULT
INDETERMINATE
ADDR[6:5] = 00
29.5 30.0 30.5 31.0 31.5 32.0 32.5 33.0
FALL TIME (ps)
�� Maxim Integrated Products 10
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
SDA
CSEL
VOUT
TOP VIEW
SCL
Pin Configuration
12
11
10
9
VCC 13
TIN+ 14
8
VCCT
7
TOUTC
6
TOUTA
5
VCCT
MAX3948
TIN- 15
*EP
2
3
4
BMON
DISABLE
1
FAULT
+
VSEL
VCC 16
TQFN
(3mm x 3mm)
*EXPOSED PAD MUST BE CONNECTED TO GROUND.
Pin Description
PIN
NAME
FUNCTION
EQUIVALENT CIRCUIT
VCC
VCC
1
DISABLE
Disable Input, CMOS. Set to logic-low for normal operation.
Logic-high or open disables both the modulation current
and the DC current. Internally pulled up by a 7.5kI resistor
to VCC.
VCC
7.5kI
DISABLE
ESD
PROTECTION
VCC
2
VSEL
4-Level Input for SPI Device Address Detection. Connecting
to VCC sets ADDR[6:5] to 11b, connecting to VCC x
2/3 sets ADDR[6:5] to 10b, connecting to VCC/3 sets
ADDR[6:5] to 01b, and connecting to GND sets ADDR[6:5]
to 00b.
VSEL
�� Maxim Integrated Products 11
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Pin Description (continued)
PIN
NAME
FUNCTION
FAULT
Fault Output, Open Drain. Logic-high indicates a fault condition has been detected. It remains high even after the
fault condition has been removed. A logic-low occurs when
the fault condition has been removed and the fault latch
has been cleared by toggling DISABLE. FAULT should be
pulled up to VCC by a 4.7kI to 10kI resistor.
EQUIVALENT CIRCUIT
FAULT
3
CLAMP
VCCT
4
5,8
BMON
VCCT
Analog Laser DC Current Monitor Output. Current out of
this pin develops a ground-referenced voltage across an
external resistor that is proportional to the VOUT pin current. The current sourced by this pin is typically 1/60 the
VOUT pin current.
Power Supply. Provides supply voltage to the output block.
R
BMON
—
VCCT
6
TOUTA
Inverting Laser Diode Modulation Current Output. Connect
this pin to the anode of the laser diode.
TOUTA
TOUTC
7
TOUTC
Noninverting Laser Diode Modulation Current Output.
Connect this pin to the cathode of the laser diode.
�� Maxim Integrated Products 12
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Pin Description (continued)
PIN
NAME
FUNCTION
EQUIVALENT CIRCUIT
VCCT
9
VOUT
Combined Current Return Path and Laser DC Current
Output
VOUT
VCC
10
CSEL
Chip-Select CMOS Input. Setting CSEL to logic-high starts
a 3-wire command cycle. Setting CSEL to logic-low ends
the cycle and resets the control state machine. Internally
pulled down to GND by a 75kI resistor.
ESD
PROTECTION
VCC
CSEL
75kI
VCC
VCC
VCC
75kI
11
SDA
Serial Data Bidirectional CMOS Input. Also an open-drain
output. This pin has a 75kI internal pullup, but requires an
external 4.7kI to 10kI pullup resistor to VCC for proper
operation.
SDA
ESD
PROTECTION
VCC
12
SCL
Serial-Clock CMOS Input. This pin has an internal 75kI
pulldown resistor to GND.
ESD
PROTECTION
VCC
SCL
75kI
�� Maxim Integrated Products 13
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Pin Description (continued)
PIN
NAME
FUNCTION
EQUIVALENT CIRCUIT
13,
16
VCC
Power Supply. Provides supply voltage to core analog and
digital circuitry.
—
VCC
14
TIN+
Noninverting Data Input. Input with internal 50I termination.
TIN+
50I
CONTROL
LOOP
50I
TIN-
15
TIN-
Inverting Data Input. Input with internal 50I termination.
GND
—
EP
Exposed Pad (Ground). This is the only electrical connection to ground on the MAX3948 and must be soldered to the
circuit board ground for proper thermal and electrical performance (see the Exposed-Pad Package section).
—
�� Maxim Integrated Products 14
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
VCCT
VCC
25I
7.5kI
TOUTA
DISABLE
TX_EN
FAULT
VCM
EYE SAFETY AND
OUTPUT CONTROL
VOUT
POWER-ON RESET
25I
50I
TIN+
TX_POL
TX_LOS
50I
TOUTC
1
EQ
TIN-
IDC
VOUT
0
VCC
CONTROL
LOGIC
75kI
SDA
3-WIRE
INTERFACE
SCL
CSEL
75kI
75kI
IMOD_DAC + IDE_DAC
IDC/60
VCC
BMON
REGISTER
2b SET_TXEQ
MAX3948
9b DAC SET_IMOD
CHANNEL
DETECTION
VSEL
7b DAC SET_TXDE
9b DAC SET_IDC
Figure 2. Functional Diagram
�� Maxim Integrated Products 15
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Detailed Description
Table 1. Input Equalization Control
Register Settings
The MAX3948 SFP+/QSFP+ laser driver is designed
to drive 5I to 10I TOSAs from 1Gbps to 11.3Gbps. It
contains an input buffer with programmable equalization,
DC and modulation current DACs, an output driver with
adjustable deemphasis, power-on-reset circuitry, DC
current monitor, programmable 3-wire address, and eye
safety circuitry with maskable fault monitors. A 3-wire
digital interface is used to control these functions.
SET_TXEQ[1:0]
0
1
0
1
3
1
1
5.5
DC Current DAC
The DC current from the device is optimized to provide up to 61mA of DC current into a laser diode with
116FA resolution (Figure 3). The DC DAC current is
controlled through the 3-wire digital interface using the
SET_IDC[8:0], IDCMAX[7:0], and DCINC[4:0] bits.
Input Buffer with Programmable Equalization
The input is internally biased and terminated with 50I to
a common-mode voltage. The first amplifier stage features a programmable equalizer for high-frequency losses including a SFP+/QSFP+ host connector. Equalization
is controlled by the SET_TXEQ register (Table 1). The
TX_POL bit in the TXCTRL register controls the polarity of
TOUTA and TOUTC vs. TIN+ and TIN-. A status indicator bit (TXSTAT1 bit 5) monitors the presence of an AC
input signal.
