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LDTC0520 / LDTC1020 LASER DIODE AND TEMPERATURE CONTROLLER
THEORY OF OPERATION
The LDTC Laser Diode Driver and Temperature Controller
combines Wavelength’s proprietary FL500 and highly
stable WTC3243 in one compact module.
The LDTC0520 employs a single FL500 laser diode control
chip; the LDTC1020 parallels two FL500 chips. The current
source continually monitors the actual output current,
compares it to the setpoint, and adjusts the current if there
is a difference between the two signals.
Itmay be usefultorememberthat you do notdirectlysetthe
drive current setpoint; instead, you adjust a voltage signal
that represents the output current. The setpoint voltage is
controlled by the onboard trimpot or by an external input.
As current is driven through the load, there is a voltage
drop across the load because of the impedance. As the
current increases, the voltage drop may increase to the
point that it reaches the Compliance Voltage limit of the
current source. Once that occurs the current source is no
longer able to increase the current driven to the load even
if you increase the setpoint.
The LDTC laser driver includes features that help protect
your laser and make the driver more versatile in a wide
array of applications:
• The current limit is set by an onboard trimpot and
protects the laser from over-current conditions. The
current limit circuit is designed to avoid overshoot,
ringing,orsaturatingthecontrolelements,andrecovers
from limit events without phase shifts or inversions.
• Slow-start delays the current ramp by 100 msec,
and then ramps the current to setpoint at a rate
of 15 mA / msec.
• The photodiode feedback control loop allows for
Constant Power operation whereby the driver adjusts
the laser forward current in order to maintain a constant
photodiode current.
• Brownout protection switches off the laser diode drive
current if VDD_FL drops below 2.7 VDC.
The WTC3243 delivers bidirectional current to a Peltier
Effect thermoelectric cooler, or unidirectional current to a
resistive heater. The controller adjusts the output current
in order to change the temperature of the sensor that is
connected to the thermal load. The goal is to make the
voltage across the sensor match the setpoint voltage,
and then keep them equal in spite of changes to ambient
conditions and variations in thermal load.
SAFETY INFORMATION
& THERMAL DESIGN
CONSIDERATIONS
SAFE OPERATING AREA — DO NOT EXCEED
INTERNAL POWER DISSIPATION LIMITS
Before attempting to operate the LDTC, it is
imperative that you first determine that the laser driver
and temperature controller will operate within the
Safe Operating Area (SOA). Operating the unit outside of
the SOAmay damage the controller or the load, and will void
the warranty.
Go to the Wavelength Electronics website for the most
accurate, up-to-date, and easy to use SOA calculators:
Laser Diode Drivers:
http://www.teamwavelength.com/support/calculator/soa/soald.php
http://www.teamwavelength.com/support/calculator/soa/soald.php
Temperature Controllers:
http://www.teamwavelength.com/support/calculator/soa/soatc.php
http://www.teamwavelength.com/support/calculator/soa/soatc.php
SOAcharts are included in this datasheet for quick reference
(page 22), but we recommend you use the online tools
instead.
TOENSURE SAFE OPERATION OF THE LDTC CONTROLLER,
IT IS IMPERATIVE THAT YOU DETERMINE IF THE UNIT IS GOING
TO BE OPERATING WITHIN THE INTERNAL HEAT DISSIPATION
SAFE OPERATING AREA (SOA).
For more information on Safe Operating Area, see our
Application Note AN-LDTC01: The Principle of the Safe
Operating Area.
When you assemble and mount the TEC (or heater),
heatsink, and temperature sensor, make sure the physical
connections between the components are solid. We
recommend using thermal paste or thermal washers at the
load/TEC and TEC/heatsink interfaces. The thermistor must
be in firm contact with the load in order to achieve stable and
reliable temperature control.
PREVENT DAMAGE FROM
ELECTROSTATIC DISCHARGE
Before proceeding, it is critical that you take precautions to
prevent electrostatic discharge (ESD) damage to the driver
and your laser. ESD damage can result from improper
handling of sensitive electronics, and is easily preventable
with simple precautions.
Enter the search phrase “ESD Precautions for Handling
Electronics” in an internet search engine to find information
on ESD-safe handling practices.
We recommend that you always observe ESD precautions
when handling the LDTC controller and loads.
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