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LTC5100

sn5100 5100fs

Reducing Im_gain slows the settling time and increasing

Im_gain speeds the settling time. For example, with Im_gain

= 1, the residual loop error is cut by 1/8 with each servo

iteration. In this case it would take 35 servo iterations

(about 140ms) to settle to 1%. With Im_gain = 7, the

residual servo loop error is cut by 7/8 with each servo

iteration. In this case it would take only three servo

iterations (about 12ms) to settle to 1%, but the servo loop

will tend to “hunt” or oscillate at a low level with such a

high loop gain.

Temperature Compensation

The set point value for the modulation current, Im_set in

Figure 3 and Figure 4, changes with temperature to com-

pensate the temperature dependence of the laser diode’s

slope efficiency. Temperature measurements are supplied

either by an on-chip temperature sensor or by an external

microprocessor, according to the setting of Ext_temp_en.

The temperature compensated expression for Im_set is

given by:

Im_

•

Im_

•

Im_

•

–

set

nom

∆

+∆ +













(18)

Im_tc1 and Im_tc2 are the first and second order tempera-

ture coefficients for the modulation current.

LASER BIAS CURRENT CONTROL IN APC MODE

Figure 3 is a functional diagram of the LTC5100 operating

in automatic power control (APC) mode. In APC mode, the

LTC5100 servo controls the average optical power with

feedback from a monitor photodiode. Setting Apc_en = 1

selects this mode. In APC mode the monitor diode current

can be temperature compensated with first and second

order temperature coefficients.

Figure 9 shows an equivalent circuit for the MD pin and

Figure 23 shows details of the monitor diode circuit. The

Md_polarity bit selects whether the monitor diode sources

or sinks current from the MD pin. A programmable attenu-

ator and logarithmic amplifier permit a very wide range of

monitor diode currents spanning 4.25

µA to 2176µA (typi-

cal) with constant 0.2% set point resolution. The attenu-

ator divides the monitor diode current by 1, 4, 16 or 64

depending on the value of Imd_rng. Two bits called Imd_rng

control the attenuator setting, selecting a full scale current

range of 34, 136, 544 or 2176

µA typical. A 5kHz lowpass

filter provides antialiasing and limits noise. The logarithmic

amplifier compresses the dynamic range of the monitor

diode current and plays a role in maintaining constant and

predictable settling times regardless of the photodiode

characteristics.

Range Selection

Figure 24 depicts the current ranges for the monitor diode

current. The full-scale range of the monitor diode current

is 34

µA•4Imd_rng typicalwhereImd_rng=0,1,2or3.The

minimum operating level should be limited to 20% of full

scale to ensure adequate settling time of the optical power

output of the laser. The range should be selected so that

the monitor diode current stays within the guaranteed

current limits over temperature.

OPERATIO

Figure 23. Detail of the Monitor Photodiode Circuit

LOG AMP

10-BIT ADC

LTC5100

VDD

5kHz

LOWPASS

FILTER

ATTENUATOR

÷1, ÷4, ÷16, ÷64

POLARITY

CONTROL

Md_polarity

Md_polarity = 1

Md_polarity = 0

5100 F32

Imd_rng

Imd_adc

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