For laser operation, the laser DC current can be set using
the 9-bit SET_IDC DAC register. The upper 8 bits are set
by the SET_IDC[8:1] register, commonly used during the
initialization procedure after power-on reset (POR). The
LSB (bit 0) of SET_IDC (DCINC[7]) is initialized to zero
after POR and can be updated using the DCINC register.
AC-COUPLING CASE
VCCT
BOOST AT 5.16GHz (dB)
0
DC-COUPLING CASE
3.3V
VCCT
TOUTA
3.3V
TOUTA
IMOD_DAC
IMOD_DAC
TOUTC
TOUTC
IBIAS_DAC
IDC_DAC
VOUT
LDMOD
VOUT
LDMOD
LDBIAS
LDMOD = K1 × IMOD_DAC
LDBIAS = IBIAS_DAC
NOTE: FIGURES ARE SIMPLIFIED TO EXPRESS AC-COUPLING vs. DC-COUPLING DIFFERENCES.
LDDC
LDBIAS
LDMOD = K1 × IMOD_DAC
LDBIAS* = IDC_DAC + K1/2 × IMOD_DAC + f(IMOD_DAC,DE,R)
*SEE THE ELECTRICAL CHARACTERISTICS TABLE, NOTES 5 AND 8.
Figure 3. AC-/DC-Coupling Cases
�� Maxim Integrated Products 16
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
The IDCMAX register limits the maximum SET_IDC[8:1]
DAC code.
After initialization the value of the SET_IDC DAC register
should be updated using the DCINC register to optimize
cycle time and enhance laser safety. The DCINC register is an 8-bit register. The first 5 bits of DCINC contain
the increment information in two’s complement format.
Increment values range from -16 to +15 LSBs. If the
updated value of SET_IDC[8:1] exceeds IDCMAX[7:0],
the IDCERR warning flag is set and SET_IDC[8:1] is set
to IDCMAX[7:0].
Modulation Current DAC
The modulation current from the MAX3948 is optimized
to provide up to 85mA of modulation current into a 5I
laser load with 210FA resolution. The modulation current
is controlled through the 3-wire digital interface using
the SET_IMOD[8:1], IMODMAX[7:0], MODINC[7:0], and
SET_TXDE registers.
For laser operation, the laser modulation current can be
set using the 9-bit SET_IMOD DAC. The upper 8 bits are
programmed through the SET_IMOD[8:1] register, commonly used during the initialization procedure after POR.
The LSB (bit 0) of SET_IMOD (MODINC[7])is initialized to
zero after POR and can be updated using the MODINC
register. The IMODMAX register limits the maximum
SET_IMOD[8:1] DAC code.
After initialization the value of the SET_IMOD DAC register should be updated using the MODINC[4:0] bits
to optimize cycle time and enhance laser safety. The
MODINC register is an 8-bit register. The first 5 bits of
MODINC contain the increment information in two’s complement format. Increment values range from -16 to +15
LSBs. If the updated value of SET_IMOD[8:1] exceeds
IMODMAX[7:0], the IMODERR warning flag is set and
SET_IMOD[8:1] is set to IMODMAX[7:0].
Effective modulation current seen by the laser is actually the combination of the DAC current generated by
the SET_IMOD[8:0] register (IMOD), deemphasis setting
(DE), and differential laser load (R). It is calculated by the
following formula:
LDMOD = IMOD x 50 x (1 - DE)/(50 + R)
Output Driver
This device is optimized to drive a differential TOSA with
a 25I flex circuit. The unique design of the output stage
enables DC-coupling to unmatched TOSAs with laser
diode impedances ranging from 5I to 10I. The output
stage also features programmable deemphasis that can
be set as a percentage of the modulation current. The
deemphasis function is controlled by the TXCTRL[4:3]
and the SET_TXDE registers.
Power-On Reset (POR)
Power-on reset ensures that the laser is off until the supply
voltage has reached a specified threshold (2.75V). After
power-on reset, TX_EN is 0 and DC current and modulation current DACs default to small codes. In the case of a
POR, all registers are reset to their default values.
BMON Function
The current out of the BMON pin is typically 1/60th the
value of the current into the VOUT pin. The total resistance to ground at BMON sets the voltage.
VSEL Function
The VSEL pin is an analog input that sets the 3-wire
address for the MAX3948. The pin can be set to either
VCC, VCC x 2/3, VCC/3, or to GND (Table 2). This allows
up to four MAX3948s to be operated on a single 3-wire
bus, each with their own address.
Table 2. 3-Wire Address Selection
VSEL
ADDR[6:5]
VCC
11b
VCC x 2/3
10b
VCC/3
01b
GND
00b
Eye Safety and Output Control Circuitry
The safety and output control circuitry includes the disable pin (DISABLE) and enable bit (TX_EN), along with
a FAULT indicator and fault detectors (Figure 4). A fault
condition triggers the FAULT pin to go high and a corresponding bit is set in the TXSTAT1 register. The MAX3948
has two types of faults: hard faults and soft faults. Hard
faults are maskable, trigger the FAULT pin (transitions
high), disable the outputs and are stored in the TXSTAT1
register. Soft faults serve as warnings, do not disable the
outputs, and are stored in the TXSTAT2 register.
�� Maxim Integrated Products 17
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
VCCT
25Ω
TOUTA
TOUTC
25Ω
VOUT
IDC
<0>
VCCT - 2.4V (FOR VOUT)
VCC - 0.3V (FOR VCCT)
IMOD
<1>
0.41V
<2>
VCCT - 1.6V
<3>
0.48V + 0.14 x SET_IMOD[8:6]d
(SELF-ADJUSTING)
<4>
LOS
CIRCUIT
FAULT REGISTER
TXSTAT1
ADDR = H0x06
<5>
UNUSED
<6>
POR
VCC
<7>
RESET
7.5kI
DISABLE
UNUSED
UNUSED
MAX3948
SET_IDC[8:1]
IDCMAX[7:0]
SET_IMOD[8:1]
IMODMAX[7:0]
<0>
<1>
WARNING REGISTER
TXSTAT2
ADDR = H0x07
OVERFLOW
<2>
UNDERFLOW
OVERFLOW
<3>
UNDERFLOW
Figure 4. Eye Safety Circuitry
�� Maxim Integrated Products 18
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
The FAULT pin is a latched output that can be cleared by
toggling the DISABLE pin. Toggling the DISABLE pin also
clears the TXSTAT1 and TXSTAT2 registers. A single-point
failure can be a short to VCC or GND. Table 3 shows the
circuit response to various single-point failures.
