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MAX3643
(Rev.:2005)

Burst-Mode Laser Driver

Maxim Integrated 

Burst-Mode Laser Driver

This approach can also be used for a conventional volt-

age DAC output, if desired. In all cases, the voltage at

MODSET must be kept ≤ 1.4V, which limits the range of

acceptable values for RMODSET depending on the maxi-

mum modulation current.

Laser diode bias current is set in the same manner as

modulation current.

LVPECL Data/Burst-Enable Inputs

The MAX3643 data and BEN inputs are biased with an

on-chip, high-impedance network. When DC-coupled,

the MAX3643 operates properly with signals that meet

the EC table input-swing and common-mode require-

ments, including LVPECL and most CML.

See Figure 3 for a termination network that can be used

to connect the data and BEN inputs to LVPECL data out-

puts. Other termination networks may also be used, as

long as both the input swing and common limits are met.

Sample-and-Hold Operation

When the MAX3643 internal sample-and-hold is not

required, the MDIN pin should be connected to ground

and the MDOUT pin unconnected. If the internal sample-

and-hold is required, then it is necessary to ensure that

the time constant resulting from the monitor diode load

resistance and the total load capacitance is compatible

with the desired minimum burst interval. It is also neces-

sary to make certain that the load at MDOUT does not

exceed the capability of the MDOUT pin.

Because the voltage at MDIN is not reflected to MDOUT

until after the end of the laser burst, systems using the

internal sample-and-hold alone cannot support continu-

ous mode operation, often a required feature for module

calibration. In this case, the voltage at MDIN can also

be connected directly to a mux input as shown in the

Typical Applications Circuit. As long as the total capaci-

tance (including monitor diode intrinsic capacitance,

MDIN capacitance, mux off-capacitance, and wiring

parasitics) is less than 50pF, and the monitor diode

load resistor is less than 2kΩ, then the sample-and-hold

captures a 576ns minimum burst. The MAX3643 typical

MDIN capacitance is 5pF, typical monitor diode maxi-

mum capacitance is 25pF, and the typical capacitance

of a mux input in the off-state is 3pF to 5pF. When the

VCC

130Ω

LVPECL

ZO = 50Ω

ZO = 50Ω

82Ω

130Ω

82Ω

MAX3643

IN+/BEN+

IN-/BEN-

Figure 3. LVPECL High-Speed Inputs

VCMBEN + (100mV to 800mV)

VCMBEN

VCMBEN - (100mV to 800mV)

3.3V

R1 = 1.65kΩ

VCMBEN = +2.0V

R2 = 2.54kΩ

IN+

IN-

BEN+

BEN-

MAX3643

Figure 4. Single-Ended Biasing for Burst Enable

mux is in the on-state, the capacitance at the input is

typically 10pF to 20pF.

If the minimum burst duration is longer than 576ns, it

may be useful to connect an external capacitor in par-

allel with the monitor diode load to limit the effects of

the data pattern on the monitor diode output.

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