Datasheet

Modulation-Current Generator
The modulation-current generator circuit provides con-
trol of the modulation-current amplitude. This amplitude
is determined by the voltage at the MODSET pin and
external resistor R
MODSET.
Do not connect bypass capacitors at the MODSET pin.
Capacitance at this pin increases high-frequency out-
put noise. The MON pin provides an optional modula-
tion-current monitor. The current sourced from the MON
pin is 1/96 of the modulation current. If used, the pin
should be connected to V
EE
through a resistor. The
resistance must be chosen so the voltage on MON
does not exceed 1.1V. If not used, leave MON open.
Prebias Current Generator
A prebias voltage (V
PREBIAS
) can be applied to the
LED to improve switching speed. The prebias current
generator creates a current that flows through the 78Ω
prebias resistor in the output stage, creating a prebias
voltage. The prebias voltage can be adjusted by selec-
tively connecting pins PB1, PB2, and PB3 to V
EE
. Table 1
describes the functions of PB1, PB2, and PB3.
Output Current Drivers
The modulation-current reference is switched and
amplified by the output stages.
LED package lead inductance causes ringing and
overshoot, which can be compensated with an RC filter
network. The MAX3967A includes 35Ω and 12pF of
MAX3967A
270Mbps SFP LED Driver
_______________________________________________________________________________________ 7
Figure 2. Functional Diagram
12pF
MODULATION-
CURRENT
GENERATOR
REFERENCE-VOLTAGE GENERATOR
PREBIAS-CURRENT
GENERATOR
COMPLEMENTARY
OUTPUT
24X
MAIN
OUTPUT
24X
V
TEMP
TCMIN
PB1 PB2 PB3
TCNOM
TC
MODSET MON
1.2V
OUT-
OUT+
IN-
IN+
INPUT
BUFFER
GAIN 4X
MAX3967A
R
MODSET
V
CCOUT
TX_DISABLE
50Ω
35Ω
5Ω
V
CC
R
PREBIAS
78Ω
10
0