Table 3. Circuit Response to Single-Point Failure
PIN
NAME
SHORT TO VCC
1
DISABLE
SHORT TO GND
OPEN
Disabled
Normal (Note 1). Can only be
disabled by other means.
Disabled
2
VSEL
Normal (Note 2)
Normal (Note 2)
Normal (Note 2)
3
FAULT
Normal (Note 2)
Normal (Note 1)
Normal (Note 2)
4
BMON
Normal (Note 2)
Normal (Note 2)
Normal (Note 2)
5, 8
VCCT
Normal
Disabled—Fault (external supply
shorted) (Note 3)
Redundant path (Note 4)
6
TOUTA
Laser modulation current is
reduced
Disabled (hard fault)
Laser modulation current is
reduced or disabled (hard fault)
7
TOUTC
Laser modulation current is
reduced or off
Disabled (hard fault)
Laser modulation current is
reduced or disabled (hard fault)
9
VOUT
IDC is on, but not delivered to
the laser; no fault
Disabled (hard fault)
Disabled (hard fault)
10
CSEL
Normal (Note 2)
Normal (Note 2)
Normal (Note 2)
11
SDA
Normal (Note 2)
Normal (Note 2)
Normal (Note 2)
12
SCL
Normal (Note 2)
Normal (Note 2)
Normal (Note 2)
13, 16
VCC
Normal
Disabled—Hard fault (external
supply shorted) (Note 3)
Redundant path (Note 4)
14
TIN+
Disabled (hard fault)
Disabled (hard fault)
Normal (Note 2) or disabled
(hard fault)
15
TIN-
Disabled (hard fault)
Disabled (hard fault)
Normal (Note 2) or disabled
(hard fault)
Note 1: Normal operation—Does not affect the laser power.
Note 2: Pin functionality might be affected, which could affect laser power/performance.
Note 3: Supply-shorted current is assumed to be primarily on the circuit board (outside this device) and the main supply is collapsed by the short.
Note 4: Normal in functionality, but performance could be affected.
Warning: Shorted to VCC or shorted to ground on some pins can violate the Absolute Maximum Ratings.
�� Maxim Integrated Products 19
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
3-Wire Interface
the transmission by setting CSEL to 0. Figure 5 shows the
interface timing.
The MAX3948 implements a proprietary 3-wire digital
interface. An external controller generates the clock. The
3-wire interface consists of an SDA bidirectional data
line, a SCL clock signal input, and a CSEL chip-select
input (active high). The external master initiates a data
transfer by asserting the CSEL pin. The master starts to
generate a clock signal after the CSEL pin has been set
to a logic-high. All data transfers are most significant bit
(MSB) first.
Read Mode (RWN = 1)
The master generates 16 clock cycles at SCL in total. The
master outputs a total of 8 bits (MSB first) to the SDA line
at the falling edge of the clock. The SDA line is released
after the RWN bit has been transmitted. The slave outputs 8 bits of data (MSB first) at the rising edge of the
clock. The master closes the transmission by setting
CSEL to 0. Figure 5 shows the interface timing.
Protocol
Each operation consists of 16-bit transfers (15-bit
address/data, 1-bit RWN). The bus master generates 16
clock cycles to SCL. All operations transfer 8 bits to the
MAX3948. The RWN bit determines if the cycle is read or
write (Table 5).
Mode Control
Normal mode allows read-only instruction for all registers.
Only the MODINC and DCINC registers can be updated
during normal mode. Doing so speeds up the laser control update through the 3-wire interface by a factor of two.
The normal mode is the default mode.
Register Addresses
The MAX3948 contains 13 registers available for programming. Table 6 shows the registers and addresses.
Setup mode allows the master to write unrestricted data
into any register except the status (TXSTAT1, TXSTAT2)
registers. To enter the setup mode, the MODECTRL
register (address = H0x0F) must be set to 12h. After the
MODECTRL register has been set to 12h, the next operation is unrestricted. The setup mode is automatically
exited after the operation is finished. This sequence
must be repeated if further unrestricted settings are
necessary.
Write Mode (RWN = 0)
The master generates 16 clock cycles at SCL in total. The
master outputs a total of 16 bits (MSB first) to the SDA
line at the falling edge of the clock. The master closes
Table 4. Broadcast Mode Register
Initialization Sequence
ADDRESS
Broadcast mode allows for faster configuration of multiple MAX3948 ICs by causing the address selection bits
(ADDR[6:5]) to be ignored so all MAX3948s on the bus
can be written to simultaneously.
NAME
H0x0F
FMSK
H0x10
SET_TXDE
H0x11
SET_TXEQ
H0x0A
IMODMAX
H0x0B
IDCMAX
H0x08
SET_IDC
H0x09
SET_IMOD
H0x05
TXCTRL
A block write in broadcast mode can start at any of the
addresses in Table 4. The block write is achieved by
holding the CSEL pin high to lengthen the SPI cycle.
The register address increments automatically through
the sequence listed in Table 4 and wraps from TXCTRL
to FMSK. The block write ends once the CSEL pin is
asserted low.
