Datasheet

MAX3967A
internal compensation. The compensation network can
be optimized by adding additional components
between V
CCOUT
and OUT+.
The MAX3967A includes a complementary output dri-
ver, which is switched 180° out of phase with the main
output. This configuration helps to maintain constant
current flow from the voltage supply, reducing noise
and EMI. A large diode and a 5Ω resistor are connect-
ed in series with the negative output (OUT-) to emulate
the LED load at OUT+.
Peaking Current
The prebias resistor provides peaking current to improve
the LED switching speed. The peaking magnitude is
given by the following equation:
The peaking amplitude is equal for rising and falling
data transitions.
Design Procedure
Select an LED
For best performance, select a high-efficiency, low-
inductance LED. LED inductance causes large voltage
swings and ringing.
Program the Modulation-Current Tempco
Select a modulation-current tempco that provides near-
ly constant LED output power as temperature varies.
For the minimum tempco, connect TCMIN to the TC pin.
For a tempco of approximately 3600ppm/°C, connect
TC to TCNOM and leave TCMIN unconnected. For the
maximum tempco, leave TCMIN, TCNOM, and TC
unconnected.
See the Modulation Current Tempco vs. R
TC
graph in
the Typical Operating Characteristics to program a cus-
tom tempco. From the graph, determine the appropriate
resistor and connect it between TCMIN and TC.
For example, if an LED requires a 5000ppm/°C tempco,
choose R
TC
of 8.3kΩ.
Program the Modulation Current
Determine the required modulation current at T
A
= +25°C.
Then select the appropriate value of R
MODSET
from the
Modulation Current vs. R
MODSET
graph in the Typical
Operating Characteristics.
For example, to program 75mA modulation current, the
graph indicates an R
MODSET
value of 750Ω for maxi-
mum tempco (12,000ppm/°C) and 1kΩ for nominal
tempco (3600ppm/°C). By interpolation, choose an
R
MODSET
of 792Ω for a tempco of 5000ppm/°C.
Program Prebias Voltage
Determine the LED prebias voltage that produces an
acceptable trade-off between peaking current and extinc-
tion ratio. See Table 1 for PB1, PB2, and PB3 settings.
Layout Considerations
For optimum performance, total load inductance should
not exceed 10nH. Load inductance includes LED
inductance, LED package lead inductance, and circuit-
board traces. Keep the connections between the
MAX3967A OUT pins and the LED as short as possible
to minimize inductance.
Chip-and-wire (hybrid) technology reduces package
inductance significantly, and provides the best possible
performance.
Use good high-frequency layout techniques and a
multilayer board with an uninterrupted ground plane.
Power supplies should be capacitively bypassed to the
ground plane with surface-mount capacitors located
near the power-supply pins.
I
PEAK
LED PREBIAS
V-V
78
=
Ω
270Mbps SFP LED Driver
8 _______________________________________________________________________________________
Table 1. LED Prebias Voltage
PB1 PB2 PB3 PREBIAS (V)
Open Open Open 0.400
V
EE
Open Open 0.475
Open V
EE
Open 0.550
V
EE
V
EE
Open 0.625
Open Open V
EE
0.700
V
EE
Open V
EE
0.775
Open V
EE
V
EE
0.850
V
EE
V
EE
V
EE
0.925