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Browse Prior Art Database

Infrared LED Thermal Droop Compensator

IP.com Disclosure Number: IPCOM000040114D
Original Publication Date: 1987-Sep-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 52K

Publishing Venue

IBM

Related People

Bustamante, CM: AUTHOR [+2]

Abstract

A technique is described whereby an electrical circuit is added to infrared light-emitting diode (LED) transmitting circuitry so as to compensate for thermal droop. Thermal droop is the loss of infrared transmitting capability of LEDs as the diode temperature rises with use. The circuit compensates for the thermal droop by controlling the current applied to the LED, thereby increasing the performance of communication packets. (Image Omitted) Since all LEDs exhibit a decrease in emitted infrared light as the diode temperature increases, bit error rates (BERs) of transmitted data also increase as the diodes heat up, placing a limit on usable packet lengths. Typical optical output power loss is approximately 1% per oC. As data is transmitted, the temperature of the LEDs will increase, reducing the light output for transmission.

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Infrared LED Thermal Droop Compensator

A technique is described whereby an electrical circuit is added to infrared light- emitting diode (LED) transmitting circuitry so as to compensate for thermal droop. Thermal droop is the loss of infrared transmitting capability of LEDs as the diode temperature rises with use. The circuit compensates for the thermal droop by controlling the current applied to the LED, thereby increasing the performance of communication packets.

(Image Omitted)

Since all LEDs exhibit a decrease in emitted infrared light as the diode temperature increases, bit error rates (BERs) of transmitted data also increase as the diodes heat up, placing a limit on usable packet lengths. Typical optical output power loss is approximately 1% per oC. As data is transmitted, the temperature of the LEDs will increase, reducing the light output for transmission. Power output at the end of a packet is then lower than at the beginning of the packet, which in turn increases the BER. The circuitry described herein provides a way of compensating for the thermal effect on the LED, thereby providing a uniform BER throughout the packet. The typical LED transmitting circuit, as shown in Fig. 1, uses a linear feedback such that the current source formed by transistor 10 and resistor 11 will provide the effect that the higher the current source value, the lower the transmitted current, and vice-versa, due to the feedback to the gate drive voltage 13. The voltage at node 14 is directly proportional to the amplitude of the current through the load. The thermal compensating circuit 15, as shown in Fig. 2, provides an effective injection of current through resistor 11, thereby causing a larger required voltage drop to occur across sense resistor 16 activating the linear feedback current source circuit made up of transistor 10 and resistor 11. A drop across resistor 16 correlates to greater drive currents through the transmit diodes. By injecting current through resistor 11, the voltage at the emitter of transistor 10 increases. Fo...