Table 5. Digital Communication Word Structure
BIT
15
14
13
12
ADDR[6:0]
11
10
9
8
RWN
7
6
5
4
3
2
1
0
DATA[7:0]
�� Maxim Integrated Products 20
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Table 6. Register Descriptions and Addresses
ADDRESS
NAME
H0x05
TXCTRL
Transmitter Control Register
FUNCTION
H0x06
TXSTAT1
Transmitter Status Register 1
H0x07
TXSTAT2
Transmitter Status Register 2
H0x08
SET_IDC
DC Current Setting Register
H0x09
SET_IMOD
Modulation Current Setting Register
H0x0A
IMODMAX
Maximum Modulation Current Setting Register
H0x0B
IDCMAX
Maximum DC Current Setting Register
H0x0C
MODINC
Modulation Current Increment Setting Register
H0x0D
DCINC
H0x0E
MODECTRL
DC Current Increment Setting Register
H0x0F
FMSK
H0x10
SET_TXDE
Transmitter Deemphasis Control Register
H0x11
SET_TXEQ
Transmitter Equalization Control Register
Mode Control Register
Fault Mask Register
WRITE MODE
CSEL
tL
tT
tCH
0
SCL
tCL
1
2
3
4
5
6
7
8
9
A4
A3
A2
A1
A0
RWN
D7
D6
10
11
12
13
14
15
tDS
SDA
A6
A5
D5
D4
D3
D2
D1
D0
M
tDH
READ MODE
CSEL
tL
tCH
SCL
tT
tCL
0
1
2
3
4
5
6
7
A4
A3
A2
A1
A0
RWN
tDS
SDA
A6
A5
M
8
tD
D7
9
10
D6
11
D5
12
D4
13
D3
14
D2
15
D1
D0
S
tDH
Figure 5. Timing for 3-Wire Digital Interface
�� Maxim Integrated Products 21
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Register Descriptions
Transmitter Control Register (TXCTRL), Address: H0x05
Bit
D7
D6
RESERVED
RESERVED
Read/Write
R/W
R/W
R/W
R/W
POR State
0
0
0
0
Bit Name
D5
D4
D3
D2
D1
D0
SOFTRES
TX_POL
TX_EN
R/W
R/W
R/W
R/W
0
0
1
0
RESERVED TXDE_MD[1] TXDE_MD[0]
The TXCTRL register sets the device’s operation.
BIT
NAME
DESCRIPTION
RESERVED
Reserved Bits. The default state for these bits is 0 and they must be kept 0 when
the register is accessed for a write operation.
TXDE_MD
Controls the mode of the transmit output deemphasis circuitry.
00 = Deemphasis is fixed at 6% of the modulation amplitude
01 = Deemphasis is fixed at 3% of the modulation amplitude
10 = Deemphasis is programmed by SET_TXDE register setting (3% to 9%)
11 = Deemphasis is at its maximum of ~9%
D2
SOFTRES
Resets all registers to their default values (TXCTRL[1:0] must be = 10b during the
write to SOFTRES for the registers to be set to their default values).
0 = Normal operation
1 = Reset
D1
TX_POL
D0
TX_EN
D[7:5]
D[4:3]
Controls the polarity of the transmit signal path.
0 = Inverse
1 = Normal operation
Enables or disables the transmit circuitry.
0 = Disabled
1 = Enabled
�� Maxim Integrated Products 22
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Transmitter Status Register 1 (TXSTAT1), Address: H0x06
Bit
Bit Name
D7
(STICKY)
D6
(STICKY)
D5
(STICKY)
D4
(STICKY)
D3
(STICKY)
D2
(STICKY)
D1
(STICKY)
D0
(STICKY)
FST[7]
FST[6]
FST[5]
FST[4]
FST[3]
FST[2]
FST[1]
FST[0]
Read/Write
R
R
R
R
R
R
R
R
POR State
1
X
X
X
X
X
X
X
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Reset Upon Read
The TXSTAT1 register is a device status register.
BIT
NAME
DESCRIPTION
D7
FST[7]
When the VCC supply voltage is below 2.3V, the POR circuitry reports a FAULT and communication to the SPI cannot be performed. Once the VCC supply voltage is above 2.75V, the
POR resets all registers to their default values and the FAULT latch is cleared.
D6
FST[6]
Reserved.
D5
FST[5]
Indicates low or no AC signal at the inputs, a hard fault is reported unless masked.
D4
FST[4]
Indicates VOUT too low condition. Intended to be used as a warning/soft fault rather than a
hard fault. In normal operation, FMSK[4] should be kept at logic 1 to convert this to a soft fault
behavior. Self-adjustable threshold = 0.48V + 0.14V x SET_IMOD[8:6] (decimal value 0 to 7). A
logic 1 can indicate marginal power-supply headroom.
D3
FST[3]
Indicates TOUTA open or shorted to GND condition, threshold = VCCT - 1.6V, a hard fault is
reported unless masked.
D2
FST[2]
Indicates TOUTC open or shorted to GND condition, threshold = 0.41V, a hard fault is reported
unless masked.
D1
FST[1]
Indicates VOUT or VCCT open or shorted to GND conditions, threshold (VCCT) = VCC - 0.3V,
threshold (VOUT) = VCCT - 2.4V, a hard fault is reported unless masked.
D0
FST[0]
Copy of a FAULT signal.
Transmitter Status Register 2 (TXSTAT2), Address: H0x07
D7
D6
D5
D4
D3
(STICKY)
D2
(STICKY)
D1
D0
Bit Name
X
X
X
X
IMODERR
IDCERR
X
X
Read/Write
X
X
X
X
R
R
X
X
POR State
X
X
X
X
0
0
X
X
Reset Upon Read
X
X
X
X
Yes
Yes
X
X
Bit
The TXSTAT2 register is a device status register.
BIT
NAME
D3
IMODERR
D2
IDCERR
DESCRIPTION
Modulation current overflow (on increment) or underflow (on decrement) error.
Overflow occurs if result > IMODMAX. In overflow condition, SET_IMOD[8:1] = IMODMAX[7:0].
Underflow occurs if result < 0. In underflow condition, SET_IMOD[8:0] = 0.
DC current overflow (on increment) or underflow (on decrement) error.
Overflow occurs if result > IDCMAX. In overflow condition, SET_IDC[8:1] = IDCMAX[7:0].
Underflow occurs if result < 0. In underflow condition, SET_IDC[8:0] = 0.
�� Maxim Integrated Products 23
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
DC Current Setting Register (SET_IDC), Address: H0x08
Bit
D7
Bit Name
D6
D5
D4
D3
D2
D1
D0
SET_IDC[8] SET_IDC[7] SET_IDC[6] SET_IDC[5] SET_IDC[4] SET_IDC[3] SET_IDC[2] SET_IDC[1]
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
POR State
0
0
0
0
0
0
0
1
The SET_IDC register sets the laser DC current DAC.
BIT
D[7:0]
NAME
DESCRIPTION
SET_IDC[8:1]
The DC current DAC is controlled by a total of 9 bits. The SET_IDC[8:1] bits are used to
set the DC current with even denominations from 0 to 510 bits. The LSB (SET_IDC[0])
bit is controlled by the DCINC register and is used to set the odd denominations in the
SET_IDC[8:0]. Any direct write to SET_IDC[8:1] resets the LSB.
