LTC5100EUF datasheet(25/52 Pages) LINER

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LTC5100

sn5100 5100fs

Equation 20 shows that the loop gain is completely inde-

pendent of the slope efficiency and monitor diode re-

sponse. Consequently the servo dynamics and settling

time are independent of these highly varying quantities.

The Apc_gain quantity can be set to compensate for the

selected values of Is_rng and Im_rng as well as the

extinction ratio, termination resistance and laser dynamic

resistance.

The step response of the APC loop is:

Imd_adcn = Imd_set • [1 – (1 – A)n]

(22)

The step response given in Equation 22 has the familiar

exponential settling characteristic of a first order system.

The step response is shown in Figure 25 for A = 0.5. The

remaining error is reduced by one-half with each servo

iteration. In seven iterations, or about 28ms, the modula-

tion current settles to under 1% in this example. The

measured step response, including the modulation enve-

lope, is shown in the Typical Performance Characteristics.

Choosing A = 0.5 is nearly optimal because it results in

smooth, exponential settling. A = 1 will settle in about two

servo iterations or 8ms, but “hunting” or low level oscil-

lation will be seen in the laser bias current. A > 1 results

in overshoot and A > 2 results in sustained high level

oscillation.

microprocessor, according to the setting of Ext_temp_en.

The temperature compensated expression for Imd_set is

given by:

Im _

•

Im _

•

Im _

•

–

d set

d nom

dtc

∆

+∆ +













(23)

Imd_tc1 and Imd_tc2 are the first and second order

temperature coefficients for the monitor diode current.

Equation 23 applies to the digital representation of the

monitor diode current. Recall that Imd_set is the digital set

point for the

logarithm of the monitor diode current. This

fact has two important implications. First, the first order

temperature coefficient in Equation 23 (Imd_tc1) results

in an exponential change in the physical monitor diode

current with temperature. However, the monitor diode

temperature drift is usually very small, and the exponential

is well approximated as linear. Second, if Imd_tc2 = 0, the

relative temperature sensitivity of the physical current is

given by:

dIMD

dtc

d nom

•

ln( )•

•Im _

•

Im _

–

1024

(24)

where IMD is the physical monitor diode current in Amps.

Equation 24 shows that the temperature coefficient of the

physical current depends on the nominal monitor diode

current. For example, if Imd_nom = 512 and Imd_tc1 = 4,

the physical temperature compensation would be:

dIMD

ppm C

•

ln( )•

• •

–

512

1024

508

(25)

The effect of Imd_tc2 on the physical monitor diode

current has no simple physical interpretation. In most

cases it will be sufficient to set Imd_tc2 to zero and use the

first order temperature coefficient, Imd_tc1 to correct

monitor diode drift.

LASER BIAS CURRENT CONTROL IN CCC MODE

Figure 4 is a functional diagram of the LTC5100 operating

in constant current control (CCC) mode. In CCC mode, the

LTC5100 sets the laser bias current directly. Setting

Apc_en = 0 selects this mode. In CCC mode the laser bias

OPERATIO

Figure 25. Step Response of the Monitor

Diode Current for a Total Loop Gain of 0.5

Im_adc

5100 F25

345678

TIME (ms)

SERVO

ITERATIONS

Imd_set

Temperature Compensation

The set point value for the monitor diode current, Imd_set

in Figure 3, can be changed with temperature to compen-

sate the temperature dependence of the monitor diode

response. Temperature measurements are supplied either

by an on-chip temperature sensor or by an external

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