Driver thermally compensates LED

Andrzej Wolczko, University of Mining and Metallurgy, Krakow, Poland -- 9/2/1999

Optical power of popular types of LEDs decreases with temperature. Optical-power measurements into a typical multimode fiber at 62.5/125 ?m for a GCA 1A194 LED indicate a temperature coefficient of approximately -0.4%/°C. In bipolar analog drivers for short- and medium-distance fiber links with a dc component, you frequently place an LED in the collector of a differential pair. For zero-input signals, the LED emits half its maximum power (zero reference), and the bipolar input signal modulates that power from zero to maximum. Temperature changes cause decreased power of zero reference and decreased slope efficiency of optical power versus input voltage.

The circuit in Figure 1 allows you to compensate for both of these temperature-dependent problems, using only the TSF-102 sensor from Texas Instruments (www.ti.com). The device is a temperature-dependent positive-temperature-coefficient resistor with a linear temperature coefficient of approximately 0.7%/°C at 25°C. For the circuit in Figure 1, you must thermally couple the sensor with the LED. IC₂ is a summing amplifier for the input voltage and the reference voltage from IC₃. The gain is -1 and increases with temperature. The temperature-independent -V_REF drives the base of transistor Q₂. The thermally stable current source, Q₃, via the differential pair Q₁-Q₂, supplies the LED. Note that you should mount Q₃, D, and D_Z on a common heat sink.

With V_IN=0V, temperature changes unbalance the Q₁-Q₂ pair, such that the optical power remains constant. Empirical measurements show that the compensation is optimal with V_REF=1.1V. IC₂ amplifies the input signal by a temperature-dependent factor. You obtain matching of the temperature coefficient of the sensor to the coefficient of the LED by inserting the additional 500? resistor in the feedback loop of IC₂. Figure 2 shows the LED current (Figure 2a) and the optical power (Figure 2b) in the fiber-versus-input voltage for 10 and 50°C. The circuit provides an approximate tenfold decrease in the thermal coefficient. The improvement comes at the cost of a limited input-signal range for linear operation. The circuit has a bandwidth of 0 Hz to more than 10 MHz. (DI #2398).

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