Modulation Current Setting Register (SET_IMOD), Address: H0x09
Bit
D7
Bit Name
D6
D5
D4
D3
D2
D1
D0
SET_IMOD[8]SET_IMOD[7]SET_IMOD[6]SET_IMOD[5]SET_IMOD[4]SET_IMOD[3]SET_IMOD[2]SET_IMOD[1]
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
POR State
0
0
0
0
0
1
0
0
The SET_IMOD register sets the laser modulation current DAC.
BIT
D[7:0]
NAME
DESCRIPTION
SET_IMOD[8:1]
The mod current DAC is controlled by a total of 9 bits. The SET_IMOD[8:1] bits are
used to set the modulation current with even denominations from 0 to 510 bits. The
LSB (SET_IMOD[0]) bit is controlled by the MODINC register and is used to set the odd
denominations in the SET_IMOD[8:0]. Any direct write to SET_IMOD[8:1] resets the LSB.
�� Maxim Integrated Products 24
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Maximum Modulation Current Setting Register (IMODMAX), Address: H0x0A
Bit
Bit Name
D7
D6
D5
D4
D3
D2
D1
D0
IMODMAX[7]
IMODMAX[6]
IMODMAX[5]
IMODMAX[4]
IMODMAX[3]
IMODMAX[2]
IMODMAX[1]
IMODMAX[0]
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
POR State
0
0
1
0
0
0
0
0
The IMODMAX register sets the upper limit of modulation current.
BIT
NAME
DESCRIPTION
D[7:0]
IMODMAX[7:0]
The IMODMAX register is an 8-bit register that can be used to limit the maximum modulation current. IMODMAX[7:0] is continuously compared to SET_IMOD[8:1].
Maximum DC Current Setting Register (IDCMAX), Address: H0x0B
Bit
Bit Name
D7
D6
D5
D4
D3
D2
D1
D0
IDCMAX[7]
IDCMAX[6]
IDCMAX[5]
IDCMAX[4]
IDCMAX[3]
IDCMAX[2]
IDCMAX[1]
IDCMAX[0]
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
POR State
0
0
1
0
0
0
0
0
The IDCMAX register sets the upper limit of DC current.
BIT
D[7:0]
NAME
DESCRIPTION
IDCMAX[7:0]
The IDCMAX register is an 8-bit register that can be used to limit the maximum DC current. IDCMAX[7:0] is continuously compared to SET_IDC[8:1].
Modulation Current Increment Setting Register (MODINC), Address: H0x0C
Bit
D7
D6
D5
D4
D3
D2
D1
D0
SET_IMOD[0]
X
X
MODINC[4]
MODINC[3]
MODINC[2]
MODINC[1]
MODINC[0]
Read/Write
R
X
X
R/W
R/W
R/W
R/W
R/W
POR State
0
X
X
0
0
0
0
0
Bit Name
The MODINC register increments/decrements the SET_IMOD register.
BIT
NAME
D7
SET_IMOD[0]
D[4:0]
MODINC
DESCRIPTION
LSB of SET_IMOD register
This string of bits is used to increment or decrement the modulation current. When
written to, the SET_IMOD[8:0] bits are updated. MODINC[4:0] are a two’s complement
string.
�� Maxim Integrated Products 25
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
DC Current Increment Setting Register (DCINC), Address: H0x0D
Bit
Bit Name
D7
D6
D5
D4
D3
D2
D1
D0
SET_IDC[0]
X
X
DCINC[4]
DCINC[3]
DCINC[2]
DCINC[1]
DCINC[0]
Read/Write
R
X
X
R/W
R/W
R/W
R/W
R/W
POR State
0
X
X
0
0
0
0
0
The DCINC register increments/decrements the SET_IDC register.
BIT
NAME
D7
SET_IDC[0]
DESCRIPTION
D[4:0]
DCINC
LSB of SET_IDC register.
This string of bits is used to increment or decrement the modulation current. When written
to, the SET_IDC[8:0] bits are updated. DCINC[4:0] are a two’s complement string.
Mode Control Register (MODECTRL), Address: H0x0E
Bit
D7
D6
D5
D4
D3
D2
D1
D0
MODECTRL[7]
MODECTRL[6]
MODECTRL[5]
MODECTRL[4]
MODECTRL[3]
MODECTRL[2]
MODECTRL[1]
MODECTRL[0]
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
POR State
0
0
0
0
0
0
0
0
Yes*
Yes*
Yes*
Yes*
Yes*
Yes*
Yes*
Yes*
Bit Name
Reset Upon Read
*All three modes reset back to 0h on the next 3-wire access.
The MODECTRL register set the operational mode of the 3-wire control for the MAX3948.
BIT
D[7:0]
NAME
DESCRIPTION
MODECTRL[7:0]
The MODECTRL register enables the user to switch between normal and setup modes.
The setup mode is achieved by setting this register to 12h. MODECTRL must be updated before each write operation. Exceptions are MODINC and DCINC, which can be
updated in normal mode.
00h: normal mode
12h: setup mode
C9h: broadcast mode
�� Maxim Integrated Products 26
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Fault Mask Register (FMSK), Address: H0x0F
Bit
D7
D6
D5
D4
D3
D2
D1
D0
Bit Name
X
RESERVED
FMSK[5]
FMSK[4]
FMSK[3]
FMSK[2]
FMSK[1]
FMSK[0]
Read/Write
X
R/W
R/W
R/W
R/W
R/W
R/W
R/W
POR State
X
1
1
0
0
0
0
0
Reset Upon Read
X
No
No
No
No
No
No
No
The FMSK register sets masking for the fault circuitry.
BIT
NAME
D6
RESERVED
D5
FMSK[5]
Input LOS FAULT condition mask.
0 = No mask
1 = Mask
D4
FMSK[4]
VOUT too low FAULT condition mask. This condition is intended to behave like a warning/soft fault in normal operation. In normal operation, FMSK[4] should be kept at
logic 1.
0 = No mask
1 = Mask
D3
FMSK[3]
TOUTA open or shorted to GND FAULT condition mask.
