MOST Automotive Transmitter Using the MAX3905 and an

Design Note:
HFDN-37.0
Rev.2; 04/08
MOST Automotive Transmitter Using the MAX3905
and an Ulm Photonics VCSEL
Functional Diagrams
Pin Configurations appear at end of data sheet.
Functional Diagrams continued at end of data sheet.
UCSP is a trademark of Maxim Integrated Products, Inc.
LE
AVAILAB
MOST Automotive Transmitter Using the MAX3905
and an Ulm Photonics VCSEL
Media-Oriented Systems Transport (MOST®) is a
growing standard for automotive multimedia
networks. The low-cost, fiber-optic-based network
interfaces to audio and video devices to provide
integration for passenger information and
entertainment networks. This discussion focuses on
the MOST transmitter. The critical requirements for
a MOST transmitter are the optical modulation
amplitude (OMA) and average power (PAVE) at the
optical output, which are specified in the MOST
Advanced Optical Physical Layer Specification
Draft 0.9 (Reference 1). For more information on
these terms, see Maxim Application Note HFAN02.2, Optical Modulation Amplitude (OMA) and
Extinction Ratio.
Lasers and VCSELs (Vertical Cavity SurfaceEmitting Lasers) suffer from threshold current and
slope efficiency variation over temperature. The
laser/VCSEL driver needs to compensate for this
variation in order to produce a constant OMA and
average power over temperature. The most common
method for producing constant PAVE is an automatic
power control (APC) loop. In an APC loop, the
monitor diode (optically coupled to the laser)
generates a current proportional to the average
power, and the laser driver adjusts its output current
to keep the monitor diode current (and therefore the
average power) constant. However, due to space
and cost constraints, a closed-loop VCSEL driver
that meets MOST specifications is difficult to
achieve. The MAX3905 VCSEL driver uses an
innovative approach to control the properties of the
output current, without the cost and complexity of an
APC loop (see MAX3905 data sheet for details). By
setting the output current control inputs on the
MAX3905 to match the properties of the VCSEL,
the VCSEL OMA and average power will meet the
MOST specifications.
Application Note HFDN-37.0 (Rev.2; 04/08)
2 Ulm Photonics’ ULM850-01-TNS05AUT VCSEL
A VCSEL with properties that match well with the
output current profile of the MAX3905 is Ulm
Photonics’ ULM850-01-TN-S05AUT. This VCSEL
is suitable for automotive use (ambient temperature
range of -40ºC to 105ºC).
Ulm Photonics
recommends (Reference 2) the VCSEL current
profile shown in Figure 1 to achieve OMA and PAVE
which meet the MOST specification.
RECOMMENDED VCSEL CURRENT vs.
TEMPERATURE
16
14
12
CURRENT (mA) )
1 Introduction
10
CURRENT FOR
HIGH STATE
8
6
4
CURRENT FOR LOW STATE = 0
2
0
-50
0
50
100
150
o
TEMPERATURE ( C)
Figure 1. Recommended VCSEL Current
Profile, as Prescribed by Ulm Photonics.
3 Achieve Similar Current Profile
with MAX3905
While the MAX3905 cannot exactly mimic the
current profile given in Figure 1, it can produce a
similar profile and hence meet the MOST
Maxim Integrated
Page 2 of 6
specification. The following is a procedure for
selecting a similar profile.
Select Current for Low State
To achieve zero low-state current, a test mode is
enabled on the MAX3905. This test mode subtracts
the constant current in the DT0 region to produce
approximately zero low-state current in that region
(Figure 2). Connecting pad 4 to VCC enables this
test mode. This pad is labeled “N.C.” in the
MAX3905 Data Sheet. On the MAX3905 EV Kit,
rev A, this test mode is activated by shunting JP4.
BIAS CURRENT vs. JUNCTION
TEMPERATURE
6
4
TEST = VCC
DT0[1,2] = [OPEN, OPEN]
3.5
TC[1,2,3] = [GND, GND, GND]
3
TC[1,2,3] = [GND, GND, OPEN]
2.5
TC[1,2,3] = [OPEN, GND, OPEN]
2
TC[1,2,3] = [OPEN, OPEN, OPEN]
1.5
1
TC[1,2,3] = [GND, GND, OPEN]
DT0[1,2] = [OPEN, OPEN]
5
BIAS CURRENT (mA) )
BIAS CURRENT vs. JUNCTION
TEMPERATURE
BIAS CURRENT (mA) )
3.1
state current from Figure 3 to produce the total high
current. This setting is MOD[1, 2] = [VCC, VCC]
(Figure 4).
0.5
LOW[1,2] = [GND, GND]
LOW[1,2] = [OPEN, GND]
0
4
-40 -20
20
40
60
80
100 120 140
o
3
Figure 3. Bias Current vs. Temperature with
Test Mode Enabled.
2
LOW[1,2] = [GND, OPEN]
LOW[1,2] = [OPEN, OPEN]
1
MODULATION CURRENT vs. JUNCTION
TEMPERATURE
TEST MODE
8
0
-40
-20
0
20
40
60
80
100 120 140
Select Temperature Coefficient
A low temperature coefficient is selected to reduce
the effect of the low current rising above the
threshold current of the VCSEL (since the VCSEL is
meant to be modulated below threshold). For this
VCSEL, TC[1, 2, 3] = [OPEN, GND, OPEN] is the
correct setting (Figure 3).
MODULATION CURRENT (mA) )
Figure 2. Bias Current vs. Temperature,
Including Test Mode.
