+3.3V, 2.5Gbps Low-Power Laser Driver

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9

optical output ringing and duty-cycle distortion. Refer to

Maxim application note HFAN 02.0, Interfacing Maxim

Laser Drivers with Laser Diodes, for more information.

At the data rate of 2.5Gbps, any capacitive load at the

cathode of a laser diode degrades the optical output

performance. Because the BIAS output is directly con-

nected to the laser cathode, the parasitic capacitance

associated with this pin is minimized by using an induc-

tor to isolate the BIAS pin from the laser cathode.

Automatic Power Control (APC)

To maintain constant average optical power, the

MAX3273 incorporates an APC loop to compensate for

the changes in laser threshold current over temperature

and lifetime. A back-facet photodiode mounted in the

laser package is used to convert the optical power into

a photocurrent. The APC loop adjusts the laser bias

current so that the monitor current is matched to a ref-

the APC loop is determined by an external capacitor

associated with the APC loop-time constant, and to

guarantee loop stability, the recommended value for

When the APC loop is functioning, the maximum allow-

An APC failure flag (FAIL) is asserted low when the bias

current can no longer be adjusted to achieve the desired

average optical power.

APC closed-loop operation requires the user to set three

currents with external resistors connected between

ground and BIASMAX, MODSET, and APCSET (see

Figure 3). Detailed guidelines for these resistor settings

are described in the Design Procedure section.

Open-Loop Operation

If necessary, the MAX3273 is fully operational without

APC. To disable the APC loop, ground the APCFILT1

pin. In this case, the laser current is directly set by two

external resistors connected from ground to BIASMAX

and MODSET. See the Design Procedure section for

more details on open-loop operation.

Optional Data Input Latch

To minimize jitter in the input data, connect a synchro-

nous differential clock signal to the CLK+ and CLK-

inputs. When the LATCH control input is tied high, the

input data is retimed on the rising edge of CLK+. If

LATCH is tied low or left floating, the retiming function is

disabled and the input data is directly connected to the

output stage. When this latch function is not used, con-

Output Enable

The MAX3273 incorporates a TTL/CMOS input to

enable the output. When EN is low, the modulation and

bias outputs are enabled. When EN is high or floating,

both the bias and modulation currents are off. The typi-

cal enable time is 364ns, and the typical disable time is

27ns when the bias is operated open loop.

Slow-Start

For laser safety reasons, the MAX3273 incorporates a

slow-start circuit that provides a delay of 364ns for

enabling a laser diode.

APC Failure Monitor

The MAX3273 provides an APC failure monitor

(TTL/CMOS) to indicate an APC loop tracking failure.

FAIL is asserted low when the APC loop no longer can

regulate the bias current to maintain the desired moni-

tor diode current. FAIL asserts low when the APC loop

is disabled.

Short-Circuit Protection

The MAX3273 provides short-circuit protection for the

modulation and bias current sources. If BIASMAX,

MODSET, or APCSET is shorted to ground, the bias

and modulation output turns off.

Design Procedure

When designing a laser transmitter, the optical output

usually is expressed in terms of average power and

extinction ratio. Table 1 gives relationships helpful in

converting between the optical average power and the

modulation current. These relationships are valid if the

mark density and duty cycle of the optical waveform

are 50%.

Programming the Modulation Current

η), and

graph in the Typical Operating Characteristics and

required current at +25°C. The equation below provides

a derivation of the modulation current using Table 1.

I

Pr

r

MOD

AVE

e

e

=×

×

−

+

2

1

1

η