MAX3668ESLASHD datasheet(9/12 Pages) MAXIM

9 / 12 page

ratio is re = 6.6 (8.2dB), the operating temperature is

-40°C to +85°C, and the laser diode has the following

characteristics:

Wavelength:

λ = 1.3µm

Threshold Current:

ΙTH = 22mA at +25°C

Threshold Temperature

Coefficient:

βTH = 1.3%/°C

Laser to Monitor Transfer:

ρMON = 0.2A/W

Laser Slope Efficiency:

η = 0.05mW/mA

at +25°C

Determine RAPCSET

The desired monitor diode current is estimated by

IMD = PAVE

× ρMON = 200µA. The Monitor Diode Current

vs. APC Set Resistor graph in the Typical Operating

Characteristics shows that RAPCSET should be 6k

Ω.

Determine RMODSET

To achieve a minimum extinction ratio (re) of 6.6 over

temperature and lifetime, calculate the required extinc-

tion ratio at +25°C. Assuming re = 20, the peak-to-peak

optical power Pp-p = 1.81mW according to Table 1. The

required modulation current is 1.81(mW) / 0.05(mW/mA)

= 36.2mA. The Modulation Current vs. Modulation Set

Resistor graph in the (see Typical Operating Characteri-

stics) shows that RMODSET should be 5k

Ω.

Determine RBIASMAX

Calculate the maximum threshold current (ITH(MAX)) at

TA = +85°C and end of life. Assuming ITH(MAX) =

50mA, the maximum bias current should be:

IBIAS = ITH(MAX) + IMOD / 2

In this example, IBIAS = 68.1mA. The Bias Current vs.

Maximum Bias Set Resistor graph in the Typical

Operating Characteristics shows that RBIASMAX should

be 3k

Ω.

Modulation Current More than 50mA

To drive modulation currents greater than 50mA at

3.3V, external pull-up inductors (Figure 5) should be

used to DC-bias the modulation output at VCC. Such a

configuration isolates the laser forward voltage from the

output circuitry and allows the output at OUT+ to swing

above and below the supply voltage VCC.

At +5V power supply, the headroom voltage for the

MAX3668 is significantly improved. In this case, it is

possible to achieve a modulation current of more than

50mA (using resistor pull-ups as shown in the Typical

Operating Circuit). The MAX3668 can also be DC-coupled

to a laser diode when operating at +5V supply; the volt-

age at OUT+ should be

≥2.0V for proper operation.

Wire Bonding Die

For high current density and reliable operation, the

MAX3668 uses gold metalization. Make connections to

the die with gold wire only, using ball-bonding tech-

niques. Wedge bonding is not recommended. Die-pad

size is 4 mils (100µm) square, and die thickness is 12

mils (300µm) mils.

Layout Considerations

To minimize inductance, keep the connections between

the MAX3668 output pins and LD as close as possible.

Optimize the laser diode performance by placing a

bypass capacitor as close as possible to the laser

anode. Use good high-frequency layout techniques

and multilayer boards with uninterrupted ground planes

to minimize EMI and crosstalk.

Laser Safety and IEC 825

Using the MAX3668 laser driver alone does not ensure

that a transmitter design is compliant with IEC 825. The

entire transmitter circuit and component selections must

be considered. Customers must determine the level of

fault tolerance required by their application, recognizing

that Maxim products are not designed or authorized for

use as components in systems intended for surgical

implant into the body, for applications intended to sup-

port or sustain life, or for any other application where the

failure of a Maxim product could create a situation

where personal injury or death may occur.

+3.3V, 622Mbps SDH/SONET

Laser Driver with Automatic Power Control

_______________________________________________________________________________________

RFILT

CFILT

1000pF

1.0

µF5Ω

Ω

VCC

FERRITE

BEADS

FERRITE BEAD

BIAS

OUT+

OUT-

MAX3668

Figure 5. Output Termination for Maximum Modulation Current

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