MOD[1,2] = [VCC, VCC]
7
o
JUNCTION TEMPERATURE ( C)
3.2
0
JUNCTION TEMPERATURE ( C)
6
MOD[1,2] = [OPEN, VCC]
5
4
3
2
MOD[1,2] = [VCC, OPEN]
1
MOD[1,2] = [OPEN, OPEN]
0
-40
-20
0
20
40
60
80
100 120 140
o
JUNCTION TEMPERATURE ( C)
3.3
Select Modulation Current
In order to emulate the current profile recommended
by Ulm Photonics, the maximum modulation current
is selected. This modulation is added to the lowApplication Note HFDN-37.0 (Rev.2; 04/08)
Figure 4. Modulation Current vs. Temperature.
Maxim Integrated
Page 3 of 6
3.4
Select DT0
OPTICAL MODULATION AMPLITUDE vs.
TEMPERATURE
The temperature DT0 is the center of the
temperature-stable low-state current region. This
should roughly correspond to the T0 of the VCSEL.
For this VCSEL, the correct setting is DT0[1, 2] =
[OPEN, OPEN].
3
2
MAX3905 + VCSEL OMA
1
3.5
Comparison of Recommended
Current and MAX3905 Output
Current
OMA (dB)
0
The output current profile of the MAX3905 is
compared with the Ulm Photonics recommended
profile in Figure 5.
-1
-2
-3
> 5dB
-4
-5
MOST OMA SPECIFICATION
-6
COMPARISON OF RECOMMENDED CURRENT
AND MAX3905 OUTPUT CURRENT
-7
-40
CURRENT (mA) )
RECOMMENDED
HIGH CURRENT
4.2
8
MAX3905 HIGH CURRENT
RECOMMENDED
LOW CURRENT
4
2
20
40
60
80
100
120
Figure 6. Optical Modulation Amplitude vs.
Temperature.
12
6
0
TEMPERATURE ( C)
14
10
-20
o
16
MAX3905 LOW CURRENT
PAVE Performance
The average power of the MAX3905 with the Ulm
Photonics VCSEL meets the MOST PAVE
specification (Figure 7).
This configuration
produces a relatively high average power with
margin for optical loss.
0
-40 -20
0
20
40
60
80
100 120 140
o
TEMPERATURE ( C)
Figure 5. Comparison of Ulm Photonics Recommended Current and MAX3905
Output Current.
4 Measured Performance
4.1
OMA Performance
The optical modulation amplitude of the MAX3905
with the Ulm Photonics VCSEL meets the MOST
OMA specification with a typical margin of greater
than 5dB (Figure 6). This gives the module designer
flexibility to account for VCSEL coupling loss and
VCSEL variation.
Application Note HFDN-37.0 (Rev.2; 04/08)
Maxim Integrated
Page 4 of 6
AVERAGE POWER vs. TEMPERATURE
3
Table 1. Wirebond Connections.
PAD
NUMBER
PAD NAME
CONNECTION
4
TEST (N.C.)
VCC
11
DT01
OPEN
12
DT02
OPEN
13
MOD2
VCC
-5
14
MOD1
VCC
-6
21
TC1
OPEN
22
TC2
GND
23
TC3
OPEN
24
LOW1
OPEN
25
LOW2
OPEN
2
1
PAVE (dBm)
0
MOST MAX PAVE SPECIFICATION
-1
-2
-3
MAX3905 + VCSEL PAVE
-4
-7
-40
-20
0
20
40
60
80
100
120
o
TEMPERATURE ( C)
Figure 7. Average Power vs. Temperature.
5 Wirebonding Connections and
Diagram
The MAX3905 chip topography is shown in Figure
8, with the TEST pad at pad 4. The wirebond
connections are shown in Table 1.
6 Conclusion
The performance of the MAX3905 VCSEL Driver
and the ULM850-01-TN-S05AUT VCSEL meets
the MOST Advanced Optical Physical Layer
specification with margin. The two devices provide
a complete physical layer solution for the MOST
automotive optical transmitter.
7 References
1. MOST Advanced Optical Physical Layer Draft
0.9.
2. Application Note: “VCSELs for Automotive
Applications,” Ulm Photonics, February 2005.
Application Note HFDN-37.0 (Rev.2; 04/08)
Maxim Integrated
Page 5 of 6
VEE
(PAD 26)
LOW2
(PAD 25)
LOW1
(PAD 24)
TC3
(PAD 23)
TC2
(PAD 22)
TC1
(PAD 21)
VEET
(PAD 1)
VEE
(PAD 20)
DIFF
(PAD 2)
3DB
(PAD 19)
IN_TTL
(PAD 3)
SQEN
(PAD 18)
TEST
(PAD 4)
OUT
(PAD 17)
60mils
1.52mm
OUT
(PAD 16)
N.C.
(PAD 5)
VCC
(PAD 15)
N.C.
(PAD 6)
MOD1
(PAD 14)
IN+
(PAD 7)
TEMPSENS
(PAD 8)
MOD2
(PAD 13)
IN(PAD 9)
LASER
TRIM
TARGET
VCCT
(PAD 10)
DT01
(PAD 11)
DT02
(PAD 12)
60mils
1.52mm
Figure 8. MAX3905 Chip Topography
MOST is a registered trademark of MOST Cooperation.
Application Note HFDN-37.0 (Rev.2; 04/08)
Maxim Integrated
Page 6 of 6