MAX3646ETG Maxim Integrated Products, MAX3646ETG Datasheet - Page 11

• Manufacturer Part Number: MAX3646ETG
• Description: Laser Driver 155Mbps to 622Mbps S FF-SFP Laser Driver
• Manufacturer: Maxim Integrated Products
• Type: Laser Driverr

Specifications of MAX3646ETG

• Data Rate: 155 Mbps to 622 Mbps
• Operating Supply Voltage: 3.3 V
• Supply Current: 47 mA
• Maximum Operating Temperature: + 85 C
• Package / Case: TQFN-24
• Maximum Power Dissipation: 1805 mW
• Minimum Operating Temperature: - 40 C
• Mounting Style: SMD/SMT

Table 3. Optical Power Relations
Note: Assuming a 50% average input duty cycle and mark
density.
The safety circuitry contains a disable input
(TX_DISABLE), a latched fault output (TX_FAULT), and
fault detectors (Figure 5). This circuitry monitors the
operation of the laser driver and forces a shutdown if a
fault is detected (Table 1). The TX_FAULT pin should
be pulled high with a 4.7kΩ to 10kΩ resistor to V
required by the SFP MSA. A single-point fault can be a
short to V
response to various single-point failure. The transmit
fault condition is latched until reset by a toggle or
TX_DISABLE or V
laser diode shutdown through the optional shutdown
circuitry as shown in the Typical Application Circuit .
This shutdown transistor prevents a single-point fault at
the laser from creating an unsafe condition.
The MAX3646 features monitors (BC_MON, PC_MON)
for bias current (I
monitors are realized by mirroring a fraction of the cur-
rents and developing voltages across external resistors
connected to ground. Voltages greater than V
PC_MON or BC_MON result in a fault state. For exam-
ple, connecting a 100Ω resistor to ground at each mon-
itor output gives the following relationships:
External sense resistors can be used for high-accuracy
measurement of bias and photodiode currents. On-chip
Average power
Extinction ratio
Optical power of a one
Optical power of a zero
Optical amplitude
Laser slope efficiency
Modulation current
Threshold current
Bias current
(AC-coupled)
Laser to monitor
transfer
PARAMETER
CC
V
Safety Circuitry Current Monitors
BC_MON
or GND. See Table 2 to view the circuit
Laser Driver with Extinction Ratio Control
V
PC_MON
CC
______________________________________________________________________________________
BIAS
. The laser driver offers redundant
= (I
SYMBOL
) and photocurrent (I
ρ
P
I
I
P
MOD
BIAS
MON
I
AVG
P
P
BIAS
= I
r
P-P
TH
η
e
1
0
MD
/ 82) x 100Ω
x 100Ω
P
1
I
P
P
BIAS
Safety Circuitry
= 2P
0
AVG
η = P
I
= 2P
MOD
P
P
RELATION
r
P-P
I
≥ I
e
MD
AVG
0
= (P
= P
at I ≥ I
AVG
TH
P-P
= P
= P
/ P
155Mbps to 622Mbps SFF/SFP
x r
0
1
+ I
/ I
1
P-P
AVG
+ P
/ P
/ (r
e
MD
MOD
- P
MOD
TH
/ (r
0
e
1
/ η
REF
0
). The
) / 2
+ 1)
CC
e
/ 2
+ 1)
as
at
isolation resistors are included to reduce the number of
components needed to implement this function.
When designing a laser transmitter, the optical output is
usually expressed in terms of average power and
extinction ratio. Table 3 shows relationships that are
helpful in converting between the optical average
power and the modulation current. These relationships
are valid if the mark density and duty cycle of the opti-
cal waveform are 50%.
For a desired laser average optical power (P
optical extinction ratio (r
lation currents can be calculated using the equations in
Table 3. Proper setting of these currents requires
knowledge of the laser to monitor transfer (ρ
slope efficiency (η).
The MAX3646 operates in APC mode at all times. The
bias current is automatically set so average laser power
is determined by the APCSET resistor:
The APCSET pin controls the set point for the monitor
diode current. An internal current regulator establishes
the APCSET current in the same manner as the
MODSET pin. See the I
Typical Operating Characteristics and select the value
of R
+25°C:
The laser driver automatically adjusts the bias to main-
tain the constant average power. For DC-coupled laser
diodes:
Determine the modulation current form the laser slope
efficiency:
The modulation current of the MAX3646 consists of a
static modulation current (I
al to I
The portion of I
an internal current regulator, which maintains the refer-
ence voltage of V
resistor. See the I
Typical Operating Characteristics and select the value
APCSET
BIAS
I
MOD
, and a current proportional to temperature.
that corresponds to the required current at
I
Programming the Monitor-Diode
= 2 x P
MD
MOD
I
AVG
REF
P
Programming the Modulation
= 1/2 x V
AVG
Current with Compensation
MOD
set by MODSET is established by
= I
AVG
across the external programming
e
= I
BIAS
MD
), the required bias and modu-
/ η x (r
vs. R
MD
MODS
Design Procedure
REF
vs. R
+ I
/ ρ
/ R
MOD
MODSET
e
), a current proportion-
MON
Current Set Point
APCSET
ACPSET
- 1)/(r
/ 2
e+
graph in the
graph in the
+ 1)
MON
AVG
) and
) and
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