ltc5100 Linear Technology Corporation, ltc5100 Datasheet - Page 23

• Manufacturer Part Number: ltc5100
• Description: 3.3v, 3.2gbps Vcsel Driver
• Manufacturer: Linear Technology Corporation

OPERATIO
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_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
Im_
set
Im_
U
nom
•
Im_
Im_
tc
LTC5100
2 2
tc
•
1 2
LOWPASS
FILTER
•
5kHz
–
Figure 23. Detail of the Monitor Photodiode Circuit
18
–
13
•
•
LOG AMP
T
2
T
ATTENUATOR
1, 4, 16, 64
1
Imd_rng
(18)
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 A to 2176 A (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 A 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 A • 4
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.
10-BIT ADC
Md_polarity
POLARITY
CONTROL
Imd_rng
typical where Imd_rng = 0, 1, 2 or 3. The
Imd_adc
MD
13
5100 F32
V
DD
Md_polarity = 1
Md_polarity = 0
LTC5100
sn5100 5100fs
23