0 = No mask
1 = Mask
D2
FMSK[2]
TOUTC open or shorted to GND FAULT condition mask.
0 = No mask
1 = Mask
D1
FMSK[1]
VOUT or VCCT open or shorted to GND FAULT conditions mask.
0 = No mask
1 = Mask
FMSK[0]
Masks the FAULT latch signal, which controls the output stage on/off behavior.
0 = No mask
1 = Mask
When FMSK[0] = 1, output stage behavior becomes independent of FAULT conditions
and is only controlled by DISABLE pin and TX_EN bit. Masking this bit has no impact on
normal reporting of fault status bits and assertion of the FAULT pin.
D0
DESCRIPTION
Reserved. This bit must be kept at logic 1 for all operations.
�� Maxim Integrated Products 27
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Transmitter Deemphasis Control Register (SET_TXDE), Address: H0x10
Bit
D7
D6
D5
D4
D3
D2
D1
D0
Bit Name
X
Read/Write
X
SET_TXDE[6] SET_TXDE[5] SET_TXDE[4] SET_TXDE[3] SET_TXDE[2] SET_TXDE[1] SET_TXDE[0]
R/W
R/W
R/W
R/W
R/W
R/W
R/W
POR State
X
0
0
0
0
0
1
0
The SET_TXDE register sets the deemphasis amount for the transmitter when TXDE_MD[1:0] is 10b.
BIT
NAME
DESCRIPTION
D[6:0]
SET_TXDE[6:0]
This is a 7-bit register used to control the amount of deemphasis on the transmitter output. When calculating the total modulation current, the amount of deemphasis must be
taken into account. Deemphasis is set as a percentage of modulation current.
Transmitter Equalization Control Register (SET_TXEQ), Address: H0x11
Bit
D7
D6
D5
D4
D3
D2
Bit Name
X
X
X
X
X
X
D1
D0
Read/Write
X
X
X
X
X
X
R/W
R/W
POR State
X
X
X
X
X
X
0
0
SET_TXEQ[1] SET_TXEQ[0]
The SET_TXEQ register sets the equalization amount for the transmitter input.
BIT
NAME
D[1:0]
SET_TXEQ
DESCRIPTION
This is a 2-bit register used to control the amount of equalization on the transmitter
input. See Table 1 for more information.
�� Maxim Integrated Products 28
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Design Procedure
Programming Modulation Current
1) IMODMAX[7:0] = Maximum_Modulation_Current_
Value
2) SET_IMODn[8:0] = Present_Modulation_Current_
Value
Note: SET_IMOD[8:1] are the bits that can be manually written. SET_IMOD[0] can only be updated using
the MODINC register.
When implementing modulation current temperature
compensation, it is recommended to use the MODINC
register, which guarantees the fastest modulation current update.
3) MODINCn[4:0] = New_Increment_Value
The device performs the following operation when
MODINCn[4:0] is written to:
If (SET_IMODn[8:1] P IMODMAX[7:0]), then
(SET_IMODn[8:0] = SET_IMODn-1[8:0] + MODINCn[4:0])
else (SET_IMODn[8:1] = IMODMAX[7:0])
The modulation DAC current can be calculated using
the following equation:
IMOD DAC Current = IMOD =
(16 + SET_IMOD[8:0]) x 247FA
The net modulation current (P-P) seen at the laser
when driven differentially is calculated using the following equation:
LDMOD = IMOD x (1 - DE) x 50/(50 + R)
6) TXCTRL[4:3] = 00, SET_TXDE can be externally set to
any value R SET_IMOD[8:3]:
IDE = (2 + SET_TXDE[6:0]) x 61.8FA
In this case DE = IDE/IMOD. The value of the DE factor
starts close to 0.03 and can go up to 0.09 as the value
of SET_TXDE[6:0] is increased. Once the DE ratio is
close to 0.09, the IDE saturates and a further increase
in SET_TXDE[6:0] value does not change IDE much.
7) TXCTRL[4:3] = 11, DE = 0.09 (~ 9% deemphasis
case). In this mode, the device calculates and sets
the SET_TXDE[6:0] = 127. SET_TXDE is not accessible for external write.
Programming DC Current
1) IDCMAX[7:0] = Maximum_DC_Current_Value
2) SET_IDCn[8:0] = Present_ DC _Current_Value
Note: SET_IDC[8:1] are the bits that can be manually
written. SET_IDC[0] can only be updated using the
DCINC register.
When implementing laser bias current temperature
compensation, it is recommended to use the DCINC
register, which guarantees the fastest modulation current update.
3) DCINCn[4:0] = New_Increment_Value
The device performs the following operation when
DCINCn[4:0] is written to:
If (SET_IDCn[8:1] P IDCMAX[7:0]), then
(SET_IDCn[8:0] = SET_IDCn-1[8:0] + DCINCn[4:0])
else (SET_IDCn[8:1] = IDCMAX[7:0])
where R is the differential load impedance of the
laser plus any added series resistance, and DE is the
deemphasis factor controlled by the TX_DEMD[1:0]
bits.
The DC DAC current can be calculated using the following equation:
4) TXCTRL[4:3] = 00, DE = 0.0625 (~ 6% deemphasis
case). In this mode, the device calculates and sets
SET_TXDE[6:0] = SET_IMOD[8:2]. SET_TXDE is not
accessible for external write.
The net DC current seen at the laser when driven differentially is calculated using the following equation:
5) TXCTRL[4:3] = 00, DE = 0.03125 (~ 3% deemphasis
case). In this mode, the device calculates and sets
SET_TXDE[6:0] = SET_IMOD[8:3]. SET_TXDE is not
accessible for external write.
DC DAC Current = IDC =
(16 + SET_IDC[8:0]) x 116FA
LDDC = IDC + IMOD x (DE + R x (1 - DE)/(50 + R)/2)
where R is the differential load impedance of the laser
plus any added series resistance, DE is the deemphasis factor controlled by the TX_DEMD[1:0] bits, and
IMOD is the modulation DAC current.
�� Maxim Integrated Products 29
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Applications Information
Laser Safety and IEC 825
Using the MAX3948 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. Each user must determine the level of
fault tolerance required by the application, recognizing
that Maxim products are neither designed nor 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 in which
the failure of a Maxim product could create a situation
where personal injury or death could occur.
