Detailed Description

The MAX3740 contains a bias generator with automatic

power control (APC), safety circuit, and a laser modula-

tor with optional peaking compensation.

Bias Generator

Figure 1 shows the bias generator circuitry that contains

a power-control amplifier and smooth-start circuitry. An

internal PNP transistor provides DC laser current to bias

the laser in a light-emitting state. The APC circuitry

adjusts the laser-bias current to maintain average power

over temperature and changing laser properties. The

smooth-start circuitry prevents current spikes to the laser

during power-up or enable, ensuring compliance with

safety requirements and extending the life of the laser.

The MD input is connected to the cathode of a monitor

diode, which is used to sense laser power. The BIAS

output is connected to the anode of the laser through an

inductor or ferrite bead. The power-control amplifier dri-

ves a current amplifier to control the laser’s bias current.

During a fault condition, the bias current is disabled.

The PWRMON output provides a voltage proportional to

average laser power given by:

The BIASMON output provides a current proportional to

the laser bias current given by:

When APC is not used (no monitor diode, open-loop

configuration) connect the COMP and PWRMON pins

to GND. In this mode, the bias current is set by the

used, connect a 1.7k

Ω resistor between ground and

BIASSET to set the maximum bias current.

Safety Circuit

The safety circuit contains an input disable

(TX_DISABLE), a latched fault output (FAULT), and fault

detectors (Figure 2). This circuit monitors the operation

of the laser driver and forces a shutdown (disables

laser) if a fault is detected (Table 1). Table 2 contains

the circuit’s response to various single-point failures.

The transmit fault condition is latched until reset by a

be pulled high with a 4.7k

Ω to 10kΩ resistor.

3.2Gbps SFP VCSEL Driver with Diagnostic

Monitors

_______________________________________________________________________________________

9

COMP

BIASSET

MD

1.8V

REF

CURRENT

AMPLIFIER

POWER-

CONTROL

AMPLIFIER

ENABLE

200

Ω

PWRMON

BIAS GENERATOR

SMOOTH-

START

BIASMON

FERRITE

BEAD

BIAS

1.6V

2X

40

9

MAX3740

0.8V

Figure 1. Bias Generator

PIN

FAULT CONDITION

BIAS

BIASMON

PWRMON

Table 1. Fault Conditions