Safety Circuitry Current Monitors

The MAX3737 features monitors (MC_MON, BC_MON,

by mirroring a fraction of the currents and developing volt-

ages across external resistors connected to ground.

BC_MON result in a fault state. For example, connecting a

Multirate Laser Driver with Extinction

Ratio Control

10

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1

exceed the programmed threshold.

2

End-of-life (EOL) condition of the laser diode. The bias current and/or the photocurrent exceed the programmed threshold.

3

Laser cathode is grounded and photocurrent exceeds the programming threshold.

4

No feedback for the APC loop (broken interconnection, defective monitor photodiode), and the bias current exceeds the

programmed threshold.

Table 1. Typical Fault Conditions

PIN

CIRCUIT RESPONSE TO OVERVOLTAGE

CIRCUIT RESPONSE TO UNDERVOLTAGE

OR SHORT TO GROUND

TX_FAULT

Does not affect laser power.

Does not effect laser power.

TX_DISABLE

Modulation and bias currents are disabled.

Normal condition for circuit operation.

IN+

The optical average power increases and a fault occurs

responds by decreasing the bias current.

The optical average power decreases and the APC loop

responds by increasing the bias current. A fault state

IN-

The optical average power decreases and the APC loop

responds by increasing the bias current. A fault state

The optical average power increases and a fault occurs

responds by decreasing the bias current.

MD

This disables bias current. A fault state occurs.

The APC circuit responds by increasing bias current

until a fault is detected, then a fault* state occurs.

SHUTDOWN

Does not affect laser power. If the shutdown circuitry is

used, laser current is disabled.

Does not affect laser power.

BIAS

In this condition, laser forward voltage is 0V and no light

is emitted.

Fault state* occurs. If the shutdown circuitry is used,

laser current is disabled.

OUT+

The APC circuit responds by increasing the bias current

until a fault is detected, then a fault state* occurs.

Fault state* occurs. If the shutdown circuitry is used,

laser current is disabled.

OUT-

Does not affect laser power.

Does not affect laser power.

PC_MON

Fault state* occurs.

Does not affect laser power.

BC_MON

Fault state* occurs.

Does not affect laser power.

MC_MON

Fault state* occurs.

Does not affect laser power.

APCFILT1

voltage.

voltage.

APCFILT2

voltage.

voltage.

MODSET

Does not affect laser power.

Fault state* occurs.

APCSET

Does not affect laser power.

Fault state* occurs.

Table 2. Circuit Responses to Various Single-Point Faults

*

A fault state asserts the TX_FAULT pin, disables the modulation and bias currents, and asserts the SHUTDOWN pin.