QuiC SLED™ Handling and Operation
These application notes give recommendations for safe handling and operation of QuiC SLED™ mid-infrared light-emitting diodes (LEDs). In particular, these notes apply to part number MIRLED04250, in TO-39 packages, operated at room temperature and emitting wavelengths near 4.25 μm. QuiC SLED™ are quantum interband-cascade superlattice-light-emitting diodes, based on InAs-GaSb superlattices tuned to a particular wavelength.
The technology spans all wavelengths ranging from about 3–15 μm, which includes the mid-wave infrared (MWIR) and long-wave infrared (LWIR) spectra. The MIRLED04250 emits milliwatt-scale powers, approximately centered on the 4.26 μm CO2 absorption lines. These devices have been developed for applications in chemical sensing, spectroscopy, infrared illumination, and telecommunications.
Figure 1 shows the pinout configuration for the TO-39 package when viewed from the bottom. Pin #1 is the positive terminal and connects to the anode (+) of the LED. Pin #2 connects the cathode (–) of the LED as well as the device case. Pin #3 is not connected (NC).
Packaged LEDs are fragile devices and should be handled with great care. Like most other electronic devices, QuiC SLED™ are susceptible to damage from electrostatic discharge (ESD) and proper ESD prevention should be exercised while handling. If the device is used without a cap, exposing the LED die and its bonding wires, then great care should be taken to avoid dropping the device or inadvertently allowing contact with other obstacles.
We recommend handling the device only with sturdy tweezers and only by the pins. Devices without a cap should never be lain down on their side, as the the TO- 39 package is very top-heavy and tends to roll over onto the top. Pins should always be safely secured, either by storage in the packaging foam that came with the box or by standard electronic sockets.
Recommended Operating Conditions
Under most practical conditions, the limiting factor against higher drive current, and thus higher output power, is heat dissipation. When driven at a constant DC current, we recommend a maximum value of 200 mA for reliable operation. If higher drive currents are desirable, it is possible to drive the device with short pulses of current at reduced duty cycle. For example, reliable operation at 300 mA is possible when driven at a 50 % duty cycle. Drive conditions such as this are often called quasi continuous wave (qCW).
When very large instantaneous drive currents are desired, nonlinearities can offset the average safe power-handling capabilities of the device. We currently recommend no more than 2.0 A in pulse amplitude, which can be safely driven for 50 μs at a 1.0 % duty cycle without heat-sinking. Further operating conditions can be extrapolated from this point by conserving the total time-averaged dissipated power.
Figure 2 summarizes a set of safe duty cycles under high-current conditions as a function of pulse amplitude.
Heat sinking is recommended for TO-39 packaged devices when used at high-drive conditions. Doing so will help to ensure longer lifetime of the devices. Other benefits include device stability, since certain device parameters can vary noticeably with temperature. For example, Figure 3 compares the normalized output spectra of a typical QuiC SLED™ , both with and without a simple heat sink clipped onto the cap. In the absence of any heat sink, higher junction temperatures manifest as a slight broadening of the spectrum with a slight shift toward longer wavelengths.
Another temperature-sensitive parameter is the current-voltage response of the device. Figure 4 compares the current-voltage characteristics that occur with and without a heat sink. Without the heat sink, we observe that the voltage required to drive a specific current tends to decrease. If one is not careful, this can lead to non-stationary behavior of the device over time. For example, when the device is driven by a constant voltage source, drift in temperatures tend to result in greater drive current for the same potential.
Higher current then leads to greater increase in temperature, and thus greater current still. Positive feedback continues to force a drift in output current until eventually settling into a thermal equilibrium. For this reason, we recommend the use of a regulated constant-current source when driving the QuiC SLED™ to maintain consistent power output.