Datasheet

ManualsBrandsMAXIM INTEGRATED ManualsPMICsPMIC - laser controller Laser diode driver (LWL) TQFN 24 Surface-mount

+3.3V, 622Mbps SDH/SONET

Laser Driver with Current Monitors and APC

Maxim Integrated 9

MAX3669

Resistor graph in the

Typical Operating Character-

istics

and select the value of R

BIASMAX

that corre-

sponds to the required current at +25°C.

When using the MAX3669 in closed-loop operation, the

BIASMAX

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 Bias Current vs. Maximum

Bias Set Resistor graph in the

Typical Operating

Characteristics

and select the value of R

BIASMAX

that

corresponds to the end-of-life bias current at +85°C.

Programming the APC Loop

When the MAX3669’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 Monitor Diode Current vs. APC Set Resistor

graph in the

Typical Operating Characteristics

and

select the value of R

APCSET

that corresponds to the

required current.

Interfacing with the Laser Diode

To minimize optical output aberrations due to the laser

parasitic inductance, an RC shunt network may be

used (see

Typical Operating Circuit

). If R

represents

the laser diode resistance, the recommended total

resistance for R

+ R

is 10Ω. Starting values for coaxi-

al lasers are R

FILT

= 20Ω and C

FILT

= 5pF. R

FILT

and

FILT

should be experimentally adjusted to optimize

the output waveform. A bypass capacitor should also

be placed as close to the laser anode as possible for

best performance.

Pattern-Dependent Jitter (PDJ)

When transmitting NRZ data with long strings of consec-

utive identical digits (CIDs), LF droop can contribute to

PDJ. To minimize this PDJ, two external components

must be properly chosen: capacitor C

APC

, which domi-

nates the APC loop time constant, and AC-coupling

capacitor C

To filter out noise effects and guarantee loop stability,

the recommended value for C

APC

is 0.1µF. This results

in an APC loop bandwidth of 20kHz. Consequently, the

PDJ associated with an APC loop time constant can be

ignored.

The time constant associated with the DC blocking

capacitor on I

MOD

will have an effect on PDJ. It is

important that this time constant produce minimum

droop for long consecutive bit streams.

Referring to Figure 4, the droop resulting from long time

periods without transitions can be represented by the

following equation:

AC-coupling of I

MOD

results in a discharge level for τ

that is equal to P

AVG

. An overall droop of 6% relative to

p-p

equates to a 12% droop relative to P

AVG

. To

ensure a droop of less than 12% (6% relative to P

p-p

this equation can be solved for τ as follows:

If t

equals 80 consecutive unit intervals without a tran-

sition, the time constant associated with the DC block-

ing capacitor needs to be longer than:

≥ R

= 7.8 (80 bits) (1.6ns/bit) = 1.0µs

FILT

can be ignored for C

FILT

<< C

; therefore, the

estimated value of R

is:

= 20Ω ⏐⏐ (R

+ r

LASER

)

Assuming R

= 5Ω, and r

LASER

= 5Ω:

= 6.7Ω

with C

= 1µF, τ

= 6.7µs.

Input Termination Requirement

The MAX3669 data inputs are PECL compatible.

However, it is not necessary to drive the MAX3669 with

a standard PECL signal. As long as the specified com-

mon-mode voltage and differential voltage swings are

met, the MAX3669 will operate properly.

(1 - 0.12)

-t

τ = = 7.8t

(100% - DROOP) = e

-t

DROOP

AVG

p-p

τ = ∞

τ << τ

Figure 4. Droop