Table 7. Register Summary
REGISTER
FUNCTION/
ADDRESS
Transmitter
Control
Register
Address =
H0x05
Transmitter
Status
Register 1
Address =
H0x06
REGISTER
NAME
TXCTRL
TXSTAT1
NORMAL
MODE
SETUP
MODE
BIT
NUMBER/
TYPE
BIT NAME
DEFAULT
VALUE
R
R/W
7
Reserved
0
Must be kept at 0
R
R/W
6
Reserved
0
Must be kept at 0
R
R/W
5
Reserved
0
Must be kept at 0
R
R/W
4
TXDE_MD[1]
0
Tx deemphasis control
R
R/W
3
TXDE_MD[0]
0
Tx deemphasis control
R
R/W
2
SOFTRES
0
Global digital reset
R
R/W
1
TX_POL
1
Tx polarity
0: inverse, 1: normal
R
R/W
0
TX_EN
0
Tx control
0: disable, 1: enable
R
R
7 (sticky)
FST[7]
1
PORàVCC low-limit
violation
R
R
6 (sticky)
FST[6]
X
Reserved
R
R
5 (sticky)
FST[5]
X
Low or no AC signal at
input
R
R
4 (sticky)
FST[4]
X
VOUT too low
R
R
3 (sticky)
FST[3]
X
TOUTA open or shorted
to GND
R
R
2 (sticky)
FST[2]
X
TOUTC open or shorted
to GND
R
R
1 (sticky)
FST[1]
X
VOUT/VCCT open or
shorted to GND
R
R
0 (sticky)
FST[0]
X
Copy of FAULT signal
NOTES
�� Maxim Integrated Products 30
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Table 7. Register Summary (continued)
REGISTER
FUNCTION/
ADDRESS
REGISTER
NAME
NORMAL
MODE
R
Transmitter
Status
Register 2
Address =
H0x07
DC Current
Setting
Register
Address =
H0x08
Modulation
Current
Setting
Register
Address =
H0x09
Maximum
Modulation
Current
Setting
Register
Address =
H0x0A
SETUP
MODE
R
BIT
NUMBER/
TYPE
3 (sticky)
BIT NAME
IMODERR
DEFAULT
VALUE
NOTES
0
Modulation current overflow
(on increment) or underflow (on decrement) error.
Overflow occurs if result
> IMODMAX. Underflow
occurs if result < 0.
TXSTAT2
SET_IDC
SET_IMOD
IMODMAX
R
R
2 (sticky)
IDCERR
0
DC current overflow (on
increment) or underflow
(on decrement) error.
Overflow occurs if result
> IDCMAX. Underflow
occurs if result < 0.
R
R/W
7
SET_IDC[8]
0
MSB DC DAC
R
R/W
6
SET_IDC[7]
0
R
R/W
5
SET_IDC[6]
0
R
R/W
4
SET_IDC[5]
0
R
R/W
3
SET_IDC[4]
0
R
R/W
2
SET_IDC[3]
0
R
R/W
1
SET_IDC[2]
0
R
R/W
0
SET_IDC[1]
1
R
R/W
7
SET_IMOD[8]
0
R
R/W
6
SET_IMOD[7]
0
R
R/W
5
SET_IMOD[6]
0
R
R/W
4
SET_IMOD[5]
0
R
R/W
3
SET_IMOD[4]
0
R
R/W
2
SET_IMOD[3]
1
R
R/W
1
SET_IMOD[2]
0
R
R/W
0
SET_IMOD[1]
0
R
R/W
7
IMODMAX[7]
0
R
R/W
6
IMODMAX[6]
0
R
R/W
5
IMODMAX[5]
1
R
R/W
4
IMODMAX[4]
0
R
R/W
3
IMODMAX[3]
0
R
R/W
2
IMODMAX[2]
0
R
R/W
1
IMODMAX[1]
0
R
R/W
0
IMODMAX[0]
0
MSB modulation DAC
MSB modulation limit
LSB modulation limit
�� Maxim Integrated Products 31
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Table 7. Register Summary (continued)
REGISTER
FUNCTION/
ADDRESS
Maximum
DC DAC
Current
Setting
Register
Address =
H0x0B
Modulation
Current
Increment
Setting
Register
Address =
H0x0C
DC Current
Increment
Setting
Register
Address =
H0x0D
Mode
Control
Register
Address =
H0x0E
REGISTER
NAME
IDCMAX
MODINC
DCINC
MODECTRL
NORMAL
MODE
SETUP
MODE
BIT
NUMBER/
TYPE
BIT NAME
DEFAULT
VALUE
R
R/W
7
IDCMAX[7]
0
R
R/W
6
IDCMAX[6]
0
R
R/W
5
IDCMAX[5]
1
R
R/W
4
IDCMAX[4]
0
R
R/W
3
IDCMAX[3]
0
R
R/W
2
IDCMAX[2]
0
R
R/W
1
IDCMAX[1]
0
R
R/W
0
IDCMAX[0]
0
LSB DC DAC limit
R
R
7
SET_IMOD[0]
0
LSB of SET_IMOD DAC
register address = H0x09
R/W
R/W
4
MODINC[4]
0
MSB MOD DAC two’s
complement
R/W
R/W
3
MODINC[3]
0
R/W
R/W
2
MODINC[2]
0
R/W
R/W
1
MODINC[1]
0
R/W
R/W
0
MODINC[0]
0
LSB MOD DAC two’s
complement
R
R
7
SET_IDC[0]
0
LSB of SET_IDC DAC
register address = H0x08
R/W
R/W
4
DCINC[4]
0
MSB DC DAC two’s
complement increment/
decrement
R/W
R/W
3
DCINC[3]
0
R/W
R/W
2
DCINC[2]
0
R/W
R/W
1
DCINC[1]
0
R/W
R/W
0
DCINC[0]
0
LSB DC DAC two’s
complement increment/
decrement
R/W
R/W
7
MODECTRL[7]
0
MSB mode control
R/W
R/W
6
MODECTRL[6]
0
R/W
R/W
5
MODECTRL[5]
0
R/W
R/W
4
MODECTRL[4]
0
R/W
R/W
3
MODECTRL[3]
0
R/W
R/W
2
MODECTRL[2]
0
R/W
R/W
1
MODECTRL[1]
0
R/W
R/W
0
MODECTRL[0]
0
NOTES
MSB DC DAC limit
LSB mode control
�� Maxim Integrated Products 32
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Table 7. Register Summary (continued)
REGISTER
FUNCTION/
ADDRESS
Fault Mask
Register
Address =
H0x0F
Transmitter
Deemphasis
Control
Register
Address =
H0x10
Transmitter
Equalization
Control
Register
Address =
H0x11
REGISTER
NAME
FMSK
SET_TXDE
NORMAL
MODE
SETUP
MODE
BIT
NUMBER/
TYPE
BIT NAME
DEFAULT
VALUE
R
R/W
6
RESERVED
1
Must be kept at logic 1
R
R/W
5
FMSK[5]
1
MSB Tx fault mask
R
R/W
4
FMSK[4]
0
R
R/W
3
FMSK[3]
0
R
R/W
2
FMSK[2]
0
R
R/W
1
FMSK[1]
0
R
R/W
0
FMSK[0]
0
