MAX3286CSLASHD datasheet(15/28 Pages) MAXIM

15 / 28 page

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3.0V to 5.5V, 1.25Gbps/2.5Gbps

LAN Laser Drivers

VCC

MAX3286/96

ONLY

SHDNDRV

CBIASDRV

BIASDRV

MON

LASER

POL

POWER-CONTROL

AMPLIFIER

IBIAS

FERRITE

BEAD

MONITOR

DIODE

1.7V

SMOOTH-

START

VCC

RDEG

RSET

POL

MAX3286

MAX3289

MAX3296

MAX3299

Figure 12. Common Anode with Photodiode

The relationship between laser bias current and RMON

is shown in Figure 11. The remaining discrete compo-

nents used with the common cathode without photodi-

ode configuration are as follows:

Q1 = general-purpose PNP, β >100, ft > 5MHz

B1 = ferrite bead (see the Bias Filter section)

M1 = general-purpose PMOS device (optional)

CBIASDRV = 0.1µF (typ)

Common Anode with Photodiode

In the common-anode configuration with photodiode, a

servo control loop is formed by an external NPN transis-

tor (Q1), the laser diode, the monitor diode, RSET, and

the power-control amplifier. The voltage at MD is stabi-

lized to 1.7V. The monitor photodiode current is set by

ID = VMD / RSET (Figure 12). Determine the desired mon-

itor current (ID), then select RSET = 1.7V / ID.

CBIASDRV and a degeneration resistor (RDEG) must be

connected to the bias transistor (in this case NPN) to

obtain the desired APC loop time constant. This

improves power-supply (and ground) noise rejection. A

capacitance of 0.1µF is sufficient to obtain time con-

stants of up to 5µs in most cases. The voltage across

RDEG should not be larger than 250mV at maximum bias

current.

The discrete components used with the common anode

with photodiode configuration are summarized as follows:

RSET = 1.7 / ID

CBIASDRV = 0.1µF (typ)

RDEG = 0.25 / IBIAS(MAX)

Q1 = general-purpose NPN, β > 100, ft > 5MHz

B1 = ferrite bead (see the Bias Filter section)

M1 = general-purpose PMOS (optional)

Programming POR Delay

A capacitor can be added to PORDLY to increase the

delay for which POR is asserted low (meaning that VCC

is within the operational range) when powering up the

part.

The delay is approximately:

See the Typical Operating Characteristics.

PORDLY

()

[]

−

14 10

100

0.1

10k

RMON (Ω)

100

RSET = 1kΩ

RMD = 5kΩ

Figure 11. Common Cathode without Photodiode Laser

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