The overshoot mirror sets the bias in the input buffer

stage (Figure 2). Reducing this current slows the input

stage and reduces overshoot in the modulation signal.

At the same time, the peak-to-peak output swing of the

input buffer stage is reduced. Careful design must be

used to ensure that the buffer stage can switch the out-

put stage completely into the nonlinear region. The

input swing required to completely switch the output

Current and Maximum Modulation Current vs. Minimum

Differential Input Signal Amplitude graphs in the

Typical

Operating Characteristics. For the output stage, the

width of the linear region is a function of the desired

modulation current. Increasing the modulation current

increases the linear region. Therefore, increases in the

modulation current require larger output levels from the

first stage.

Failure to ensure that the output stage switches com-

pletely results in a loss of modulation current (and

extinction ratio). In addition, if the modulation port does

not switch completely off, the modulation current will

contribute to the bias current, and may complicate

module assembly.

Automatic Power Control

The automatic power control (APC) feature allows an

optical transmitter to maintain constant power, despite

changes in laser efficiency with temperature or age. The

APC requires the use of a monitor photodiode.

The APC circuit incorporates the laser diode, the monitor

photodiode, the pin set current mirror, a transconduc-

tance amplifier, the bias set current mirror, and the laser

fail comparator (Figure 1). Light produced by the laser

diode generates an average current in the monitor pho-

todiode. This current flows into the MAX3263’s IPIN

input. The IPINSET current mirror draws current away

from the IPIN node. When the current into the IPIN node

equals the current drawn away by IPINSET, the node

ductance amplifier. When the monitor current exceeds

IPINSET, the IPIN node voltage will be forced higher. If

the monitor current decreases, the IPIN node voltage is

decreased. In either case, the voltage change is ampli-

fied by the transconductance amplifier, and results in a

feedback current at the IBIASFB node. Under normal

APC operation, IBIASFB is summed with IBIASSET, and

the laser bias level is adjusted to maintain constant out-

put power. This feedback process continues until the

monitor-diode current equals IPINSET.

If the monitor-diode current is sufficiently less than IPIN-

SET (i.e., the laser stops functioning), the voltage on the

IPIN node drops below 2.6V. This triggers the failout

comparator, which provides a TTL signal indicating laser

failure. The FAILOUT output asserts only if the monitor-

diode current is low, not in the reverse situation where

the monitor current exceeds IPINSET. FAILOUT is an

open-collector output that requires an external pull-up

resistor of 2.7k

The transconductance amplifier can source or sink cur-

rents up to approximately 1mA. Since the laser bias gen-

erator has a gain of approximately 40, the APC function

has a limit of approximately 40mA (up or down) from the

initial set point. To take full advantage of this adjustment

range, it may be prudent to program the laser bias cur-

rent slightly higher than required for normal operation.

mum rating of 75mA.

To maintain APC loop stability, a 0.1µF bypass capaci-

tor may be required across the photodiode. If the APC

function is not used, disconnect the IBIASFB pin.

Enable Inputs

The MAX3263 provides complementary enable inputs

(ENB+, ENB-). The laser is disabled by reducing the ref-

erence voltage outputs (VREF1, VREF2). Only one logic

state enables laser operation (Figure 3 and Table 1).

Single +5V, Fully Integrated,

155Mbps Laser Diode Driver

6

_______________________________________________________________________________________

OUTPUTS

280

Ω

280

Ω

9

Ω

400

Ω

9

Ω

INPUT BUFFER

OUTPUT STAGE

INPUTS

MAX3263

Figure 2. MAX3263 Modulation Driver (Simplified)