Laser Current Requirements

Bias and modulation current requirements can be

determined from the laser threshold current and slope

efficiency. The modulation and bias currents under a

single operating condition are:

• For DC-coupled laser diodes:

• For AC-coupled laser diodes:

Given the desired parameters for operation of the laser

diode, the programming of the MAX3865 is explained

in the following text.

Current Limits

To keep the modulation current in compliance with the

programmed value, the following constraint on the total

modulation current must be made:

DC-Coupled Laser Diodes:

≥ 1.8V

(1.2V typ)

RL—Laser diode bias-point resistance (5

Ω typ)

RD—Series matching resistor (15

Ω typ)

AC-Coupled Laser Diodes:

To allow larger modulation current, the laser can be

AC-coupled to the MAX3865 as shown in the Typical

Application Circuit. In this configuration, a constant cur-

for compliance in the AC-coupled circuit is as follows:

The AC-coupling capacitor and bias inductor form a

second-order high-pass circuit. Pattern-dependent jitter

results from the low-frequency cutoff of this high-pass

circuit. To prevent ringing:

For deviation from 50% duty cycle or for runs of con-

secutive identical digits (CID), the low-frequency corner

formed by the LC circuit must be low enough to limit

the droop.

If droop = 6.7%, number_CID = 100 and data_rate =

increased in value to reduce droop without ringing.

Programming the Maximum Bias Current

In AMC (or APC) mode, the bias current needs a limit if

allowed bias current. The bias current is proportional to

lator maintains the band-gap voltage of 1.2V across the

gramming resistor as follows:

BIASMAX pin may set the current maximum:

When the AMC or APC loop is enabled, the actual bias

current is reduced below the maximum value to main-

tain a constant average current from the monitor diode.

With closed-loop control, the bias current will be deter-

mined by the transfer function of the monitor diode to

laser-diode current. For example, if the transfer function

Programming the Average Monitor

Diode-Current Set Point

The APCSET pin controls the set point for the average

monitor diode current when in AMC or APC mode. The

APCSET current is externally established in the same

manner as the BIASMAX pin. The average monitor

as follows:

Alternatively, a current DAC at the APCSET pin can set

the monitor diode current by:

average I

V

Rk

MD

APCSET

_

.

=×

+Ω

5

12

2

I

V

Rk

BIASMAX

BIASMAX

=×

+Ω

480

12

2

.

Droop

Number CID

Data Rate

L

C

P

=

××

_

_

RR

L

C

DL

P

+

2

V

I

RR

V

CC

MOD

DL

−×

+

2

18

.

II

I

BIAS

TH

MOD

>+

2

I

Pr

r

MOD

AVG

e

e

=×

× −

+

2

1

1

η

2.5Gbps Laser Driver with Automatic

Modulation Control

_______________________________________________________________________________________

9