MAX3795ETG+ Maxim Integrated Products, MAX3795ETG+ Datasheet - Page 12

• Manufacturer Part Number: MAX3795ETG+
• Description: IC DRIVER VCSEL W/MON 24-TQFN
• Manufacturer: Maxim Integrated Products
• Type: Laser Diode Driverr

Specifications of MAX3795ETG+

• Data Rate: 4.25Gbps
• Number Of Channels: 1
• Voltage - Supply: 2.97 V ~ 3.63 V
• Current - Supply: 71mA
• Current - Modulation: 15mA
• Current - Bias: 15mA
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 24-TQFN Exposed Pad
• Mounting Type: Surface Mount
• Operating Supply Voltage: 3.3 V
• Minimum Operating Temperature: - 40 C
• Mounting Style: SMD/SMT
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

1Gbps to 4.25Gbps Multirate VCSEL Driver
with Diagnostic Monitors
current. For appropriate R
the Typical Operating Characteristics.
Select a communications-grade laser with a rise time of
90ps or better for 4.25Gbps applications. Use a high-
efficiency 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.
A resistor (R
and ground controls the modulation current out of the
MAX3795 to the VCSEL. The modulation current is
given by the following:
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. R
graph in the Typical Operating Characteristics shows the
current into a 50Ω load. Capacitance at the MODSET pin
should be ≤20pF.
The bias current output of the MAX3795 is controlled by
a resistor (R
and ground. In open-loop operation, BIASSET controls
the bias current level of the VCSEL. In closed-loop
operation (APC); the R
allowed bias current. The open-loop bias current is
given by the following:
The Bias Current vs. R
Operating Characteristics shows the current into a 50Ω
load. Capacitance at the BIASSET pin should be
≤20pF.
12
I
MOD
______________________________________________________________________________________
=







200
I
MODSET
BIASSET
BIAS
Programming Modulation Current
+
R
=
MODSET
1



) placed between the BIASSET pin
) placed between the MODSET pin
200
Programming Bias Current
BIASSET
+
BIASSET
R
TC
1 2
Design Procedure

 ×

BIASSET
.
and R
40
controls the maximum




graph in the Typical
×
MODSET

 ×




R
OUT
34
Select Laser
R
OUT
+
values, see
R
LOAD
MODSET



Compute the required modulation tempco from the
slope efficiency of the laser at T
higher temperature. Then select the value of R
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:
From the Typical Operating Characteristics, the value
of R
modulation temperature compensation is not desired,
short TC1 and TC2.
Program the average optical power by adjusting
R
desired monitor current to be maintained over tempera-
ture and lifetime. See the Monitor Diode Current vs.
R
and select the value of R
required current.
Figure 4. Simplified Input Structure
Programming Modulation-Current Tempco
PWRSET
PWRSET
IN+
IN-
TC
PACKAGE
, which offsets the tempco of the laser, is 9kΩ. If
MAX3795
Laser tempco
1nH
1nH
graph in the Typical Operating Characteristics,
. To select the resistance, determine the
0.5pF
0.5pF
Programming the APC Loop
V
V
=
CC
CC
PWRSET
SE
=
(
SE
50Ω
50Ω
−
25
2380
85
×
1kΩ
−
(
ppm C
85 25
that corresponds to the
SE
A
−
25
V
= +25°C and at a
/
CC
°
)
)
×
15pF
10
6
TC
from