Since the IBIAS output is also connected directly to the

laser cathode, any parasitic capacitance associated

with this output must not be allowed to significantly load

the response. To resolve this problem, place an R-L

compensation network in series with the IBIAS output.

The additional high-frequency impedance of this net-

work will help maintain a high impedance at this node.

The recommended values for this resistance and

inductance are 100

Ω and 470nH, respectively.

Optimize the laser diode performance by placing a

bypass capacitor as close to the anode pin as possi-

ble. Use good high-frequency layout techniques and

multilayer boards with uninterrupted ground planes.

Input Termination Requirements

The MAX3667 data inputs are PECL compatible.

Standard PECL levels require 50

Ω terminations to

swing of 620mVp-p. The MAX3667’s inputs need not be

driven with standard PECL signals; as long as the com-

mon-mode voltage and differential swing is met, the

device will operate properly. 50

Ω input termination is

also not required, but is recommended for good high-

frequency termination.

Wire Bonding

For high current density and reliable operation, the

MAX3667 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 (100mm) square, and die thickness is 12

mils (300µm).

__________Applications Information

DC-Coupled Operation

and Output Current Limits

To improve headroom conditions for the MAX3667, AC

coupling of the modulation current is required at +3.3V

operation. At +5.0V operation, AC coupling is suggest-

ed but not required.

For AC-coupled operation, the total output current

is equal to IBIAS + IMOD / 2. For DC-coupled mod-

ulation currents, the total output current is equal to

IBIAS + IMOD.

Optimizing Performance

for Low Modulation Currents

The MAX3667’s dynamic range and headroom require-

ments are such that, in order to meet these spec-

ifications, low-current performance is compromised.

If continual operation at low modulation currents

(

≤ 20mA) is the intended application, the MAX3667’s

high-frequency performance can be improved with an

external pull-up resistor. By shunting the AC current

away form the laser diode, this technique reduces the

output swing without reducing the operating current of

the output transistor. Maintaining a higher modulation

operating current level preserves the high-frequency

performance of the output device. A suggested starting

point for the external pull-up resistor value is 100

Ω.

Modulation Currents Greater than 60mA

At +5.0V operation, the headroom conditions for the

MAX3667 are improved significantly. In this mode, it is

possible to achieve modulation currents greater than

60mA by floating PULLUP and driving the laser diode

directly (DC-coupled IMOD).

Laser Safety and IEC 825

Using the MAX3667 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. Each customer 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 support 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

10

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