+3.3V, 622Mbps SDH/SONET

Laser Driver with Automatic Power Control

6

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MAX3668

DATA+

OUT+

OUT-

DATA-

100k

ENABLE

APCSET

CAPC

BIASMAX

MD

MODSET

FAIL

BIAS

165X

FAILURE

DETECTOR

40X

5X

Figure 3. Functional Diagram

Detailed Description

The MAX3668 laser driver consists of two main parts: a

high-speed modulation driver and a laser-biasing block

with Automatic Power Control (APC). The circuit is opti-

mized for low-voltage (+3.3V) operation.

The output stage is composed of a high-speed differential

pair and a programmable modulation current source.

Since the modulation output drives a maximum current

of 75mA into the laser with a 230ps edge speed, large

transient voltage spikes can be generated due to the

parasitic inductance. These transients and the laser for-

ward voltage leave insufficient headroom for the proper

operation of the laser driver if the modulation output is

DC-coupled to the laser diode. To solve this problem,

the MAX3668’s modulation output is designed to be

AC-coupled to the cathode of a laser diode. A simpli-

fied functional diagram is shown in Figure 3.

The MAX3668 modulation output is optimized for driv-

ing a 20

Ω 10Ω load; the minimum required voltage at

OUT+ is 2.0V. Modulation current swings of 75mA are

possible. To interface with the laser diode, a damping

RC shunt network may be used to compensate for the

laser-diode parasitic inductance, thereby improving the

optical output aberrations and duty-cycle distortion.

At a 622Mbps data rate, any capacitive load at the cath-

ode of a laser diode degrades the optical output perfor-

mance. Since the BIAS output is directly connected to the

laser cathode, minimize the parasitic capacitance associ-

ated with this pin by using an inductor to isolate the BIAS

pin from the laser cathode.