LSB Tx fault mask
R
R/W
6
SET_TXDE[6]
0
MSB Tx deemphasis
R
R/W
5
SET_TXDE[5]
0
R
R/W
4
SET_TXDE[4]
0
R
R/W
3
SET_TXDE[3]
0
R
R/W
2
SET_TXDE[2]
0
R
R/W
1
SET_TXDE[1]
1
R
R/W
0
SET_TXDE[0]
0
LSB Tx deemphasis
R
R/W
1
SET_TXEQ[1]
0
Tx equalization control
R
R/W
0
SET_TXEQ[0]
0
Tx equalization control
NOTES
SET_TXEQ
Layout Considerations
The data inputs and outputs are the most critical paths
for the MAX3948 and great care should be taken to
minimize discontinuities on these transmission lines. The
following are some suggestions for maximizing the performance of the IC:
• Use good high-frequency layout techniques and multilayer boards with an uninterrupted ground plane to
minimize EMI and crosstalk.
• The data inputs should be wired directly between the
module connector and IC without stubs.
• Maintain 100I differential transmission line impedance into the IC.
• The data transmission lines to the laser should be kept
as short as possible, and must be designed for 50I differential or 25I single-ended characteristic impedance.
• An uninterrupted ground plane should be positioned
beneath the high-speed I/Os.
• Ground path vias should be placed close to the IC
and the input/output interfaces to allow a return current path to the IC and the laser.
Refer to the schematic and board layers of MAX3948
Evaluation Kit for more information.
Exposed-Pad Package and
Thermal Considerations
The exposed pad on the 16-pin TQFN package provides
a very low-thermal resistance path for heat removal
from the IC. The pad is the only electrical ground on
the MAX3948 and must be soldered to the circuit board
ground for proper thermal and electrical performance.
Refer to Application Note 862: HFAN-08.1: Thermal
Considerations for QFN and Other Exposed-Paddle
Packages for additional information.
�� Maxim Integrated Products 33
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Typical Application Circuits
HOST BOARD
SFP+
CONNECTOR
SFP+ OPTICAL TRANSCEIVER
HOST FILTER
SUPPLY FILTER
VCC_TX
VCC
VCCT
ZDIFF = 100Ω
FR4 MICROSTRIP UP TO 12in
TOUTA
0.1µF
Z0 = 25Ω
TIN+
TIN0.1µF
MAX3948
TOUTC
Z0 = 25Ω
10G
DFB-TOSA
0.1µF
VOUT
VSEL
3-WIRE
INTERFACE
FAULT
SCL
SDA
CSEL
DISABLE
BMON
EP
SerDes
2.38V TO 3.46V
4.7kΩ TO 10kΩ
TX_FAULT
TX_DISABLE
RATE SELECT
DS1878
MODE_DEF1 (SCL)
MODE_DEF2 (SDA)
VCC_RX
HOST FILTER
ZDIFF = 100Ω
FR4 MICROSTRIP UP TO 12in
ADC
I2C
VCC (3.3V)
RMON1
SOFTWARE
3-WIRE
INTERFACE
SUPPLY FILTER
10G
LINEAR PIN ROSA
0.1µF
0.1µF
RMON2
�� Maxim Integrated Products 34
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Typical Application Circuits (continued)
VCC (+3.3V)
VCCT
25I
VSEL
TOUTA
MD
R1
MAX3948
DFB
25I
25I
DS4830
SCL
SCL
TOUTC
SDA
SDA
VOUT
CSEL
CSEL
VCCT
VSEL
SLAVE
I2C
MODE_DEF2 (SDA)
MODE_DEF1 (SCL)
13-BIT ADC
TOUTA
MD
R2
MAX3948
DFB
BIAS
MONITOR
SCL
25I
25I
TOUTC
SDA
VOUT
CSEL
VCCT
VSEL
RSSI
MONITOR
TOUTA
MD
R3
MAX3948
DFB
SCL
25I
25I
TOUTC
SDA
VOUT
CSEL
VCCT
VSEL
TOUTA
MD
MAX3948
DFB
SCL
25I
TOUTC
SDA
VOUT
CSEL
�� Maxim Integrated Products 35
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Ordering Information
PART
MAX3948ETE+
TEMP RANGE
PIN-PACKAGE
-40°C to +85°C
16 TQFN-EP*
Note: Parts are guaranteed by design and characterization to
operate over the -40°C to +95°C ambient temperature range
(TA) and are tested up to +85°C.
+Denotes a lead(Pb)-free/RoHS-compliant package.
*Exposed pad.
Chip Information
Package Information
For the latest package outline information and land patterns
(footprints), go to www.maxim-ic.com/packages. Note that a
“+”, “#”, or “-” in the package code indicates RoHS status only.
Package drawings may show a different suffix character, but
the drawing pertains to the package regardless of RoHS status.
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
16 TQFN-EP
T1633+5
21-0136
90-0032
PROCESS: SiGe BiPOLAR
�� Maxim Integrated Products 36
MAX3948
11.3Gbps, Low-Power, DC-Coupled Laser Driver
Revision History
REVISION
NUMBER
REVISION
DATE
0
6/11
DESCRIPTION
Initial release
PAGES
CHANGED
—
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. The parametric values (min and max limits) shown in the Electrical
Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
©
2011 Maxim Integrated Products
37
Maxim is a registered trademark of Maxim Integrated Products, Inc.

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