Double the protection of a laser driver using a 1V power supply

Tai-Shan Liao, National Applied Research Laboratories, Instrument Technology Research Center, Hsinchu, Taiwan

EDN

An excessive level of light from a laser pointer, even if only for a short duration, can be harmful if it enters the human eye either directly or through reflection from a shiny object. Most countries, therefore, have laser safety requirements that limit the maximum emission level. This Design Idea describes a laser driver that works even with a single 1.5 V cell discharged to 1 V, and uses dual current-control transistors to improve reliability against shorting and allowing excessive laser current and light emission.

Figure 1.	You can use this dc-dc step-up circuit and dual current-control transistors to safely power this laser from an almost-discharged battery.

In Figure 1, the transistors Q₁, Q₂, and Q₃ compose a negative impedance, which can be described approximately as

Assume that all of the transistors have the same current gain (β), and V_BE is the base-to-emitter voltage of all transistors. Feedback is provided through R_F, and R₁ bias controls the collector current of Q₁. Inductor L₁ and parasitic capacitance form a resonant circuit that oscillates due to the negative impedance, resulting in about 3.5V pk-pk at Q₁’s collector, with the battery at 1 V. Schottky diode D₁ and C₁ form a half-wave rectifier that provides about −3 V for the laser cathode; with V_DD at 1 V, this provides a 4 V working range to overcome the laser threshold.

Q₅ and Q₆ control the laser current. The photodiode built into the laser assembly monitors the light intensity and sends negative feedback through Q₄ to bias Q₅ and Q₆ to the proper collector current for the constant desired laser intensity. The Q₅ and Q₆ pair is series connected so that if one should fail shorted, the other will still maintain the laser current at a safe level. The probability of failure of two transistors at the same time is far lower than the probability of failure of a single transistor.

Editor’s note:

Due to variations in laser and photodiode efficiency, R₇ might need to be adjusted to ensure the laser output is within safety-regulation limits.