MAX3643(2005) データシートの表示（PDF） - Maxim Integrated

部品番号

コンポーネント説明

メーカー

MAX3643
(Rev.:2005)

Burst-Mode Laser Driver

Maxim Integrated 

Burst-Mode Laser Driver

Detailed Description

The MAX3643 laser driver includes a bias current gen-

erator, bias current monitor, modulation current genera-

tor, laser drive outputs, and monitor diode sample and

hold. LVPECL-compatible inputs are provided for both

high-speed data and burst enable. The high-speed

burst-enable input signal is replicated on an LVCMOS

output for use by the controller.

Laser Diode Modulation and

Bias Current Generators

Laser diode modulation current amplitude is controlled

by the current out of the MODSET pin, and bias current

by the current out of the BIASSET pin, according to:

IMOD = IMODSET x 88

IBIAS = IBIASSET x 88

A voltage source and two op amps are provided to

enable IMODSET and IBIASSET to be set using either a

resistor to ground or a current digital-to-analog convert-

er (DAC). The high-impedance op amp reference input

can be externally controlled, so that the modulation and

bias currents can also be set using voltage DACs.

Laser Diode Modulation and

Bias Current Limiter

Typical laser diodes have an absolute maximum rating

of 150mA. To reduce the possibility of laser damage,

the modulation current and bias current are shut off if

the sum IMOD + IBIAS attempts to exceed the limit set

by RIMAX; see the Typical Operating Characteristics.

Bias Current Monitor

The laser diode bias current can be monitored by measur-

ing the voltage across an external load resistor connected

from BCMON to ground. For example, a 1kΩ resistor from

BCMON to ground gives the following relationship:

VBCMON = IBIAS × GBSM × 1kΩ

The voltage at BCMON must be below 1.4V for proper

operation.

Output Drivers

The modulation current ranges from 10mA to 85mA, as

set by the current through MODSET. The laser modula-

tion current output OUT+ is optimized to drive a 15Ω

load, and must be DC-coupled. A damping resistor,

RD, provides impedance matching to the laser diode.

The combined value of the series damping resistor and

the laser diode equivalent series resistance should be

close to 15Ω. An RC shunt network, RCOMP/CCOMP,

should also be provided to reduce optical output aber-

rations and duty-cycle distortion. The values of RCOMP

and CCOMP can be adjusted to match the laser and PC

board layout characteristics for optimal optical eye per-

formance. The OUT- pin is normally connected through

a 15Ω resistor to VCC or through a switching diode and

series resistor to VCC. With some laser diodes, the use

of a switching diode at OUT can improve the optical

output eye by better matching the laser characteristics.

The bias current ranges from 1mA to 70mA, as set by

the current through BIASSET. Current in the BIAS out-

put also switches at high speed when bursting; there-

fore, the BIAS+ pin should be connected directly

through a resistor, equal to RD as determined above, to

the laser cathode. The BIAS- pin must also be connect-

ed through a 15Ω resistor or through a switching diode

and series resistor to VCC.

When the BEN input is high, the laser driver sinks bias

and modulation current according to the settings at

MODSET and BIASSET. When the BEN input is low, the

BIAS+ and OUT+ currents both shut off within 2ns. Note

that when BEN is low, the bias current is shunted

through the BIAS- output and the modulation current

through the OUT- output.

Monitor Diode Sample and Hold

Laser monitor diode current is only generated when

there is an optical output (BEN is active). When BEN is

inactive, the monitor current is zero, reflecting the fact

that the laser is off. A sample-and-hold circuit, trig-

gered by the state of the BEN input, is provided in the

MAX3643. During the burst-enable active period, the

voltage present at MDIN is stored on an internal sam-

ple-and-hold capacitor; and during the burst-enable

inactive period, that voltage is output on MDOUT; see

the timing diagram in Figure 2.

While the internal sample-and-hold is sampling (BEN

active), MDOUT voltage takes a 1.2V reference level.

_______________________________________________________________________________________ 9

Share Link: