MAX3273 Просмотр технического описания (PDF) - Maxim Integrated

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MAX3273

+3.3V, 2.5Gbps Low-Power Laser Driver

Maxim Integrated 

+3.3V, 2.5Gbps Low-Power Laser Driver

Programming the Bias Current

with APC Disabled

When using the MAX3273 in open-loop operation, the

bias current is determined by the RBIASMAX resistor. To

select this resistor, see the IBIASMAX vs. RBIASMAX graph

in the Typical Operating Characteristics and select the

value of RBIASMAX that corresponds to the required

IBIASMAX at +25°C. Ground the APCFILT1 pin for open-

loop operation.

Programming the Bias Current

with APC Enabled

When the MAX3273’s APC feature is used, program the

average optical power by adjusting the APCSET resis-

tor. To select this resistor, determine the desired moni-

tor current to be maintained over temperature and life.

See the IMD vs. RAPCSET graph in the Typical

Operating Characteristics and select the value of RAPC-

SET that corresponds to the required current.

When using the MAX3273 in closed-loop operation, the

RBIASMAX resistor sets the maximum bias current avail-

able to the laser diode over temperature and life. The

APC loop can subtract from this maximum value but

cannot add to it. See the IBIASMAX vs. RBIASMAX graph

in the Typical Operating Characteristics and select the

value of RBIASMAX that corresponds to the end-of-life

bias current at +85°C.

Interfacing with Laser Diodes

To minimize optical output aberrations caused by sig-

nal reflections at the electrical interface to the laser

diode, a series-damping resistor (RD) is required (see

the Typical Application Circuit). Additionally, the

MAX3273 outputs are optimized for a 25Ω load.

Therefore, the series combination of RD and RL (where

RL represents the laser-diode resistance) should equal

25Ω. Typical values for RD are 18Ω to 23Ω. For best

performance, a bypass capacitor (0.01µF typical)

should be placed as close as possible to the anode of

the laser diode. Depending on the exact characteristics

of the laser diode and PC board layout, a resistor (RP)

of 50Ω to 100Ω in parallel with pullup inductor LP1 can

be useful in damping overshoot and ringing in the opti-

cal output.

In some applications (depending on laser-diode para-

sitic inductance), an RC-shunt network between the

laser cathode and ground helps minimize optical out-

put aberrations. Starting values for most coaxial lasers

are R = 75Ω in series with C = 3.3pF. These values

should be experimentally adjusted until the optical out-

put waveform is optimized.

Pattern-Dependent Jitter

When transmitting NRZ data with long strings of con-

secutive identical digits (CIDs), LF droop can occur

and contribute to pattern-dependent jitter (PDJ). To

minimize this PDJ, three external components must be

properly chosen: capacitor (CAPCFILT), which domi-

nates the APC loop time constant; pullup inductor (LP);

and AC-coupling capacitor (CD).

To filter out noise effects and guarantee loop stability,

the recommended value for CAPCFILT is 0.01µF. This

results in an APC loop bandwidth of 100kHz or a time

constant of 15µs. As a result, the PDJ associated with

an APC loop time constant can be ignored.

The time constant associated with the output pullup

inductor (LP ≈ LP2) and the AC-coupling capacitor (CD)

affects the PDJ. For such a second-order network, the

PDJ is dominated by LP because of the low frequency

cutoff. For a data rate of 2.5Gbps, the recommended

value for CD is 0.056µF. During the maximum CID peri-

od, limit the peak voltage droop to less than 12% of the

average (6% of the amplitude). The time constant can

be estimated by:

−t

12% = 1 − e τLP

τLP = 7.8t

If τLP = LP / 25Ω, and t = 100UI ≈ 40ns, then LP =

7.8µH. To reduce the physical size of this element (LP),

use of SMD ferrite beads is recommended (Figure 2).

To achieve even greater immunity to droop, use an

optional third inductor (33µH, LP3 in Figure 2).

Input Termination Requirement

The MAX3273 data and clock inputs are CML compati-

ble. However, it is not necessary to drive the IC with a

standard CML signal. As long as the specified differen-

tial voltage swings are met, the MAX3273 operates

properly.

Calculating Power Consumption

The junction temperature of the MAX3273 dice must be

kept below +150°C at all times. The total power dissipa-

tion of the MAX3273 can be estimated by the following:

P = VCC × ICC + (VCC - Vf) ✕ IBIAS + IMOD ✕

(VCC - 25 ✕ IMOD / 2)

where IBIAS is the maximum bias current set by

RBIASMAX, IMOD is the modulation current, and Vf is the

typical laser forward voltage.

Junction temperature = P(W) ✕ 37 (°C/W)

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