© 2018
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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.
It may be useful to remember that you do not directly set the
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, or
saturating the control elements, and recovers 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 V
DD_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 SOA may 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:
www.teamwavelength.com/support/design-tools/soa-ld-calculator/
Temperature Controllers:
www.teamwavelength.com/support/design-tools/soa-tc-calculator/
SOA charts are included in this datasheet for quick reference
(
), but we recommend you use the online tools
instead.
To ensure 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
or our video,
.
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.
For more information regarding ESD, see our Application
Note
AN-LDTC06: Electrostatic Discharge Basics
We recommend that you always observe ESD precautions
when handling the LDTC controller and loads.
!
