LTC5100EUF#PBF Linear Technology, LTC5100EUF#PBF Datasheet - Page 25

• Manufacturer Part Number: LTC5100EUF#PBF
• Description: IC DRIVER VCSEL 3.2GBPS 16QFN
• Manufacturer: Linear Technology
• Type: Laser Diode Driverr

Specifications of LTC5100EUF#PBF

• Data Rate: 3.2Gbps
• Number Of Channels: 1
• Voltage - Supply: 3.135 V ~ 3.465 V
• Current - Supply: 54mA
• Current - Modulation: 12mA
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 16-QFN
• Mounting Type: Surface Mount
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

OPERATIO
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
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:
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.
Imd_adc
Im_adc
0
Figure 25. Step Response of the Monitor
Diode Current for a Total Loop Gain of 0.5
n
4
= Imd_set • [1 – (1 – A)
1
2
8
U
12
3
16
4
20
5
24
6
28
Imd_set
7
n
]
32
8
SERVO
ITERATIONS
TIME (ms)
5100 F25
(22)
microprocessor, according to the setting of Ext_temp_en.
The temperature compensated expression for Imd_set is
given by:
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:
where I
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:
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
dI
dI
dT
dT
Im _
Im _
MD
MD
d set
d nom
•
•
MD
I
I
MD
MD
1
1
is the physical monitor diode current in Amps.
•
ln( )•
ln( )•
Im _
8 2
8 2
Im _
d tc
d tc
–
–
13
13
2 2
•
• •
1 2
•Im _
4
•
–
18
–
d tc
1024
13
512
•
•
T
1
2
•
T
Im _
508
LTC5100
1024
1
d nom
ppm C
/
sn5100 5100fs
25
(23)
(24)
(25)